id
stringlengths 14
28
| title
stringclasses 18
values | content
stringlengths 2
999
| contents
stringlengths 19
1.02k
|
|---|---|---|---|
Biochemistry_Lippincott_483 | Biochemistry_Lippinco | Lactose is a disaccharide that consists of a molecule of β-galactose attached by a β(1→4) linkage to glucose. Therefore, lactose is galactosyl β(1→4)-glucose. Because lactose (milk sugar) is made by lactating (milk-producing) mammary glands, milk and other dairy products are the dietary sources of lactose. Lactose is synthesized in the Golgi by lactose synthase (UDP-galactose:glucose galactosyltransferase), which transfers galactose from UDP-galactose to glucose, releasing UDP (Fig. 12.7). This enzyme is composed of two proteins, A and B. Protein A is a β-D-galactosyltransferase and is found in a number of body tissues. In tissues other than the lactating mammary gland, this enzyme transfers galactose from UDP-galactose to N-acetyl-D-glucosamine, forming the same β(1→4) linkage found in lactose, and producing N-acetyllactosamine, a component of the structurally important N-linked glycoproteins (see p. 167). In contrast, protein B is found only in lactating mammary glands. It is | Biochemistry_Lippinco. Lactose is a disaccharide that consists of a molecule of β-galactose attached by a β(1→4) linkage to glucose. Therefore, lactose is galactosyl β(1→4)-glucose. Because lactose (milk sugar) is made by lactating (milk-producing) mammary glands, milk and other dairy products are the dietary sources of lactose. Lactose is synthesized in the Golgi by lactose synthase (UDP-galactose:glucose galactosyltransferase), which transfers galactose from UDP-galactose to glucose, releasing UDP (Fig. 12.7). This enzyme is composed of two proteins, A and B. Protein A is a β-D-galactosyltransferase and is found in a number of body tissues. In tissues other than the lactating mammary gland, this enzyme transfers galactose from UDP-galactose to N-acetyl-D-glucosamine, forming the same β(1→4) linkage found in lactose, and producing N-acetyllactosamine, a component of the structurally important N-linked glycoproteins (see p. 167). In contrast, protein B is found only in lactating mammary glands. It is |
Biochemistry_Lippincott_484 | Biochemistry_Lippinco | in lactose, and producing N-acetyllactosamine, a component of the structurally important N-linked glycoproteins (see p. 167). In contrast, protein B is found only in lactating mammary glands. It is αlactalbumin, and its synthesis is stimulated by the peptide hormone prolactin. Protein B forms a complex with the enzyme, protein A, changing the specificity of that transferase (by decreasing the Km for glucose) so that lactose, rather than | Biochemistry_Lippinco. in lactose, and producing N-acetyllactosamine, a component of the structurally important N-linked glycoproteins (see p. 167). In contrast, protein B is found only in lactating mammary glands. It is αlactalbumin, and its synthesis is stimulated by the peptide hormone prolactin. Protein B forms a complex with the enzyme, protein A, changing the specificity of that transferase (by decreasing the Km for glucose) so that lactose, rather than |
Biochemistry_Lippincott_485 | Biochemistry_Lippinco | N-acetyllactosamine, is produced (see Fig. 12.7). V. CHAPTER SUMMARY | Biochemistry_Lippinco. N-acetyllactosamine, is produced (see Fig. 12.7). V. CHAPTER SUMMARY |
Biochemistry_Lippincott_486 | Biochemistry_Lippinco | The major source of fructose is the disaccharide sucrose, which, when cleaved, releases equimolar amounts of fructose and glucose (Fig. 12.8). Transport of fructose into cells is insulin independent. Fructose is first phosphorylated to fructose 1-phosphate by fructokinase and then cleaved by aldolase B to dihydroxyacetone phosphate and glyceraldehyde. These enzymes are found in the liver, kidneys, and small intestine. A deficiency of fructokinase causes a benign condition (essential fructosuria), whereas a deficiency of aldolase B causes hereditary fructose intolerance (HFI), in which severe hypoglycemia and liver failure lead to death if fructose (and sucrose) is not removed from the diet. Mannose, an important component of glycoproteins, is phosphorylated by hexokinase to mannose 6phosphate, which is reversibly isomerized to fructose 6-phosphate by phosphomannose isomerase. Glucose can be reduced to sorbitol (glucitol) by aldose reductase in many tissues, including the lens, retina, | Biochemistry_Lippinco. The major source of fructose is the disaccharide sucrose, which, when cleaved, releases equimolar amounts of fructose and glucose (Fig. 12.8). Transport of fructose into cells is insulin independent. Fructose is first phosphorylated to fructose 1-phosphate by fructokinase and then cleaved by aldolase B to dihydroxyacetone phosphate and glyceraldehyde. These enzymes are found in the liver, kidneys, and small intestine. A deficiency of fructokinase causes a benign condition (essential fructosuria), whereas a deficiency of aldolase B causes hereditary fructose intolerance (HFI), in which severe hypoglycemia and liver failure lead to death if fructose (and sucrose) is not removed from the diet. Mannose, an important component of glycoproteins, is phosphorylated by hexokinase to mannose 6phosphate, which is reversibly isomerized to fructose 6-phosphate by phosphomannose isomerase. Glucose can be reduced to sorbitol (glucitol) by aldose reductase in many tissues, including the lens, retina, |
Biochemistry_Lippincott_487 | Biochemistry_Lippinco | which is reversibly isomerized to fructose 6-phosphate by phosphomannose isomerase. Glucose can be reduced to sorbitol (glucitol) by aldose reductase in many tissues, including the lens, retina, peripheral nerves, kidneys, ovaries, and seminal vesicles. In the liver, ovaries, and seminal vesicles, a second enzyme, sorbitol dehydrogenase, can oxidize sorbitol to produce fructose. Hyperglycemia results in the accumulation of sorbitol in those cells lacking sorbitol dehydrogenase. The resulting osmotic events cause cell swelling and may contribute to the cataract formation, peripheral neuropathy, nephropathy, and retinopathy seen in diabetes. The major dietary source of galactose is lactose. The transport of galactose into cells is insulin independent. Galactose is first phosphorylated by galactokinase (a deficiency results in cataracts) to galactose 1phosphate. This compound is converted to uridine diphosphate (UDP)galactose by galactose 1-phosphate uridylyltransferase (GALT), with the | Biochemistry_Lippinco. which is reversibly isomerized to fructose 6-phosphate by phosphomannose isomerase. Glucose can be reduced to sorbitol (glucitol) by aldose reductase in many tissues, including the lens, retina, peripheral nerves, kidneys, ovaries, and seminal vesicles. In the liver, ovaries, and seminal vesicles, a second enzyme, sorbitol dehydrogenase, can oxidize sorbitol to produce fructose. Hyperglycemia results in the accumulation of sorbitol in those cells lacking sorbitol dehydrogenase. The resulting osmotic events cause cell swelling and may contribute to the cataract formation, peripheral neuropathy, nephropathy, and retinopathy seen in diabetes. The major dietary source of galactose is lactose. The transport of galactose into cells is insulin independent. Galactose is first phosphorylated by galactokinase (a deficiency results in cataracts) to galactose 1phosphate. This compound is converted to uridine diphosphate (UDP)galactose by galactose 1-phosphate uridylyltransferase (GALT), with the |
Biochemistry_Lippincott_488 | Biochemistry_Lippinco | (a deficiency results in cataracts) to galactose 1phosphate. This compound is converted to uridine diphosphate (UDP)galactose by galactose 1-phosphate uridylyltransferase (GALT), with the nucleotide supplied by UDP-glucose. A deficiency of this enzyme causes classic galactosemia. Galactose 1-phosphate accumulates, and excess galactose is converted to galactitol by aldose reductase. This causes liver damage, brain damage, and cataracts. Treatment requires removal of galactose (and lactose) from the diet. For UDP-galactose to enter the mainstream of glucose metabolism, it must first be isomerized to UDP-glucose by UDP-hexose 4-epimerase. This enzyme can also be used to produce UDP-galactose from UDP-glucose when the former is required for glycoprotein and glycolipid synthesis. Lactose is a disaccharide of galactose and glucose. Milk and other dairy products are the dietary sources of lactose. Lactose is synthesized by lactose synthase from UDP-galactose and glucose in the lactating | Biochemistry_Lippinco. (a deficiency results in cataracts) to galactose 1phosphate. This compound is converted to uridine diphosphate (UDP)galactose by galactose 1-phosphate uridylyltransferase (GALT), with the nucleotide supplied by UDP-glucose. A deficiency of this enzyme causes classic galactosemia. Galactose 1-phosphate accumulates, and excess galactose is converted to galactitol by aldose reductase. This causes liver damage, brain damage, and cataracts. Treatment requires removal of galactose (and lactose) from the diet. For UDP-galactose to enter the mainstream of glucose metabolism, it must first be isomerized to UDP-glucose by UDP-hexose 4-epimerase. This enzyme can also be used to produce UDP-galactose from UDP-glucose when the former is required for glycoprotein and glycolipid synthesis. Lactose is a disaccharide of galactose and glucose. Milk and other dairy products are the dietary sources of lactose. Lactose is synthesized by lactose synthase from UDP-galactose and glucose in the lactating |
Biochemistry_Lippincott_489 | Biochemistry_Lippinco | a disaccharide of galactose and glucose. Milk and other dairy products are the dietary sources of lactose. Lactose is synthesized by lactose synthase from UDP-galactose and glucose in the lactating mammary gland. The enzyme has two subunits, protein A (which is a galactosyltransferase found in most cells where it synthesizes N-acetyllactosamine) and protein B (α-lactalbumin, which is found only in lactating mammary glands, and whose synthesis is stimulated by the peptide hormone prolactin). When both subunits are present, the transferase produces lactose. | Biochemistry_Lippinco. a disaccharide of galactose and glucose. Milk and other dairy products are the dietary sources of lactose. Lactose is synthesized by lactose synthase from UDP-galactose and glucose in the lactating mammary gland. The enzyme has two subunits, protein A (which is a galactosyltransferase found in most cells where it synthesizes N-acetyllactosamine) and protein B (α-lactalbumin, which is found only in lactating mammary glands, and whose synthesis is stimulated by the peptide hormone prolactin). When both subunits are present, the transferase produces lactose. |
Biochemistry_Lippincott_490 | Biochemistry_Lippinco | Choose the ONE best answer. 2.1. A nursing female with classic galactosemia is on a galactose-free diet. She is able to produce lactose in breast milk because: A. galactose can be produced from fructose by isomerization. B. galactose can be produced from a glucose metabolite by epimerization. C. hexokinase can efficiently phosphorylate galactose to galactose 1phosphate. D. the enzyme affected in galactosemia is activated by a hormone produced in the mammary gland. Correct answer = B. Uridine diphosphate (UDP)-glucose is converted to UDPgalactose by UDP-hexose 4-epimerase, thereby providing the appropriate form of galactose for lactose synthesis. Isomerization of fructose to galactose does not occur in the human body. Galactose is not converted to galactose 1phosphate by hexokinase. A galactose-free diet provides no galactose. Galactosemia is the result of an enzyme (galactose 1-phosphate uridylyltransferase) deficiency. | Biochemistry_Lippinco. Choose the ONE best answer. 2.1. A nursing female with classic galactosemia is on a galactose-free diet. She is able to produce lactose in breast milk because: A. galactose can be produced from fructose by isomerization. B. galactose can be produced from a glucose metabolite by epimerization. C. hexokinase can efficiently phosphorylate galactose to galactose 1phosphate. D. the enzyme affected in galactosemia is activated by a hormone produced in the mammary gland. Correct answer = B. Uridine diphosphate (UDP)-glucose is converted to UDPgalactose by UDP-hexose 4-epimerase, thereby providing the appropriate form of galactose for lactose synthesis. Isomerization of fructose to galactose does not occur in the human body. Galactose is not converted to galactose 1phosphate by hexokinase. A galactose-free diet provides no galactose. Galactosemia is the result of an enzyme (galactose 1-phosphate uridylyltransferase) deficiency. |
Biochemistry_Lippincott_491 | Biochemistry_Lippinco | 2.2. A 5-month-old boy is brought to his physician because of vomiting, night sweats, and tremors. History revealed that these symptoms began after fruit juices were introduced to his diet as he was being weaned off breast milk. The physical examination was remarkable for hepatomegaly. Tests on the baby’s urine were positive for reducing sugar but negative for glucose. The infant most likely suffers from a deficiency of: A. aldolase B. B. fructokinase. C. galactokinase. D. β-galactosidase. | Biochemistry_Lippinco. 2.2. A 5-month-old boy is brought to his physician because of vomiting, night sweats, and tremors. History revealed that these symptoms began after fruit juices were introduced to his diet as he was being weaned off breast milk. The physical examination was remarkable for hepatomegaly. Tests on the baby’s urine were positive for reducing sugar but negative for glucose. The infant most likely suffers from a deficiency of: A. aldolase B. B. fructokinase. C. galactokinase. D. β-galactosidase. |
Biochemistry_Lippincott_492 | Biochemistry_Lippinco | A. aldolase B. B. fructokinase. C. galactokinase. D. β-galactosidase. Correct answer = A. The symptoms suggest hereditary fructose intolerance, a deficiency in aldolase B. Deficiencies in fructokinase or galactokinase result in relatively benign conditions characterized by elevated levels of fructose or galactose in the blood and urine. Deficiency in β-galactosidase (lactase) results in a decreased ability to degrade lactose (milk sugar). Congenital lactase deficiency is quite rare and would have presented much earlier in this baby (and with different symptoms). Typical lactase deficiency (adult hypolactasia) presents at a later age. 2.3. Lactose synthesis is essential in the production of milk by mammary glands. In lactose synthesis: A. galactose from galactose 1-phosphate is transferred to glucose by galactosyltransferase (protein A), generating lactose. B. protein A is used exclusively in lactose synthesis. | Biochemistry_Lippinco. A. aldolase B. B. fructokinase. C. galactokinase. D. β-galactosidase. Correct answer = A. The symptoms suggest hereditary fructose intolerance, a deficiency in aldolase B. Deficiencies in fructokinase or galactokinase result in relatively benign conditions characterized by elevated levels of fructose or galactose in the blood and urine. Deficiency in β-galactosidase (lactase) results in a decreased ability to degrade lactose (milk sugar). Congenital lactase deficiency is quite rare and would have presented much earlier in this baby (and with different symptoms). Typical lactase deficiency (adult hypolactasia) presents at a later age. 2.3. Lactose synthesis is essential in the production of milk by mammary glands. In lactose synthesis: A. galactose from galactose 1-phosphate is transferred to glucose by galactosyltransferase (protein A), generating lactose. B. protein A is used exclusively in lactose synthesis. |
Biochemistry_Lippincott_493 | Biochemistry_Lippinco | A. galactose from galactose 1-phosphate is transferred to glucose by galactosyltransferase (protein A), generating lactose. B. protein A is used exclusively in lactose synthesis. C. α-lactalbumin (protein B) regulates the specificity of protein A by decreasing its affinity for glucose. D. protein B expression is stimulated by prolactin. Correct answer = D. α-Lactalbumin (protein B) expression is increased by the hormone prolactin. Uridine diphosphate–galactose is the form used by the galactosyltransferase (protein A). Protein A is also involved in the synthesis of the amino sugar N-acetyllactosamine. Protein B decreases the Michaelis constant (Km) and, so, increases the affinity of protein A for glucose. | Biochemistry_Lippinco. A. galactose from galactose 1-phosphate is transferred to glucose by galactosyltransferase (protein A), generating lactose. B. protein A is used exclusively in lactose synthesis. C. α-lactalbumin (protein B) regulates the specificity of protein A by decreasing its affinity for glucose. D. protein B expression is stimulated by prolactin. Correct answer = D. α-Lactalbumin (protein B) expression is increased by the hormone prolactin. Uridine diphosphate–galactose is the form used by the galactosyltransferase (protein A). Protein A is also involved in the synthesis of the amino sugar N-acetyllactosamine. Protein B decreases the Michaelis constant (Km) and, so, increases the affinity of protein A for glucose. |
Biochemistry_Lippincott_494 | Biochemistry_Lippinco | 2.4. A 3-month-old girl is developing cataracts. Other than not having a social smile or being able to track objects visually, all other aspects of the girl’s examination are normal. Tests on the baby’s urine are positive for reducing sugar but negative for glucose. Which enzyme is most likely deficient in this girl? A. Aldolase B B. Fructokinase C. Galactokinase D. Galactose 1-phosphate uridylyltransferase | Biochemistry_Lippinco. 2.4. A 3-month-old girl is developing cataracts. Other than not having a social smile or being able to track objects visually, all other aspects of the girl’s examination are normal. Tests on the baby’s urine are positive for reducing sugar but negative for glucose. Which enzyme is most likely deficient in this girl? A. Aldolase B B. Fructokinase C. Galactokinase D. Galactose 1-phosphate uridylyltransferase |
Biochemistry_Lippincott_495 | Biochemistry_Lippinco | A. Aldolase B B. Fructokinase C. Galactokinase D. Galactose 1-phosphate uridylyltransferase Correct answer = C. The girl is deficient in galactokinase and is unable to appropriately phosphorylate galactose. Galactose accumulates in the blood (and urine). In the lens of the eye, galactose is reduced by aldose reductase to galactitol, a sugar alcohol, which causes osmotic effects that result in cataract formation. Deficiency of galactose 1-phosphate uridylyltransferase also results in cataracts but is characterized by liver damage and neurologic effects. Fructokinase deficiency is a benign condition. Aldolase B deficiency is severe, with effects on several tissues. Cataracts are not typically seen. For additional ancillary materials related to this chapter, please visit thePoint. I. OVERVIEW | Biochemistry_Lippinco. A. Aldolase B B. Fructokinase C. Galactokinase D. Galactose 1-phosphate uridylyltransferase Correct answer = C. The girl is deficient in galactokinase and is unable to appropriately phosphorylate galactose. Galactose accumulates in the blood (and urine). In the lens of the eye, galactose is reduced by aldose reductase to galactitol, a sugar alcohol, which causes osmotic effects that result in cataract formation. Deficiency of galactose 1-phosphate uridylyltransferase also results in cataracts but is characterized by liver damage and neurologic effects. Fructokinase deficiency is a benign condition. Aldolase B deficiency is severe, with effects on several tissues. Cataracts are not typically seen. For additional ancillary materials related to this chapter, please visit thePoint. I. OVERVIEW |
Biochemistry_Lippincott_496 | Biochemistry_Lippinco | I. OVERVIEW The pentose phosphate pathway (or, hexose monophosphate shunt) occurs in the cytosol. It includes an irreversible oxidative phase, followed by a series of reversible sugar–phosphate interconversions (Fig. 13.1). In the oxidative phase, carbon 1 of a glucose 6-phosphate molecule is released as carbon dioxide (CO2), and one pentose sugar-phosphate plus two reduced nicotinamide adenine dinucleotide phosphates (NADPH) are produced. The rate and direction of the reversible reactions are determined by the supply of and demand for intermediates of the pathway. The pentose phosphate pathway provides a major portion of the body’s NADPH, which functions as a biochemical reductant. It also produces ribose 5-phosphate, required for nucleotide biosynthesis (see p. 293), and provides a mechanism for the conversion of pentose sugars to triose and hexose intermediates of glycolysis. No ATP is directly consumed or produced in the pathway. II. IRREVERSIBLE OXIDATIVE REACTIONS | Biochemistry_Lippinco. I. OVERVIEW The pentose phosphate pathway (or, hexose monophosphate shunt) occurs in the cytosol. It includes an irreversible oxidative phase, followed by a series of reversible sugar–phosphate interconversions (Fig. 13.1). In the oxidative phase, carbon 1 of a glucose 6-phosphate molecule is released as carbon dioxide (CO2), and one pentose sugar-phosphate plus two reduced nicotinamide adenine dinucleotide phosphates (NADPH) are produced. The rate and direction of the reversible reactions are determined by the supply of and demand for intermediates of the pathway. The pentose phosphate pathway provides a major portion of the body’s NADPH, which functions as a biochemical reductant. It also produces ribose 5-phosphate, required for nucleotide biosynthesis (see p. 293), and provides a mechanism for the conversion of pentose sugars to triose and hexose intermediates of glycolysis. No ATP is directly consumed or produced in the pathway. II. IRREVERSIBLE OXIDATIVE REACTIONS |
Biochemistry_Lippincott_497 | Biochemistry_Lippinco | II. IRREVERSIBLE OXIDATIVE REACTIONS The oxidative portion of the pentose phosphate pathway consists of three irreversible reactions that lead to the formation of ribulose 5-phosphate, CO2, and two molecules of NADPH for each molecule of glucose 6-phosphate oxidized (Fig. 13.2). This portion of the pathway is particularly important in the liver, lactating mammary glands, and adipose tissue for the NADPH-dependent biosynthesis of fatty acids (see p. 186); in the testes, ovaries, placenta, and adrenal cortex for the NADPH-dependent biosynthesis of steroid hormones (see p. 237); and in red blood cells (RBC) for the NADPH-dependent reduction of glutathione (see p. 148). A. Glucose 6-phosphate dehydrogenation | Biochemistry_Lippinco. II. IRREVERSIBLE OXIDATIVE REACTIONS The oxidative portion of the pentose phosphate pathway consists of three irreversible reactions that lead to the formation of ribulose 5-phosphate, CO2, and two molecules of NADPH for each molecule of glucose 6-phosphate oxidized (Fig. 13.2). This portion of the pathway is particularly important in the liver, lactating mammary glands, and adipose tissue for the NADPH-dependent biosynthesis of fatty acids (see p. 186); in the testes, ovaries, placenta, and adrenal cortex for the NADPH-dependent biosynthesis of steroid hormones (see p. 237); and in red blood cells (RBC) for the NADPH-dependent reduction of glutathione (see p. 148). A. Glucose 6-phosphate dehydrogenation |
Biochemistry_Lippincott_498 | Biochemistry_Lippinco | A. Glucose 6-phosphate dehydrogenation Glucose 6-phosphate dehydrogenase (G6PD) catalyzes the oxidation of glucose 6-phosphate to 6-phosphogluconolactone as the coenzyme NADP+ gets reduced to NADPH. This initial reaction is the committed, rate-limiting, and regulated step of the pathway. NADPH is a potent competitive inhibitor of G6PD, and the ratio of NADPH/NADP+ is sufficiently high to substantially inhibit the enzyme under most metabolic conditions. However, with increased demand for NADPH, the ratio of NADPH/NADP+ decreases, and flux through the pathway increases in response to the enhanced activity of G6PD. [Note: Insulin upregulates expression of the gene for G6PD, and flux through the pathway increases in the absorptive state (see p. 323).] | Biochemistry_Lippinco. A. Glucose 6-phosphate dehydrogenation Glucose 6-phosphate dehydrogenase (G6PD) catalyzes the oxidation of glucose 6-phosphate to 6-phosphogluconolactone as the coenzyme NADP+ gets reduced to NADPH. This initial reaction is the committed, rate-limiting, and regulated step of the pathway. NADPH is a potent competitive inhibitor of G6PD, and the ratio of NADPH/NADP+ is sufficiently high to substantially inhibit the enzyme under most metabolic conditions. However, with increased demand for NADPH, the ratio of NADPH/NADP+ decreases, and flux through the pathway increases in response to the enhanced activity of G6PD. [Note: Insulin upregulates expression of the gene for G6PD, and flux through the pathway increases in the absorptive state (see p. 323).] |
Biochemistry_Lippincott_499 | Biochemistry_Lippinco | B. Ribulose 5-phosphate formation 6-Phosphogluconolactone is hydrolyzed by 6-phosphogluconolactone hydrolase in the second step. The oxidative decarboxylation of the product, 6-phosphogluconate, is catalyzed by 6-phosphogluconate dehydrogenase. This third irreversible step produces ribulose 5-phosphate (a pentose sugar– phosphate), CO2 (from carbon 1 of glucose), and a second molecule of NADPH (see Fig. 13.2). III. REVERSIBLE NONOXIDATIVE REACTIONS | Biochemistry_Lippinco. B. Ribulose 5-phosphate formation 6-Phosphogluconolactone is hydrolyzed by 6-phosphogluconolactone hydrolase in the second step. The oxidative decarboxylation of the product, 6-phosphogluconate, is catalyzed by 6-phosphogluconate dehydrogenase. This third irreversible step produces ribulose 5-phosphate (a pentose sugar– phosphate), CO2 (from carbon 1 of glucose), and a second molecule of NADPH (see Fig. 13.2). III. REVERSIBLE NONOXIDATIVE REACTIONS |
Biochemistry_Lippincott_500 | Biochemistry_Lippinco | The nonoxidative reactions of the pentose phosphate pathway occur in all cell types synthesizing nucleotides and nucleic acids. These reactions catalyze the interconversion of sugars containing three to seven carbons (see Fig. 13.2). These reversible reactions permit ribulose 5-phosphate (produced by the oxidative portion of the pathway) to be converted either to ribose 5-phosphate (needed for nucleotide synthesis; see p. 293) or to intermediates of glycolysis (that is, fructose 6-phosphate and glyceraldehyde 3-phosphate). For example, many cells that carry out reductive biosynthetic reactions have a greater need for NADPH than for ribose 5-phosphate. In this case, transketolase (which transfers two-carbon units in a thiamine pyrophosphate [TPP]-requiring reaction) and transaldolase (which transfers three-carbon units) convert the ribulose 5phosphate produced as an end product of the oxidative phase to glyceraldehyde 3-phosphate and fructose 6-phosphate, which are glycolytic | Biochemistry_Lippinco. The nonoxidative reactions of the pentose phosphate pathway occur in all cell types synthesizing nucleotides and nucleic acids. These reactions catalyze the interconversion of sugars containing three to seven carbons (see Fig. 13.2). These reversible reactions permit ribulose 5-phosphate (produced by the oxidative portion of the pathway) to be converted either to ribose 5-phosphate (needed for nucleotide synthesis; see p. 293) or to intermediates of glycolysis (that is, fructose 6-phosphate and glyceraldehyde 3-phosphate). For example, many cells that carry out reductive biosynthetic reactions have a greater need for NADPH than for ribose 5-phosphate. In this case, transketolase (which transfers two-carbon units in a thiamine pyrophosphate [TPP]-requiring reaction) and transaldolase (which transfers three-carbon units) convert the ribulose 5phosphate produced as an end product of the oxidative phase to glyceraldehyde 3-phosphate and fructose 6-phosphate, which are glycolytic |
Biochemistry_Lippincott_501 | Biochemistry_Lippinco | (which transfers three-carbon units) convert the ribulose 5phosphate produced as an end product of the oxidative phase to glyceraldehyde 3-phosphate and fructose 6-phosphate, which are glycolytic intermediates. In contrast, when the demand for ribose for nucleotides and nucleic acids is greater than the need for NADPH, the nonoxidative reactions can provide the ribose 5phosphate from glyceraldehyde 3-phosphate and fructose 6-phosphate in the absence of the oxidative steps (Fig. 13.3). | Biochemistry_Lippinco. (which transfers three-carbon units) convert the ribulose 5phosphate produced as an end product of the oxidative phase to glyceraldehyde 3-phosphate and fructose 6-phosphate, which are glycolytic intermediates. In contrast, when the demand for ribose for nucleotides and nucleic acids is greater than the need for NADPH, the nonoxidative reactions can provide the ribose 5phosphate from glyceraldehyde 3-phosphate and fructose 6-phosphate in the absence of the oxidative steps (Fig. 13.3). |
Biochemistry_Lippincott_502 | Biochemistry_Lippinco | In addition to transketolase, TPP is required by the multienzyme complexes pyruvate dehydrogenase (see p. 110), α-ketoglutarate dehydrogenase of the tricarboxylic acid cycle (see p. 112), and branched-chain α-keto acid dehydrogenase of branched-chain amino acid catabolism (see p. 266). IV. NADPH USES The coenzyme NADPH differs from nicotinamide adenine dinucleotide (NADH) only by the presence of a phosphate group on one of the ribose units (Fig. 13.4). This seemingly small change in structure allows NADPH to interact with NADPH-specific enzymes that have unique roles in the cell. For example, in the cytosol of hepatocytes, the steady-state NADP+/NADPH ratio is ~0.1, which favors the use of NADPH in reductive biosynthetic reactions. This contrasts with the high NAD+/NADH ratio (~1,000), which favors an oxidative role for NAD+. This section summarizes some important NADPH-specific functions in reductive biosynthesis and detoxification reactions. A. Reductive biosynthesis | Biochemistry_Lippinco. In addition to transketolase, TPP is required by the multienzyme complexes pyruvate dehydrogenase (see p. 110), α-ketoglutarate dehydrogenase of the tricarboxylic acid cycle (see p. 112), and branched-chain α-keto acid dehydrogenase of branched-chain amino acid catabolism (see p. 266). IV. NADPH USES The coenzyme NADPH differs from nicotinamide adenine dinucleotide (NADH) only by the presence of a phosphate group on one of the ribose units (Fig. 13.4). This seemingly small change in structure allows NADPH to interact with NADPH-specific enzymes that have unique roles in the cell. For example, in the cytosol of hepatocytes, the steady-state NADP+/NADPH ratio is ~0.1, which favors the use of NADPH in reductive biosynthetic reactions. This contrasts with the high NAD+/NADH ratio (~1,000), which favors an oxidative role for NAD+. This section summarizes some important NADPH-specific functions in reductive biosynthesis and detoxification reactions. A. Reductive biosynthesis |
Biochemistry_Lippincott_503 | Biochemistry_Lippinco | A. Reductive biosynthesis Like NADH, NADPH can be thought of as a high-energy molecule. However, the electrons of NADPH are used for reductive biosynthesis, rather than for transfer to the electron transport chain as is seen with NADH (see p. 74). Thus, in the metabolic transformations of the pentose phosphate pathway, part of the energy of glucose 6-phosphate is conserved in NADPH, a molecule with a negative reduction potential (see p. 76), that, therefore, can be used in reactions requiring an electron donor, such as fatty acid (see p. 186), cholesterol (see p. 221), and steroid hormone (see p. 237) synthesis. B. Hydrogen peroxide reduction | Biochemistry_Lippinco. A. Reductive biosynthesis Like NADH, NADPH can be thought of as a high-energy molecule. However, the electrons of NADPH are used for reductive biosynthesis, rather than for transfer to the electron transport chain as is seen with NADH (see p. 74). Thus, in the metabolic transformations of the pentose phosphate pathway, part of the energy of glucose 6-phosphate is conserved in NADPH, a molecule with a negative reduction potential (see p. 76), that, therefore, can be used in reactions requiring an electron donor, such as fatty acid (see p. 186), cholesterol (see p. 221), and steroid hormone (see p. 237) synthesis. B. Hydrogen peroxide reduction |
Biochemistry_Lippincott_504 | Biochemistry_Lippinco | B. Hydrogen peroxide reduction Hydrogen peroxide (H2O2) is one of a family of reactive oxygen species (ROS) that are formed from the partial reduction of molecular oxygen ([O2], Fig. 13.5A). These compounds are formed continuously as byproducts of aerobic metabolism, through reactions with drugs and environmental toxins, or when the level of antioxidants is diminished, all creating the condition of oxidative stress. These highly reactive oxygen intermediates can cause serious chemical damage to DNA, proteins, and unsaturated lipids and can lead to cell death. ROS have been implicated in a number of pathologic processes, including reperfusion injury, cancer, inflammatory disease, and aging. The cell has several protective mechanisms that minimize the toxic potential of these compounds. [Note: ROS can also be generated in the killing of microbes by white blood cells (WBC; see D. below).] | Biochemistry_Lippinco. B. Hydrogen peroxide reduction Hydrogen peroxide (H2O2) is one of a family of reactive oxygen species (ROS) that are formed from the partial reduction of molecular oxygen ([O2], Fig. 13.5A). These compounds are formed continuously as byproducts of aerobic metabolism, through reactions with drugs and environmental toxins, or when the level of antioxidants is diminished, all creating the condition of oxidative stress. These highly reactive oxygen intermediates can cause serious chemical damage to DNA, proteins, and unsaturated lipids and can lead to cell death. ROS have been implicated in a number of pathologic processes, including reperfusion injury, cancer, inflammatory disease, and aging. The cell has several protective mechanisms that minimize the toxic potential of these compounds. [Note: ROS can also be generated in the killing of microbes by white blood cells (WBC; see D. below).] |
Biochemistry_Lippincott_505 | Biochemistry_Lippinco | B. Actions of antioxidant enzymes. G-SH = reduced glutathione; G-S-S-G = oxidized glutathione. [Note: See Fig. 13.6B for the regeneration of G-SH.] 1. Enzymes that catalyze antioxidant reactions Reduced glutathione (GSH), a tripeptide-thiol (γ-glutamylcysteinylglycine) present in most cells, can chemically detoxify H2O2 (Fig. 13.5B). This reaction, catalyzed by the selenoprotein (see p. 407) glutathione peroxidase, forms oxidized glutathione (G-S-S-G), which no longer has protective properties. The cell regenerates G-SH in a reaction catalyzed by glutathione reductase, using NADPH as a source of reducing equivalents. Thus, NADPH indirectly provides electrons for the reduction of H2O2 (Fig. 13.6). Additional enzymes, such as superoxide dismutase and catalase, catalyze the conversion of other ROS to harmless products (see Fig. 13.5B). As a group, these enzymes serve as a defense system to guard against the toxic effects of ROS. glutathione. | Biochemistry_Lippinco. B. Actions of antioxidant enzymes. G-SH = reduced glutathione; G-S-S-G = oxidized glutathione. [Note: See Fig. 13.6B for the regeneration of G-SH.] 1. Enzymes that catalyze antioxidant reactions Reduced glutathione (GSH), a tripeptide-thiol (γ-glutamylcysteinylglycine) present in most cells, can chemically detoxify H2O2 (Fig. 13.5B). This reaction, catalyzed by the selenoprotein (see p. 407) glutathione peroxidase, forms oxidized glutathione (G-S-S-G), which no longer has protective properties. The cell regenerates G-SH in a reaction catalyzed by glutathione reductase, using NADPH as a source of reducing equivalents. Thus, NADPH indirectly provides electrons for the reduction of H2O2 (Fig. 13.6). Additional enzymes, such as superoxide dismutase and catalase, catalyze the conversion of other ROS to harmless products (see Fig. 13.5B). As a group, these enzymes serve as a defense system to guard against the toxic effects of ROS. glutathione. |
Biochemistry_Lippincott_506 | Biochemistry_Lippinco | 2. Antioxidant chemicals A number of intracellular reducing agents, such as ascorbate (see p. 381), vitamin E (see p. 395), and β-carotene (see p. 386), are able to reduce and, thereby, detoxify ROS in the laboratory. Consumption of foods rich in these antioxidant compounds has been correlated with a reduced risk for certain types of cancers as well as decreased frequency of certain other chronic health problems. Therefore, it is tempting to speculate that the effects of these compounds are, in part, an expression of their ability to quench the toxic effect of ROS. However, clinical trials with antioxidants as dietary supplements have failed to show clear beneficial effects. In the case of dietary supplementation with β-carotene, the rate of lung cancer in smokers increased rather than decreased. Thus, the health-promoting effects of dietary fruits and vegetables likely reflect a complex interaction among many naturally occurring compounds, which has not been duplicated by consumption | Biochemistry_Lippinco. 2. Antioxidant chemicals A number of intracellular reducing agents, such as ascorbate (see p. 381), vitamin E (see p. 395), and β-carotene (see p. 386), are able to reduce and, thereby, detoxify ROS in the laboratory. Consumption of foods rich in these antioxidant compounds has been correlated with a reduced risk for certain types of cancers as well as decreased frequency of certain other chronic health problems. Therefore, it is tempting to speculate that the effects of these compounds are, in part, an expression of their ability to quench the toxic effect of ROS. However, clinical trials with antioxidants as dietary supplements have failed to show clear beneficial effects. In the case of dietary supplementation with β-carotene, the rate of lung cancer in smokers increased rather than decreased. Thus, the health-promoting effects of dietary fruits and vegetables likely reflect a complex interaction among many naturally occurring compounds, which has not been duplicated by consumption |
Biochemistry_Lippincott_507 | Biochemistry_Lippinco | Thus, the health-promoting effects of dietary fruits and vegetables likely reflect a complex interaction among many naturally occurring compounds, which has not been duplicated by consumption of isolated antioxidant compounds. | Biochemistry_Lippinco. Thus, the health-promoting effects of dietary fruits and vegetables likely reflect a complex interaction among many naturally occurring compounds, which has not been duplicated by consumption of isolated antioxidant compounds. |
Biochemistry_Lippincott_508 | Biochemistry_Lippinco | C. Cytochrome P450 monooxygenase system | Biochemistry_Lippinco. C. Cytochrome P450 monooxygenase system |
Biochemistry_Lippincott_509 | Biochemistry_Lippinco | Monooxygenases (mixed-function oxidases) incorporate one atom from O2 into a substrate (creating a hydroxyl group), with the other atom being reduced to water (H2O). In the cytochrome P450 (CYP) monooxygenase system, NADPH provides the reducing equivalents required by this series of reactions (Fig. 13.7). This system performs different functions in two separate locations in cells. The overall reaction catalyzed by a CYP enzyme is where R may be a steroid, drug, or other chemical. [Note: CYP enzymes are actually a superfamily of related, heme-containing monooxygenases that participate in a broad variety of reactions. The P450 in the name reflects the absorbance at 450 nm by the protein.] 1. Mitochondrial system An important function of the CYP monooxygenase system found associated with the inner mitochondrial membrane is the biosynthesis of steroid hormones. In steroidogenic tissues, such as the placenta, ovaries, testes, and adrenal cortex, it is used to hydroxylate intermediates in | Biochemistry_Lippinco. Monooxygenases (mixed-function oxidases) incorporate one atom from O2 into a substrate (creating a hydroxyl group), with the other atom being reduced to water (H2O). In the cytochrome P450 (CYP) monooxygenase system, NADPH provides the reducing equivalents required by this series of reactions (Fig. 13.7). This system performs different functions in two separate locations in cells. The overall reaction catalyzed by a CYP enzyme is where R may be a steroid, drug, or other chemical. [Note: CYP enzymes are actually a superfamily of related, heme-containing monooxygenases that participate in a broad variety of reactions. The P450 in the name reflects the absorbance at 450 nm by the protein.] 1. Mitochondrial system An important function of the CYP monooxygenase system found associated with the inner mitochondrial membrane is the biosynthesis of steroid hormones. In steroidogenic tissues, such as the placenta, ovaries, testes, and adrenal cortex, it is used to hydroxylate intermediates in |
Biochemistry_Lippincott_510 | Biochemistry_Lippinco | inner mitochondrial membrane is the biosynthesis of steroid hormones. In steroidogenic tissues, such as the placenta, ovaries, testes, and adrenal cortex, it is used to hydroxylate intermediates in the conversion of cholesterol to steroid hormones, a process that makes these hydrophobic compounds more water soluble (see p. 237). The liver uses this same system in bile acid synthesis (see p. | Biochemistry_Lippinco. inner mitochondrial membrane is the biosynthesis of steroid hormones. In steroidogenic tissues, such as the placenta, ovaries, testes, and adrenal cortex, it is used to hydroxylate intermediates in the conversion of cholesterol to steroid hormones, a process that makes these hydrophobic compounds more water soluble (see p. 237). The liver uses this same system in bile acid synthesis (see p. |
Biochemistry_Lippincott_511 | Biochemistry_Lippinco | 224) and the hydroxylation of cholecalciferol to 25hydroxycholecalciferol ([vitamin D3] see p. 390), and the kidney uses it to hydroxylate vitamin D3 to its biologically active 1,25-dihydroxylated form. | Biochemistry_Lippinco. 224) and the hydroxylation of cholecalciferol to 25hydroxycholecalciferol ([vitamin D3] see p. 390), and the kidney uses it to hydroxylate vitamin D3 to its biologically active 1,25-dihydroxylated form. |
Biochemistry_Lippincott_512 | Biochemistry_Lippinco | 2. Microsomal system The microsomal CYP monooxygenase system found associated with the membrane of the smooth endoplasmic reticulum (particularly in the liver) functions primarily in the detoxification of foreign compounds (xenobiotics). These include numerous drugs and such varied pollutants as petroleum products and pesticides. CYP enzymes of the microsomal system (for example, CYP3A4) can be used to hydroxylate these toxins (phase I). The purpose of these modifications is two-fold. First, it may itself activate or inactivate a drug and second, make a toxic compound more soluble, thereby facilitating its excretion in the urine or feces. Frequently, however, the new hydroxyl group will serve as a site for conjugation with a polar molecule, such as glucuronic acid (see p. 161), which will significantly increase the compound’s solubility (phase II). [Note: Polymorphisms (see p. 491) in the genes for CYP enzymes can lead to differences in drug metabolism.] | Biochemistry_Lippinco. 2. Microsomal system The microsomal CYP monooxygenase system found associated with the membrane of the smooth endoplasmic reticulum (particularly in the liver) functions primarily in the detoxification of foreign compounds (xenobiotics). These include numerous drugs and such varied pollutants as petroleum products and pesticides. CYP enzymes of the microsomal system (for example, CYP3A4) can be used to hydroxylate these toxins (phase I). The purpose of these modifications is two-fold. First, it may itself activate or inactivate a drug and second, make a toxic compound more soluble, thereby facilitating its excretion in the urine or feces. Frequently, however, the new hydroxyl group will serve as a site for conjugation with a polar molecule, such as glucuronic acid (see p. 161), which will significantly increase the compound’s solubility (phase II). [Note: Polymorphisms (see p. 491) in the genes for CYP enzymes can lead to differences in drug metabolism.] |
Biochemistry_Lippincott_513 | Biochemistry_Lippinco | D. White blood cell phagocytosis and microbe killing Phagocytosis is the ingestion by receptor-mediated endocytosis of microorganisms, foreign particles, and cellular debris by WBC (leukocytes) such as neutrophils and macrophages (monocytes). It is an important defense mechanism, particularly in bacterial infections. Neutrophils and monocytes are armed with both oxygen-independent and oxygen-dependent mechanisms for killing bacteria. 1. Oxygen-independent Oxygen-independent mechanisms use pH changes in phagolysosomes and lysosomal enzymes to destroy pathogens. 2. | Biochemistry_Lippinco. D. White blood cell phagocytosis and microbe killing Phagocytosis is the ingestion by receptor-mediated endocytosis of microorganisms, foreign particles, and cellular debris by WBC (leukocytes) such as neutrophils and macrophages (monocytes). It is an important defense mechanism, particularly in bacterial infections. Neutrophils and monocytes are armed with both oxygen-independent and oxygen-dependent mechanisms for killing bacteria. 1. Oxygen-independent Oxygen-independent mechanisms use pH changes in phagolysosomes and lysosomal enzymes to destroy pathogens. 2. |
Biochemistry_Lippincott_514 | Biochemistry_Lippinco | Oxygen-dependent Oxygen-dependent mechanisms include the enzymes NADPH oxidase and myeloperoxidase (MPO) that work together in killing bacteria (Fig. 13.8). Overall, the MPO system is the most potent of the bactericidal mechanisms. An invading bacterium is recognized by the immune system and attacked by antibodies that bind it to a receptor on a phagocytic cell. After internalization of the microorganism has occurred, NADPH oxidase, located in the leukocyte cell membrane, activated and reduces O2 from the surrounding tissue to superoxide ( ), a free radical ROS, as NADPH is oxidized. The rapid consumption of O2 that accompanies formation of is referred to as the respiratory burst. [Note: Active NADPH oxidase is a membrane-associated complex containing a flavocytochrome plus additional peptides that translocate from the cytoplasm upon activation of the leukocyte. Electrons move from NADPH to O2 via flavin adenine nucleotide (FAD) and heme, . Rare genetic deficiencies in NADPH oxidase | Biochemistry_Lippinco. Oxygen-dependent Oxygen-dependent mechanisms include the enzymes NADPH oxidase and myeloperoxidase (MPO) that work together in killing bacteria (Fig. 13.8). Overall, the MPO system is the most potent of the bactericidal mechanisms. An invading bacterium is recognized by the immune system and attacked by antibodies that bind it to a receptor on a phagocytic cell. After internalization of the microorganism has occurred, NADPH oxidase, located in the leukocyte cell membrane, activated and reduces O2 from the surrounding tissue to superoxide ( ), a free radical ROS, as NADPH is oxidized. The rapid consumption of O2 that accompanies formation of is referred to as the respiratory burst. [Note: Active NADPH oxidase is a membrane-associated complex containing a flavocytochrome plus additional peptides that translocate from the cytoplasm upon activation of the leukocyte. Electrons move from NADPH to O2 via flavin adenine nucleotide (FAD) and heme, . Rare genetic deficiencies in NADPH oxidase |
Biochemistry_Lippincott_515 | Biochemistry_Lippinco | that translocate from the cytoplasm upon activation of the leukocyte. Electrons move from NADPH to O2 via flavin adenine nucleotide (FAD) and heme, . Rare genetic deficiencies in NADPH oxidase cause chronic granulomatous disease (CGD) characterized by severe, persistent infections and the formation of granulomas (nodular areas of inflammation) that sequester the bacteria that were not destroyed.] Next, is converted to H2O2 (also a ROS), either spontaneously or catalyzed by superoxide dismutase. In the presence of MPO, a heme-containing lysosomal enzyme present within the phagolysosome, peroxide plus chloride ions are converted to hypochlorous acid ([HOCl] the major component of household bleach), which kills the bacteria. The peroxide can also be partially reduced to the hydroxyl radical (OH•), a | Biochemistry_Lippinco. that translocate from the cytoplasm upon activation of the leukocyte. Electrons move from NADPH to O2 via flavin adenine nucleotide (FAD) and heme, . Rare genetic deficiencies in NADPH oxidase cause chronic granulomatous disease (CGD) characterized by severe, persistent infections and the formation of granulomas (nodular areas of inflammation) that sequester the bacteria that were not destroyed.] Next, is converted to H2O2 (also a ROS), either spontaneously or catalyzed by superoxide dismutase. In the presence of MPO, a heme-containing lysosomal enzyme present within the phagolysosome, peroxide plus chloride ions are converted to hypochlorous acid ([HOCl] the major component of household bleach), which kills the bacteria. The peroxide can also be partially reduced to the hydroxyl radical (OH•), a |
Biochemistry_Lippincott_516 | Biochemistry_Lippinco | ROS, or be fully reduced to H2O by catalase or glutathione peroxidase. [Note: Deficiencies in MPO do not confer increased susceptibility to infection because peroxide from NADPH oxidase is bactericidal.] G; NADP(H) = nicotinamide adenine dinucleotide phosphate; = superoxide; H2O2 = hydrogen peroxide; HOCl = hypochlorous acid; OH• = hydroxyl radical. E. Nitric oxide synthesis | Biochemistry_Lippinco. ROS, or be fully reduced to H2O by catalase or glutathione peroxidase. [Note: Deficiencies in MPO do not confer increased susceptibility to infection because peroxide from NADPH oxidase is bactericidal.] G; NADP(H) = nicotinamide adenine dinucleotide phosphate; = superoxide; H2O2 = hydrogen peroxide; HOCl = hypochlorous acid; OH• = hydroxyl radical. E. Nitric oxide synthesis |
Biochemistry_Lippincott_517 | Biochemistry_Lippinco | Nitric oxide (NO) is recognized as a mediator in a broad array of biologic systems. NO is the endothelium-derived relaxing factor that causes vasodilation by relaxing vascular smooth muscle. It also acts as a neurotransmitter, prevents platelet aggregation, and plays an essential role in macrophage function. It has a very short half-life in tissues (3–10 seconds) because it reacts with O2 and and is converted into nitrates and nitrites including peroxynitrite (O=NOO−), a reactive nitrogen species (RNS). [Note: NO is a free radical gas that is often confused with nitrous oxide (N2O), the “laughing gas” that is used as an anesthetic and is chemically stable.] 1. Nitric oxide synthase Arginine, O2, and NADPH are substrates for cytosolic NO synthase ([NOS], Fig. 13.9). Flavin mononucleotide (FMN), FAD, heme, and tetrahydrobiopterin (see p. 268) are coenzymes, and NO and citrulline are products of the reaction. Three NOS isozymes, each the product of a different gene, have been identified. | Biochemistry_Lippinco. Nitric oxide (NO) is recognized as a mediator in a broad array of biologic systems. NO is the endothelium-derived relaxing factor that causes vasodilation by relaxing vascular smooth muscle. It also acts as a neurotransmitter, prevents platelet aggregation, and plays an essential role in macrophage function. It has a very short half-life in tissues (3–10 seconds) because it reacts with O2 and and is converted into nitrates and nitrites including peroxynitrite (O=NOO−), a reactive nitrogen species (RNS). [Note: NO is a free radical gas that is often confused with nitrous oxide (N2O), the “laughing gas” that is used as an anesthetic and is chemically stable.] 1. Nitric oxide synthase Arginine, O2, and NADPH are substrates for cytosolic NO synthase ([NOS], Fig. 13.9). Flavin mononucleotide (FMN), FAD, heme, and tetrahydrobiopterin (see p. 268) are coenzymes, and NO and citrulline are products of the reaction. Three NOS isozymes, each the product of a different gene, have been identified. |
Biochemistry_Lippincott_518 | Biochemistry_Lippinco | FAD, heme, and tetrahydrobiopterin (see p. 268) are coenzymes, and NO and citrulline are products of the reaction. Three NOS isozymes, each the product of a different gene, have been identified. Two are constitutive (synthesized at a constant rate), calcium (Ca2+)–calmodulin (CaM)-dependent enzymes (see p. 133). They are found primarily in endothelium (eNOS) and neural tissue (nNOS) and constantly produce very low levels of NO for vasodilation and neurotransmission. An inducible, Ca2+-independent enzyme (iNOS) can be expressed in many cells, including macrophages and neutrophils, as an early defense against pathogens. The specific inducers for iNOS vary with cell type and include proinflammatory cytokines, such as tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ), and bacterial endotoxins such as lipopolysaccharide (LPS). These compounds promote synthesis of iNOS, which can result in large amounts of NO being produced over hours or even days. | Biochemistry_Lippinco. FAD, heme, and tetrahydrobiopterin (see p. 268) are coenzymes, and NO and citrulline are products of the reaction. Three NOS isozymes, each the product of a different gene, have been identified. Two are constitutive (synthesized at a constant rate), calcium (Ca2+)–calmodulin (CaM)-dependent enzymes (see p. 133). They are found primarily in endothelium (eNOS) and neural tissue (nNOS) and constantly produce very low levels of NO for vasodilation and neurotransmission. An inducible, Ca2+-independent enzyme (iNOS) can be expressed in many cells, including macrophages and neutrophils, as an early defense against pathogens. The specific inducers for iNOS vary with cell type and include proinflammatory cytokines, such as tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ), and bacterial endotoxins such as lipopolysaccharide (LPS). These compounds promote synthesis of iNOS, which can result in large amounts of NO being produced over hours or even days. |
Biochemistry_Lippincott_519 | Biochemistry_Lippinco | 2. | Biochemistry_Lippinco. 2. |
Biochemistry_Lippincott_520 | Biochemistry_Lippinco | Nitric oxide and vascular endothelium NO is an important mediator in the control of vascular smooth muscle tone. NO is synthesized by eNOS in endothelial cells and diffuses to vascular smooth muscle, where it activates the cytosolic form of guanylyl cyclase (or, guanylate cyclase) to form cyclic guanosine monophosphate (cGMP). [Note: This reaction is analogous to the formation of cyclic adenosine monophosphate (cAMP) by adenylyl cyclase (see p. 95).] The resultant rise in cGMP causes activation of protein kinase G, which phosphorylates Ca2+ channels, causing decreased entry of Ca2+ into smooth muscle cells. This decreases the Ca2+–CaM activation of myosin light-chain kinase, thereby decreasing smooth muscle contraction and favoring relaxation. Vasodilator nitrates, such as nitroglycerin, are metabolized to NO, which causes relaxation of vascular smooth muscle and, therefore, lowers blood pressure. Thus, NO can be envisioned as an endogenous nitrovasodilator. [Note: Under hypoxic | Biochemistry_Lippinco. Nitric oxide and vascular endothelium NO is an important mediator in the control of vascular smooth muscle tone. NO is synthesized by eNOS in endothelial cells and diffuses to vascular smooth muscle, where it activates the cytosolic form of guanylyl cyclase (or, guanylate cyclase) to form cyclic guanosine monophosphate (cGMP). [Note: This reaction is analogous to the formation of cyclic adenosine monophosphate (cAMP) by adenylyl cyclase (see p. 95).] The resultant rise in cGMP causes activation of protein kinase G, which phosphorylates Ca2+ channels, causing decreased entry of Ca2+ into smooth muscle cells. This decreases the Ca2+–CaM activation of myosin light-chain kinase, thereby decreasing smooth muscle contraction and favoring relaxation. Vasodilator nitrates, such as nitroglycerin, are metabolized to NO, which causes relaxation of vascular smooth muscle and, therefore, lowers blood pressure. Thus, NO can be envisioned as an endogenous nitrovasodilator. [Note: Under hypoxic |
Biochemistry_Lippincott_521 | Biochemistry_Lippinco | are metabolized to NO, which causes relaxation of vascular smooth muscle and, therefore, lowers blood pressure. Thus, NO can be envisioned as an endogenous nitrovasodilator. [Note: Under hypoxic conditions, nitrite (NO2−) can be reduced to NO, which binds to deoxyhemoglobin. The NO is released into the blood, causing vasodilation and increasing blood flow.] 3. | Biochemistry_Lippinco. are metabolized to NO, which causes relaxation of vascular smooth muscle and, therefore, lowers blood pressure. Thus, NO can be envisioned as an endogenous nitrovasodilator. [Note: Under hypoxic conditions, nitrite (NO2−) can be reduced to NO, which binds to deoxyhemoglobin. The NO is released into the blood, causing vasodilation and increasing blood flow.] 3. |
Biochemistry_Lippincott_522 | Biochemistry_Lippinco | Nitric oxide and macrophage bactericidal activity In macrophages, iNOS activity is normally low, but synthesis of the enzyme is significantly stimulated by bacterial LPS and by release of IFN-γ response to infection. Activated macrophages form radicals that combine with NO to form intermediates that decompose, producing the highly bactericidal OH• radical. 4. Additional functions NO is a potent inhibitor of platelet adhesion and aggregation (by activating the cGMP pathway). It is also characterized as a neurotransmitter in the central and peripheral nervous systems. V. G6PD DEFICIENCY | Biochemistry_Lippinco. Nitric oxide and macrophage bactericidal activity In macrophages, iNOS activity is normally low, but synthesis of the enzyme is significantly stimulated by bacterial LPS and by release of IFN-γ response to infection. Activated macrophages form radicals that combine with NO to form intermediates that decompose, producing the highly bactericidal OH• radical. 4. Additional functions NO is a potent inhibitor of platelet adhesion and aggregation (by activating the cGMP pathway). It is also characterized as a neurotransmitter in the central and peripheral nervous systems. V. G6PD DEFICIENCY |
Biochemistry_Lippincott_523 | Biochemistry_Lippinco | G6PD deficiency is a hereditary condition characterized by hemolytic anemia caused by the inability to detoxify oxidizing agents. G6PD deficiency is the most common disease-producing enzyme abnormality in humans, affecting >400 million individuals worldwide. This deficiency has the highest prevalence in the Middle East, tropical Africa and Asia, and parts of the Mediterranean. G6PD deficiency is X linked and is, in fact, a family of deficiencies caused by a number of different mutations in the gene encoding G6PD. Only some of the resulting protein variants cause clinical symptoms. [Note: In addition to hemolytic anemia, a clinical manifestation of G6PD deficiency is neonatal jaundice appearing 1–4 days after birth. The jaundice, which may be severe, typically results from increased production of unconjugated bilirubin (see p. 285).] The life span of individuals with a severe form of G6PD deficiency may be somewhat shortened as a result of complications arising from chronic hemolysis. | Biochemistry_Lippinco. G6PD deficiency is a hereditary condition characterized by hemolytic anemia caused by the inability to detoxify oxidizing agents. G6PD deficiency is the most common disease-producing enzyme abnormality in humans, affecting >400 million individuals worldwide. This deficiency has the highest prevalence in the Middle East, tropical Africa and Asia, and parts of the Mediterranean. G6PD deficiency is X linked and is, in fact, a family of deficiencies caused by a number of different mutations in the gene encoding G6PD. Only some of the resulting protein variants cause clinical symptoms. [Note: In addition to hemolytic anemia, a clinical manifestation of G6PD deficiency is neonatal jaundice appearing 1–4 days after birth. The jaundice, which may be severe, typically results from increased production of unconjugated bilirubin (see p. 285).] The life span of individuals with a severe form of G6PD deficiency may be somewhat shortened as a result of complications arising from chronic hemolysis. |
Biochemistry_Lippincott_524 | Biochemistry_Lippinco | of unconjugated bilirubin (see p. 285).] The life span of individuals with a severe form of G6PD deficiency may be somewhat shortened as a result of complications arising from chronic hemolysis. This negative effect of G6PD deficiency has been balanced in evolution by an advantage in survival, an increased resistance to Plasmodium falciparum malaria. [Note: Sickle cell trait and the thalassemias also confer resistance to malaria.] | Biochemistry_Lippinco. of unconjugated bilirubin (see p. 285).] The life span of individuals with a severe form of G6PD deficiency may be somewhat shortened as a result of complications arising from chronic hemolysis. This negative effect of G6PD deficiency has been balanced in evolution by an advantage in survival, an increased resistance to Plasmodium falciparum malaria. [Note: Sickle cell trait and the thalassemias also confer resistance to malaria.] |
Biochemistry_Lippincott_525 | Biochemistry_Lippinco | A. G6PD role in red blood cells | Biochemistry_Lippinco. A. G6PD role in red blood cells |
Biochemistry_Lippincott_526 | Biochemistry_Lippinco | Diminished G6PD activity impairs the ability of the cell to form the NADPH that is essential for the maintenance of the G-SH pool. This results in a decrease in the detoxification of free radicals and peroxides formed within the cell (Fig. 13.10). G-SH also helps maintain the reduced states of sulfhydryl groups in proteins, including hemoglobin. Oxidation of those sulfhydryl groups leads to the formation of denatured proteins that form insoluble masses (called Heinz bodies) that attach to RBC membranes (Fig. 13.11). Additional oxidation of membrane proteins causes RBC to be rigid (less deformable), and they are removed from the circulation by macrophages in the spleen and liver. Although G6PD deficiency occurs in all cells of the affected individual, it is most severe in RBC, where the pentose phosphate pathway provides the only means of generating NADPH. Additionally, the RBC has no nucleus or ribosomes and cannot renew its supply of the enzyme. Thus, RBC are particularly vulnerable | Biochemistry_Lippinco. Diminished G6PD activity impairs the ability of the cell to form the NADPH that is essential for the maintenance of the G-SH pool. This results in a decrease in the detoxification of free radicals and peroxides formed within the cell (Fig. 13.10). G-SH also helps maintain the reduced states of sulfhydryl groups in proteins, including hemoglobin. Oxidation of those sulfhydryl groups leads to the formation of denatured proteins that form insoluble masses (called Heinz bodies) that attach to RBC membranes (Fig. 13.11). Additional oxidation of membrane proteins causes RBC to be rigid (less deformable), and they are removed from the circulation by macrophages in the spleen and liver. Although G6PD deficiency occurs in all cells of the affected individual, it is most severe in RBC, where the pentose phosphate pathway provides the only means of generating NADPH. Additionally, the RBC has no nucleus or ribosomes and cannot renew its supply of the enzyme. Thus, RBC are particularly vulnerable |
Biochemistry_Lippincott_527 | Biochemistry_Lippinco | phosphate pathway provides the only means of generating NADPH. Additionally, the RBC has no nucleus or ribosomes and cannot renew its supply of the enzyme. Thus, RBC are particularly vulnerable to enzyme variants with diminished stability. [Note: Other tissues have an alternative source of NADPH (NADP+-dependent malate dehydrogenase [malic enzyme]; see p. 186) that can keep G-SH reduced.] pentose phosphate pathway. | Biochemistry_Lippinco. phosphate pathway provides the only means of generating NADPH. Additionally, the RBC has no nucleus or ribosomes and cannot renew its supply of the enzyme. Thus, RBC are particularly vulnerable to enzyme variants with diminished stability. [Note: Other tissues have an alternative source of NADPH (NADP+-dependent malate dehydrogenase [malic enzyme]; see p. 186) that can keep G-SH reduced.] pentose phosphate pathway. |
Biochemistry_Lippincott_528 | Biochemistry_Lippinco | B. Precipitating factors in G6PD deficiency Most individuals who have inherited one of the G6PD mutations do not show clinical manifestations (that is, they are asymptomatic). However, some patients with G6PD deficiency develop hemolytic anemia if they are treated with an oxidant drug, ingest fava beans, or contract a severe infection. 1. Oxidant drugs Commonly used drugs that produce hemolytic anemia in patients with G6PD deficiency are best remembered from the mnemonic AAA: antibiotics (for example, sulfamethoxazole and chloramphenicol), antimalarials (for example, primaquine but not chloroquine or quinine), and antipyretics (for example, acetanilide but not acetaminophen). 2. | Biochemistry_Lippinco. B. Precipitating factors in G6PD deficiency Most individuals who have inherited one of the G6PD mutations do not show clinical manifestations (that is, they are asymptomatic). However, some patients with G6PD deficiency develop hemolytic anemia if they are treated with an oxidant drug, ingest fava beans, or contract a severe infection. 1. Oxidant drugs Commonly used drugs that produce hemolytic anemia in patients with G6PD deficiency are best remembered from the mnemonic AAA: antibiotics (for example, sulfamethoxazole and chloramphenicol), antimalarials (for example, primaquine but not chloroquine or quinine), and antipyretics (for example, acetanilide but not acetaminophen). 2. |
Biochemistry_Lippincott_529 | Biochemistry_Lippinco | 2. Favism Some forms of G6PD deficiency, for example, the Mediterranean variant, are particularly susceptible to the hemolytic effect of the fava (broad) bean, a dietary staple in the Mediterranean region. Favism, the hemolytic effect of ingesting fava beans, is not observed in all individuals with G6PD deficiency, but all patients with favism have G6PD deficiency. 3. Infection Infection is the most common precipitating factor of hemolysis in G6PD deficiency. The inflammatory response to infection results in the generation of free radicals in macrophages. The radicals can diffuse into the RBC and cause oxidative damage. C. Variant G6PD properties | Biochemistry_Lippinco. 2. Favism Some forms of G6PD deficiency, for example, the Mediterranean variant, are particularly susceptible to the hemolytic effect of the fava (broad) bean, a dietary staple in the Mediterranean region. Favism, the hemolytic effect of ingesting fava beans, is not observed in all individuals with G6PD deficiency, but all patients with favism have G6PD deficiency. 3. Infection Infection is the most common precipitating factor of hemolysis in G6PD deficiency. The inflammatory response to infection results in the generation of free radicals in macrophages. The radicals can diffuse into the RBC and cause oxidative damage. C. Variant G6PD properties |
Biochemistry_Lippincott_530 | Biochemistry_Lippinco | Almost all G6PD variants are caused by point mutations (see p. 449) in the gene for G6PD. Some mutations do not affect enzymic activity. However, other mutations result in decreased catalytic activity, decreased stability, or an alteration of binding affinity for NADP+ or glucose 6-phosphate. [Note: Active G6PD exists as a homodimer or tetramer. Mutations at the interface between subunits can affect stability.] The severity of the disease usually correlates with the amount of residual enzyme activity in the patient’s RBC. For example, variants can be classified as shown in Figure 13.12. G6PD A– is the prototype of the moderate (class III) form of the disease. The RBC contain an unstable but kinetically normal G6PD, with most of the enzyme activity present in the reticulocytes and younger RBC (Fig. 13.13). Therefore, the oldest RBC have the lowest level of enzyme activity and are preferentially removed in a hemolytic episode. Because hemolysis does not affect younger cells, the | Biochemistry_Lippinco. Almost all G6PD variants are caused by point mutations (see p. 449) in the gene for G6PD. Some mutations do not affect enzymic activity. However, other mutations result in decreased catalytic activity, decreased stability, or an alteration of binding affinity for NADP+ or glucose 6-phosphate. [Note: Active G6PD exists as a homodimer or tetramer. Mutations at the interface between subunits can affect stability.] The severity of the disease usually correlates with the amount of residual enzyme activity in the patient’s RBC. For example, variants can be classified as shown in Figure 13.12. G6PD A– is the prototype of the moderate (class III) form of the disease. The RBC contain an unstable but kinetically normal G6PD, with most of the enzyme activity present in the reticulocytes and younger RBC (Fig. 13.13). Therefore, the oldest RBC have the lowest level of enzyme activity and are preferentially removed in a hemolytic episode. Because hemolysis does not affect younger cells, the |
Biochemistry_Lippincott_531 | Biochemistry_Lippinco | RBC (Fig. 13.13). Therefore, the oldest RBC have the lowest level of enzyme activity and are preferentially removed in a hemolytic episode. Because hemolysis does not affect younger cells, the episodes are self-limiting. G6PD Mediterranean is the prototype of a more severe (class II) deficiency. Class I mutations (rare) are the most severe and are associated with chronic nonspherocytic hemolytic anemia, which occurs even in the absence of oxidative stress. | Biochemistry_Lippinco. RBC (Fig. 13.13). Therefore, the oldest RBC have the lowest level of enzyme activity and are preferentially removed in a hemolytic episode. Because hemolysis does not affect younger cells, the episodes are self-limiting. G6PD Mediterranean is the prototype of a more severe (class II) deficiency. Class I mutations (rare) are the most severe and are associated with chronic nonspherocytic hemolytic anemia, which occurs even in the absence of oxidative stress. |
Biochemistry_Lippincott_532 | Biochemistry_Lippinco | D. G6PD molecular biology The cloning of the gene for G6PD and the sequencing of its DNA (see Chapter 34) have permitted the identification of mutations that cause G6PD deficiency. More than 400 G6PD variants have been identified, a finding that explains the numerous biochemical and clinical phenotypes that have been described. Most mutations that result in enzymic deficiency are missense mutations (see p. 449) in the coding region. Both G6PD A− and G6PD Mediterranean represent mutant enzymes that differ from the respective normal variants by a single amino acid. Large deletions or frameshift mutations have not been identified, suggesting that complete absence of G6PD activity is likely lethal. VI. CHAPTER SUMMARY | Biochemistry_Lippinco. D. G6PD molecular biology The cloning of the gene for G6PD and the sequencing of its DNA (see Chapter 34) have permitted the identification of mutations that cause G6PD deficiency. More than 400 G6PD variants have been identified, a finding that explains the numerous biochemical and clinical phenotypes that have been described. Most mutations that result in enzymic deficiency are missense mutations (see p. 449) in the coding region. Both G6PD A− and G6PD Mediterranean represent mutant enzymes that differ from the respective normal variants by a single amino acid. Large deletions or frameshift mutations have not been identified, suggesting that complete absence of G6PD activity is likely lethal. VI. CHAPTER SUMMARY |
Biochemistry_Lippincott_533 | Biochemistry_Lippinco | The pentose phosphate pathway includes an irreversible oxidative phase followed by a series of reversible sugar–phosphate interconversions (Fig. 13.14). No ATP is directly consumed or produced in the pathway. The reduced nicotinamide adenine dinucleotide phosphate (NADPH)-producing oxidative portion of the pathway is important in providing reducing equivalents for reductive biosynthesis and detoxification reactions. In this part of the pathway, glucose 6-phosphate is irreversibly converted to ribulose 5-phosphate, and two NADPH are produced. The regulated step is catalyzed by glucose 6-phosphate dehydrogenase (G6PD), which is strongly inhibited by a rise in the NADPH/NADP+ ratio. Reversible nonoxidative reactions interconvert sugars. This part of the pathway converts ribulose 5-phosphate to ribose 5-phosphate, required for nucleotide and nucleic acid synthesis, or to fructose 6-phosphate and glyceraldehyde 3-phosphate (glycolytic intermediates). NADPH is a source of reducing | Biochemistry_Lippinco. The pentose phosphate pathway includes an irreversible oxidative phase followed by a series of reversible sugar–phosphate interconversions (Fig. 13.14). No ATP is directly consumed or produced in the pathway. The reduced nicotinamide adenine dinucleotide phosphate (NADPH)-producing oxidative portion of the pathway is important in providing reducing equivalents for reductive biosynthesis and detoxification reactions. In this part of the pathway, glucose 6-phosphate is irreversibly converted to ribulose 5-phosphate, and two NADPH are produced. The regulated step is catalyzed by glucose 6-phosphate dehydrogenase (G6PD), which is strongly inhibited by a rise in the NADPH/NADP+ ratio. Reversible nonoxidative reactions interconvert sugars. This part of the pathway converts ribulose 5-phosphate to ribose 5-phosphate, required for nucleotide and nucleic acid synthesis, or to fructose 6-phosphate and glyceraldehyde 3-phosphate (glycolytic intermediates). NADPH is a source of reducing |
Biochemistry_Lippincott_534 | Biochemistry_Lippinco | to ribose 5-phosphate, required for nucleotide and nucleic acid synthesis, or to fructose 6-phosphate and glyceraldehyde 3-phosphate (glycolytic intermediates). NADPH is a source of reducing equivalents in reductive biosynthesis, such as the production of fatty acids in liver, adipose tissue, and the mammary gland; cholesterol in the liver; and steroid hormones in the placenta, ovaries, testes, and adrenal cortex. It is also required by red blood cells (RBC) for hydrogen peroxide reduction. Reduced glutathione (G-SH) is used by glutathione peroxidase to reduce the peroxide to water. The oxidized glutathione (G-S-S-G) produced is reduced by glutathione reductase, using NADPH as the source of electrons. NADPH provides reducing equivalents for the mitochondrial cytochrome P450 monooxygenase system, which is used in steroid hormone synthesis in steroidogenic tissue, bile acid synthesis in the liver, and vitamin D activation in the liver and kidneys. The microsomal system uses NADPH to | Biochemistry_Lippinco. to ribose 5-phosphate, required for nucleotide and nucleic acid synthesis, or to fructose 6-phosphate and glyceraldehyde 3-phosphate (glycolytic intermediates). NADPH is a source of reducing equivalents in reductive biosynthesis, such as the production of fatty acids in liver, adipose tissue, and the mammary gland; cholesterol in the liver; and steroid hormones in the placenta, ovaries, testes, and adrenal cortex. It is also required by red blood cells (RBC) for hydrogen peroxide reduction. Reduced glutathione (G-SH) is used by glutathione peroxidase to reduce the peroxide to water. The oxidized glutathione (G-S-S-G) produced is reduced by glutathione reductase, using NADPH as the source of electrons. NADPH provides reducing equivalents for the mitochondrial cytochrome P450 monooxygenase system, which is used in steroid hormone synthesis in steroidogenic tissue, bile acid synthesis in the liver, and vitamin D activation in the liver and kidneys. The microsomal system uses NADPH to |
Biochemistry_Lippincott_535 | Biochemistry_Lippinco | system, which is used in steroid hormone synthesis in steroidogenic tissue, bile acid synthesis in the liver, and vitamin D activation in the liver and kidneys. The microsomal system uses NADPH to detoxify foreign compounds (xenobiotics), such as drugs and a variety of pollutants. NADPH provides the reducing equivalents for phagocytes in the process of eliminating invading microorganisms. NADPH oxidase uses molecular oxygen (O2) and electrons from NADPH to produce superoxide radicals, which, in turn, can be converted to peroxide by superoxide dismutase. Myeloperoxidase catalyzes the formation of bactericidal hypochlorous acid from peroxide and chloride ions. Rare genetic defects in NADPH oxidase cause chronic granulomatous disease characterized by severe, persistent, infections and granuloma formation. NADPH is required for the synthesis of nitric oxide (NO), an important free radical gas that causes vasodilation by relaxing vascular smooth muscle, acts as a neurotransmitter, prevents | Biochemistry_Lippinco. system, which is used in steroid hormone synthesis in steroidogenic tissue, bile acid synthesis in the liver, and vitamin D activation in the liver and kidneys. The microsomal system uses NADPH to detoxify foreign compounds (xenobiotics), such as drugs and a variety of pollutants. NADPH provides the reducing equivalents for phagocytes in the process of eliminating invading microorganisms. NADPH oxidase uses molecular oxygen (O2) and electrons from NADPH to produce superoxide radicals, which, in turn, can be converted to peroxide by superoxide dismutase. Myeloperoxidase catalyzes the formation of bactericidal hypochlorous acid from peroxide and chloride ions. Rare genetic defects in NADPH oxidase cause chronic granulomatous disease characterized by severe, persistent, infections and granuloma formation. NADPH is required for the synthesis of nitric oxide (NO), an important free radical gas that causes vasodilation by relaxing vascular smooth muscle, acts as a neurotransmitter, prevents |
Biochemistry_Lippincott_536 | Biochemistry_Lippinco | formation. NADPH is required for the synthesis of nitric oxide (NO), an important free radical gas that causes vasodilation by relaxing vascular smooth muscle, acts as a neurotransmitter, prevents platelet aggregation, and helps mediate macrophage bactericidal activity. NO is made from arginine and O2 by three different NADPH-dependent NO synthases (NOS). The endothelial (eNOS) and neuronal (nNOS) isozymes constantly produce very low levels of NO for vasodilation and neurotransmission, respectively. The inducible isozyme (iNOS) produces large amounts of NO for defense against pathogens. G6PD deficiency impairs the ability of the cell to form the NADPH that is essential for the maintenance of the G-SH pool. The cells most affected are RBC because they do not have additional sources of NADPH. G6PD deficiency is an X-linked disease characterized by hemolytic anemia caused by the production of free radicals and peroxides following administration of oxidant drugs, ingestion of fava beans, | Biochemistry_Lippinco. formation. NADPH is required for the synthesis of nitric oxide (NO), an important free radical gas that causes vasodilation by relaxing vascular smooth muscle, acts as a neurotransmitter, prevents platelet aggregation, and helps mediate macrophage bactericidal activity. NO is made from arginine and O2 by three different NADPH-dependent NO synthases (NOS). The endothelial (eNOS) and neuronal (nNOS) isozymes constantly produce very low levels of NO for vasodilation and neurotransmission, respectively. The inducible isozyme (iNOS) produces large amounts of NO for defense against pathogens. G6PD deficiency impairs the ability of the cell to form the NADPH that is essential for the maintenance of the G-SH pool. The cells most affected are RBC because they do not have additional sources of NADPH. G6PD deficiency is an X-linked disease characterized by hemolytic anemia caused by the production of free radicals and peroxides following administration of oxidant drugs, ingestion of fava beans, |
Biochemistry_Lippincott_537 | Biochemistry_Lippinco | G6PD deficiency is an X-linked disease characterized by hemolytic anemia caused by the production of free radicals and peroxides following administration of oxidant drugs, ingestion of fava beans, or severe infections. The extent of the anemia depends on the amount of residual enzyme. Class I variants, the most severe (and least common), are associated with chronic nonspherocytic hemolytic anemia. Babies with G6PD deficiency may experience neonatal jaundice. | Biochemistry_Lippinco. G6PD deficiency is an X-linked disease characterized by hemolytic anemia caused by the production of free radicals and peroxides following administration of oxidant drugs, ingestion of fava beans, or severe infections. The extent of the anemia depends on the amount of residual enzyme. Class I variants, the most severe (and least common), are associated with chronic nonspherocytic hemolytic anemia. Babies with G6PD deficiency may experience neonatal jaundice. |
Biochemistry_Lippincott_538 | Biochemistry_Lippinco | Choose the ONE best answer. 3.1. In preparation for a trip to an area of India where chloroquine-resistant malaria is endemic, a young man is given primaquine prophylactically. Soon thereafter, he develops a hemolytic condition due to a deficiency in glucose 6-phosphate dehydrogenase. A less-than-normal level of which of the following is a consequence of the enzyme deficiency and the underlying cause of the hemolysis? A. Glucose 6-phosphate B. Oxidized form of nicotinamide adenine dinucleotide C. Reduced form of glutathione D. Ribose 5-phosphate | Biochemistry_Lippinco. Choose the ONE best answer. 3.1. In preparation for a trip to an area of India where chloroquine-resistant malaria is endemic, a young man is given primaquine prophylactically. Soon thereafter, he develops a hemolytic condition due to a deficiency in glucose 6-phosphate dehydrogenase. A less-than-normal level of which of the following is a consequence of the enzyme deficiency and the underlying cause of the hemolysis? A. Glucose 6-phosphate B. Oxidized form of nicotinamide adenine dinucleotide C. Reduced form of glutathione D. Ribose 5-phosphate |
Biochemistry_Lippincott_539 | Biochemistry_Lippinco | A. Glucose 6-phosphate B. Oxidized form of nicotinamide adenine dinucleotide C. Reduced form of glutathione D. Ribose 5-phosphate Correct answer = C. Glutathione (G-SH) is essential for red cell integrity and is maintained in this reduced (functional) form by nicotinamide adenine dinucleotide phosphate (NADPH)-dependent glutathione reductase. The NADPH is from the oxidative portion of the pentose phosphate pathway. Individuals with a deficiency of the regulated enzyme of this pathway, glucose 6-phosphate dehydrogenase (G6PD), have a decreased ability to generate NADPH and, therefore, a decreased ability to keep G-SH reduced. When treated with an oxidant drug such as primaquine, some patients with G6PD deficiency develop a hemolytic anemia. Primaquine does not affect glucose 6phosphate levels. Nicotinamide adenine dinucleotide (NAD[H]) is neither produced by the pathway nor used as a coenzyme by G-SH reductase. A decrease in ribose 5-phosphate does not cause hemolysis. | Biochemistry_Lippinco. A. Glucose 6-phosphate B. Oxidized form of nicotinamide adenine dinucleotide C. Reduced form of glutathione D. Ribose 5-phosphate Correct answer = C. Glutathione (G-SH) is essential for red cell integrity and is maintained in this reduced (functional) form by nicotinamide adenine dinucleotide phosphate (NADPH)-dependent glutathione reductase. The NADPH is from the oxidative portion of the pentose phosphate pathway. Individuals with a deficiency of the regulated enzyme of this pathway, glucose 6-phosphate dehydrogenase (G6PD), have a decreased ability to generate NADPH and, therefore, a decreased ability to keep G-SH reduced. When treated with an oxidant drug such as primaquine, some patients with G6PD deficiency develop a hemolytic anemia. Primaquine does not affect glucose 6phosphate levels. Nicotinamide adenine dinucleotide (NAD[H]) is neither produced by the pathway nor used as a coenzyme by G-SH reductase. A decrease in ribose 5-phosphate does not cause hemolysis. |
Biochemistry_Lippincott_540 | Biochemistry_Lippinco | 3.2. Septic shock, a state of acute circulatory failure characterized by persistent arterial hypotension (low blood pressure) and inadequate organ perfusion refractory to fluid resuscitation, results from a severe inflammatory response to bacterial infection. It has a high mortality rate and is associated with changes in the level of nitric oxide. Which statement concerning septic shock is most likely correct? A. Activation of endothelial nitric oxide synthase causes an increase in nitric oxide. B. High mortality is the result of the long half-life of nitric oxide. C. Lysine, the nitrogen source for nitric oxide synthesis, is deaminated by bacteria. D. Overproduction of nitric oxide by a calcium-independent enzyme is the cause of the hypotension. | Biochemistry_Lippinco. 3.2. Septic shock, a state of acute circulatory failure characterized by persistent arterial hypotension (low blood pressure) and inadequate organ perfusion refractory to fluid resuscitation, results from a severe inflammatory response to bacterial infection. It has a high mortality rate and is associated with changes in the level of nitric oxide. Which statement concerning septic shock is most likely correct? A. Activation of endothelial nitric oxide synthase causes an increase in nitric oxide. B. High mortality is the result of the long half-life of nitric oxide. C. Lysine, the nitrogen source for nitric oxide synthesis, is deaminated by bacteria. D. Overproduction of nitric oxide by a calcium-independent enzyme is the cause of the hypotension. |
Biochemistry_Lippincott_541 | Biochemistry_Lippinco | C. Lysine, the nitrogen source for nitric oxide synthesis, is deaminated by bacteria. D. Overproduction of nitric oxide by a calcium-independent enzyme is the cause of the hypotension. Correct answer = D. Overproduction of short-lived (not long-lived) nitric oxide (NO) by calcium-independent, inducible nitric oxide synthase (iNOS) results in excessive vasodilation leading to hypotension. The endothelial enzyme (eNOS) is constitutive and produces low levels of NO at a consistent rate. NOS use arginine, not lysine, as the source of the nitrogen. | Biochemistry_Lippinco. C. Lysine, the nitrogen source for nitric oxide synthesis, is deaminated by bacteria. D. Overproduction of nitric oxide by a calcium-independent enzyme is the cause of the hypotension. Correct answer = D. Overproduction of short-lived (not long-lived) nitric oxide (NO) by calcium-independent, inducible nitric oxide synthase (iNOS) results in excessive vasodilation leading to hypotension. The endothelial enzyme (eNOS) is constitutive and produces low levels of NO at a consistent rate. NOS use arginine, not lysine, as the source of the nitrogen. |
Biochemistry_Lippincott_542 | Biochemistry_Lippinco | 3.3. An individual who has recently been prescribed a drug (atorvastatin) to lower cholesterol levels is advised to limit consumption of grapefruit juice, because high intake of the juice reportedly results in an increased level of the drug in the blood, increasing the risk of side effects. Atorvastatin is a substrate for the cytochrome P450 enzyme CYP3A4, and grapefruit juice inhibits the enzyme. Which statement concerning CYP enzymes is most likely correct? They: A. accept electrons from reduced nicotinamide adenine dinucleotide. B. catalyze the hydroxylation of hydrophobic molecules. C. differ from nitric oxide synthase in that they contain heme. D. function in association with an oxidase. | Biochemistry_Lippinco. 3.3. An individual who has recently been prescribed a drug (atorvastatin) to lower cholesterol levels is advised to limit consumption of grapefruit juice, because high intake of the juice reportedly results in an increased level of the drug in the blood, increasing the risk of side effects. Atorvastatin is a substrate for the cytochrome P450 enzyme CYP3A4, and grapefruit juice inhibits the enzyme. Which statement concerning CYP enzymes is most likely correct? They: A. accept electrons from reduced nicotinamide adenine dinucleotide. B. catalyze the hydroxylation of hydrophobic molecules. C. differ from nitric oxide synthase in that they contain heme. D. function in association with an oxidase. |
Biochemistry_Lippincott_543 | Biochemistry_Lippinco | B. catalyze the hydroxylation of hydrophobic molecules. C. differ from nitric oxide synthase in that they contain heme. D. function in association with an oxidase. Correct answer = B. The CYP enzymes hydroxylate hydrophobic compounds, making them more water soluble. Reduced nicotinamide adenine dinucleotide phosphate (NADPH) from the pentose phosphate pathway is the electron donor. Both the CYP enzymes and the nitric oxide synthase isozymes contain heme. 3.4. In male patients who are hemizygous for X-linked glucose 6-phosphate dehydrogenase deficiency, pathophysiologic consequences are more apparent in red blood cells (RBC) than in other cells such as in the liver. Which one of the following provides the most reasonable explanation for this different response? A. Excess glucose 6-phosphate in the liver, but not in RBC, can be channeled to glycogen, thereby averting cellular damage. | Biochemistry_Lippinco. B. catalyze the hydroxylation of hydrophobic molecules. C. differ from nitric oxide synthase in that they contain heme. D. function in association with an oxidase. Correct answer = B. The CYP enzymes hydroxylate hydrophobic compounds, making them more water soluble. Reduced nicotinamide adenine dinucleotide phosphate (NADPH) from the pentose phosphate pathway is the electron donor. Both the CYP enzymes and the nitric oxide synthase isozymes contain heme. 3.4. In male patients who are hemizygous for X-linked glucose 6-phosphate dehydrogenase deficiency, pathophysiologic consequences are more apparent in red blood cells (RBC) than in other cells such as in the liver. Which one of the following provides the most reasonable explanation for this different response? A. Excess glucose 6-phosphate in the liver, but not in RBC, can be channeled to glycogen, thereby averting cellular damage. |
Biochemistry_Lippincott_544 | Biochemistry_Lippinco | A. Excess glucose 6-phosphate in the liver, but not in RBC, can be channeled to glycogen, thereby averting cellular damage. B. Liver cells, in contrast to RBC, have alternative mechanisms for supplying the reduced nicotinamide adenine dinucleotide phosphate required for maintaining cell integrity. C. Because RBC do not have mitochondria, production of ATP required to maintain cell integrity depends exclusively on the shunting of glucose 6-phosphate to the pentose phosphate pathway. D. In RBC, in contrast to liver cells, glucose 6-phosphatase activity decreases the level of glucose 6-phosphate, resulting in cell damage. | Biochemistry_Lippinco. A. Excess glucose 6-phosphate in the liver, but not in RBC, can be channeled to glycogen, thereby averting cellular damage. B. Liver cells, in contrast to RBC, have alternative mechanisms for supplying the reduced nicotinamide adenine dinucleotide phosphate required for maintaining cell integrity. C. Because RBC do not have mitochondria, production of ATP required to maintain cell integrity depends exclusively on the shunting of glucose 6-phosphate to the pentose phosphate pathway. D. In RBC, in contrast to liver cells, glucose 6-phosphatase activity decreases the level of glucose 6-phosphate, resulting in cell damage. |
Biochemistry_Lippincott_545 | Biochemistry_Lippinco | D. In RBC, in contrast to liver cells, glucose 6-phosphatase activity decreases the level of glucose 6-phosphate, resulting in cell damage. Correct answer = B. Cellular damage is directly related to decreased ability of the cell to regenerate reduced glutathione, for which large amounts of reduced nicotinamide adenine dinucleotide phosphate (NADPH) are needed, and RBC have no means other than the pentose phosphate pathway of generating NADPH. It is decreased product (NADPH), not increased substrate (glucose 6phosphate), that is the problem. RBC do not have glucose 6-phosphatase. The pentose phosphate pathway does not generate ATP. 3.5. An essential coenzyme for several enzymes of metabolism is derived from the vitamin thiamine. Measurement of the activity of what enzyme in red blood cells could be used to determine thiamine status in the body? | Biochemistry_Lippinco. D. In RBC, in contrast to liver cells, glucose 6-phosphatase activity decreases the level of glucose 6-phosphate, resulting in cell damage. Correct answer = B. Cellular damage is directly related to decreased ability of the cell to regenerate reduced glutathione, for which large amounts of reduced nicotinamide adenine dinucleotide phosphate (NADPH) are needed, and RBC have no means other than the pentose phosphate pathway of generating NADPH. It is decreased product (NADPH), not increased substrate (glucose 6phosphate), that is the problem. RBC do not have glucose 6-phosphatase. The pentose phosphate pathway does not generate ATP. 3.5. An essential coenzyme for several enzymes of metabolism is derived from the vitamin thiamine. Measurement of the activity of what enzyme in red blood cells could be used to determine thiamine status in the body? |
Biochemistry_Lippincott_546 | Biochemistry_Lippinco | Red blood cells do not have mitochondria and, so, do not contain mitochondrial enzymes such as pyruvate dehydrogenase that require the thiamine-derived coenzyme thiamine pyrophosphate (TPP). However, they do contain the cytosolic TPP-requiring transketolase, whose activity is used clinically to assess thiamine status. Glycosaminoglycans, Proteoglycans, and Glycoproteins 14 For additional ancillary materials related to this chapter, please visit thePoint. I. GLYCOSAMINOGLYCAN OVERVIEW | Biochemistry_Lippinco. Red blood cells do not have mitochondria and, so, do not contain mitochondrial enzymes such as pyruvate dehydrogenase that require the thiamine-derived coenzyme thiamine pyrophosphate (TPP). However, they do contain the cytosolic TPP-requiring transketolase, whose activity is used clinically to assess thiamine status. Glycosaminoglycans, Proteoglycans, and Glycoproteins 14 For additional ancillary materials related to this chapter, please visit thePoint. I. GLYCOSAMINOGLYCAN OVERVIEW |
Biochemistry_Lippincott_547 | Biochemistry_Lippinco | I. GLYCOSAMINOGLYCAN OVERVIEW Glycosaminoglycans (GAG) are large complexes of negatively charged heteropolysaccharide chains. They are generally associated with a small amount of protein (core protein), forming proteoglycans, which typically consist of up to 95% carbohydrate. GAG have the special ability to bind large amounts of water, thereby producing the gel-like matrix that forms the basis of the body’s ground substance, which, along with fibrous structural proteins such as collagen, elastin, and fibrillin-1, and adhesive proteins such as fibronectin, makes up the extracellular matrix (ECM). The hydrated GAG serve as a flexible support for the ECM, interacting with the structural and adhesive proteins, and as a molecular sieve, influencing movement of materials through the ECM. The viscous, lubricating properties of mucous secretions also result from the presence of GAG, which led to the original naming of these compounds as mucopolysaccharides. II. STRUCTURE | Biochemistry_Lippinco. I. GLYCOSAMINOGLYCAN OVERVIEW Glycosaminoglycans (GAG) are large complexes of negatively charged heteropolysaccharide chains. They are generally associated with a small amount of protein (core protein), forming proteoglycans, which typically consist of up to 95% carbohydrate. GAG have the special ability to bind large amounts of water, thereby producing the gel-like matrix that forms the basis of the body’s ground substance, which, along with fibrous structural proteins such as collagen, elastin, and fibrillin-1, and adhesive proteins such as fibronectin, makes up the extracellular matrix (ECM). The hydrated GAG serve as a flexible support for the ECM, interacting with the structural and adhesive proteins, and as a molecular sieve, influencing movement of materials through the ECM. The viscous, lubricating properties of mucous secretions also result from the presence of GAG, which led to the original naming of these compounds as mucopolysaccharides. II. STRUCTURE |
Biochemistry_Lippincott_548 | Biochemistry_Lippinco | II. STRUCTURE GAG are long, unbranched, heteropolysaccharide chains composed of a repeating disaccharide unit [acidic sugar–amino sugar]n (Fig. 14.1). [Note: A single exception is keratan sulfate, which contains galactose rather than an acidic sugar.] The amino sugar is either D-glucosamine or D-galactosamine, in which the amino group is usually acetylated, thus eliminating its positive charge. The amino sugar may also be sulfated on carbon 4 or 6 or on a nonacetylated nitrogen. The acidic sugar is either D-glucuronic acid or its C-5 epimer Liduronic acid (Fig. 14.2). These uronic sugars contain carboxyl groups that are negatively charged at physiologic pH and, together with the sulfate groups (−SO42−), give GAG their strongly negative nature. A. Structure–function relationship | Biochemistry_Lippinco. II. STRUCTURE GAG are long, unbranched, heteropolysaccharide chains composed of a repeating disaccharide unit [acidic sugar–amino sugar]n (Fig. 14.1). [Note: A single exception is keratan sulfate, which contains galactose rather than an acidic sugar.] The amino sugar is either D-glucosamine or D-galactosamine, in which the amino group is usually acetylated, thus eliminating its positive charge. The amino sugar may also be sulfated on carbon 4 or 6 or on a nonacetylated nitrogen. The acidic sugar is either D-glucuronic acid or its C-5 epimer Liduronic acid (Fig. 14.2). These uronic sugars contain carboxyl groups that are negatively charged at physiologic pH and, together with the sulfate groups (−SO42−), give GAG their strongly negative nature. A. Structure–function relationship |
Biochemistry_Lippincott_549 | Biochemistry_Lippinco | A. Structure–function relationship Because of the high concentration of negative charges, these heteropolysaccharide chains tend to be extended in solution. They repel each other and are surrounded by a shell of water molecules. When brought together, they slide past each other, much as two magnets with the same polarity seem to slide past each other. This produces the slippery consistency of mucous secretions and synovial fluid. When a solution of GAG is compressed, the water is squeezed out, and the GAG are forced to occupy a smaller volume. When the compression is released, the GAG spring back to their original, hydrated volume because of the repulsion of their negative charges. This property contributes to the resilience of synovial fluid and the vitreous humor of the eye (Fig. 14.3). B. Classification | Biochemistry_Lippinco. A. Structure–function relationship Because of the high concentration of negative charges, these heteropolysaccharide chains tend to be extended in solution. They repel each other and are surrounded by a shell of water molecules. When brought together, they slide past each other, much as two magnets with the same polarity seem to slide past each other. This produces the slippery consistency of mucous secretions and synovial fluid. When a solution of GAG is compressed, the water is squeezed out, and the GAG are forced to occupy a smaller volume. When the compression is released, the GAG spring back to their original, hydrated volume because of the repulsion of their negative charges. This property contributes to the resilience of synovial fluid and the vitreous humor of the eye (Fig. 14.3). B. Classification |
Biochemistry_Lippincott_550 | Biochemistry_Lippinco | B. Classification The six major types of GAG are divided according to monomeric composition, type of glycosidic linkages, and degree and location of sulfate units. The structure of the GAG and their distribution in the body is illustrated in Figure 14.4. All GAG, except for hyaluronic acid, are sulfated and are found covalently attached to protein, forming proteoglycan monomers. C. Proteoglycans Proteoglycans are found in the ECM and on the outer surface of cells. 1. | Biochemistry_Lippinco. B. Classification The six major types of GAG are divided according to monomeric composition, type of glycosidic linkages, and degree and location of sulfate units. The structure of the GAG and their distribution in the body is illustrated in Figure 14.4. All GAG, except for hyaluronic acid, are sulfated and are found covalently attached to protein, forming proteoglycan monomers. C. Proteoglycans Proteoglycans are found in the ECM and on the outer surface of cells. 1. |
Biochemistry_Lippincott_551 | Biochemistry_Lippinco | C. Proteoglycans Proteoglycans are found in the ECM and on the outer surface of cells. 1. Monomer structure: A proteoglycan monomer found in cartilage consists of a core protein to which up to 100 linear chains of GAG are covalently attached. These chains, which may each be composed of up to 200 disaccharide units, extend out from the core protein and remain separated from each other because of charge repulsion. The resulting structure resembles a bottle brush (Fig. 14.5). In cartilage proteoglycans, the species of GAG include chondroitin sulfate and keratan sulfate. [Note: Proteoglycans are grouped into gene families that encode core proteins with common structural features. The aggrecan family (aggrecan, versican, neurocan, and brevican), abundant in cartilage, is an example.] 2. | Biochemistry_Lippinco. C. Proteoglycans Proteoglycans are found in the ECM and on the outer surface of cells. 1. Monomer structure: A proteoglycan monomer found in cartilage consists of a core protein to which up to 100 linear chains of GAG are covalently attached. These chains, which may each be composed of up to 200 disaccharide units, extend out from the core protein and remain separated from each other because of charge repulsion. The resulting structure resembles a bottle brush (Fig. 14.5). In cartilage proteoglycans, the species of GAG include chondroitin sulfate and keratan sulfate. [Note: Proteoglycans are grouped into gene families that encode core proteins with common structural features. The aggrecan family (aggrecan, versican, neurocan, and brevican), abundant in cartilage, is an example.] 2. |
Biochemistry_Lippincott_552 | Biochemistry_Lippinco | GAG–protein linkage: This covalent linkage is most commonly through a trihexoside (galactose-galactose-xylose) and a serine residue in the protein. An O-glycosidic bond (see p. 86) is formed between the xylose and the hydroxyl group of the serine (Fig. 14.6). 3. Aggregate formation: Many proteoglycan monomers can associate with one molecule of hyaluronic acid to form proteoglycan aggregates. The association is not covalent and occurs primarily through ionic interactions between the core protein and the hyaluronic acid. The association is stabilized by additional small proteins called link proteins (Fig. 14.7). III. SYNTHESIS | Biochemistry_Lippinco. GAG–protein linkage: This covalent linkage is most commonly through a trihexoside (galactose-galactose-xylose) and a serine residue in the protein. An O-glycosidic bond (see p. 86) is formed between the xylose and the hydroxyl group of the serine (Fig. 14.6). 3. Aggregate formation: Many proteoglycan monomers can associate with one molecule of hyaluronic acid to form proteoglycan aggregates. The association is not covalent and occurs primarily through ionic interactions between the core protein and the hyaluronic acid. The association is stabilized by additional small proteins called link proteins (Fig. 14.7). III. SYNTHESIS |
Biochemistry_Lippincott_553 | Biochemistry_Lippinco | III. SYNTHESIS The heteropolysaccharide chains are elongated by the sequential addition of alternating acidic and amino sugars donated primarily by their uridine diphosphate (UDP) derivatives. The reactions are catalyzed by a family of specific glycosyltransferases. Because GAG are produced for export from the cell, their synthesis occurs primarily in the Golgi and not in the cytosol. A. Amino sugar synthesis Amino sugars are essential components of glycoconjugates such as proteoglycans, glycoproteins, and glycolipids. The synthetic pathway of amino sugars (hexosamines) is very active in connective tissues, where as much as 20% of glucose flows through this pathway. | Biochemistry_Lippinco. III. SYNTHESIS The heteropolysaccharide chains are elongated by the sequential addition of alternating acidic and amino sugars donated primarily by their uridine diphosphate (UDP) derivatives. The reactions are catalyzed by a family of specific glycosyltransferases. Because GAG are produced for export from the cell, their synthesis occurs primarily in the Golgi and not in the cytosol. A. Amino sugar synthesis Amino sugars are essential components of glycoconjugates such as proteoglycans, glycoproteins, and glycolipids. The synthetic pathway of amino sugars (hexosamines) is very active in connective tissues, where as much as 20% of glucose flows through this pathway. |
Biochemistry_Lippincott_554 | Biochemistry_Lippinco | 1. N-Acetylglucosamine and N-acetylgalactosamine: The monosaccharide fructose 6-phosphate is the precursor of N-acetylglucosamine (GlcNAc) and N-acetylgalactosamine (GalNAc). A hydroxyl group on the fructose is replaced by the amide nitrogen of a glutamine, and the glucosamine 6phosphate product gets acetylated, isomerized, and activated, producing the nucleotide sugar UDP-GlcNAc (Fig. 14.8). UDP-GalNAc is generated by the epimerization of UDP-GlcNAc. It is these nucleotide sugar forms of the amino sugars that are used to elongate the carbohydrate chains. pyrophosphate. | Biochemistry_Lippinco. 1. N-Acetylglucosamine and N-acetylgalactosamine: The monosaccharide fructose 6-phosphate is the precursor of N-acetylglucosamine (GlcNAc) and N-acetylgalactosamine (GalNAc). A hydroxyl group on the fructose is replaced by the amide nitrogen of a glutamine, and the glucosamine 6phosphate product gets acetylated, isomerized, and activated, producing the nucleotide sugar UDP-GlcNAc (Fig. 14.8). UDP-GalNAc is generated by the epimerization of UDP-GlcNAc. It is these nucleotide sugar forms of the amino sugars that are used to elongate the carbohydrate chains. pyrophosphate. |
Biochemistry_Lippincott_555 | Biochemistry_Lippinco | pyrophosphate. 2. N-Acetylneuraminic acid: NANA, a nine-carbon, acidic monosaccharide (see Fig. 17.15, p. 209), is a member of the family of sialic acids, each of which is acylated at a different site. These compounds are usually found as terminal carbohydrate residues of oligosaccharide side chains of glycoproteins, of glycolipids, or, less frequently, of GAG. N-Acetylmannosamine 6-phosphate (derived from fructose 6-phosphate) and phosphoenolpyruvate (an intermediate in glycolysis; see p. 102) are the immediate sources of the carbons and nitrogens for NANA synthesis (see Fig. 14.8). Before NANA can be added to a growing oligosaccharide, it must be activated to cytidine monophosphate (CMP)NANA by reacting with cytidine triphosphate (CTP). CMP-NANA synthetase catalyzes the reaction. [Note: CMP-NANA is the only nucleotide sugar in human metabolism in which the carrier nucleotide is a monophosphate rather than a diphosphate.] B. Acidic sugar synthesis | Biochemistry_Lippinco. pyrophosphate. 2. N-Acetylneuraminic acid: NANA, a nine-carbon, acidic monosaccharide (see Fig. 17.15, p. 209), is a member of the family of sialic acids, each of which is acylated at a different site. These compounds are usually found as terminal carbohydrate residues of oligosaccharide side chains of glycoproteins, of glycolipids, or, less frequently, of GAG. N-Acetylmannosamine 6-phosphate (derived from fructose 6-phosphate) and phosphoenolpyruvate (an intermediate in glycolysis; see p. 102) are the immediate sources of the carbons and nitrogens for NANA synthesis (see Fig. 14.8). Before NANA can be added to a growing oligosaccharide, it must be activated to cytidine monophosphate (CMP)NANA by reacting with cytidine triphosphate (CTP). CMP-NANA synthetase catalyzes the reaction. [Note: CMP-NANA is the only nucleotide sugar in human metabolism in which the carrier nucleotide is a monophosphate rather than a diphosphate.] B. Acidic sugar synthesis |
Biochemistry_Lippincott_556 | Biochemistry_Lippinco | B. Acidic sugar synthesis D-Glucuronic acid, whose structure is that of glucose with an oxidized carbon 6 (−CH2OH → −COOH), and its C-5 epimer, L-iduronic acid, are essential components of GAG. Glucuronic acid is also required for the detoxification of lipophilic compounds, such as bilirubin (see p. 282), steroids (see p. 240), and many drugs, including the statins (see p. 224), because conjugation with glucuronate (glucuronidation) increases water solubility. In plants and mammals (other than guinea pigs and primates, including humans), glucuronic acid is a precursor of ascorbic acid (vitamin C) as shown in Figure 14.9. This uronic acid pathway also provides a mechanism by which dietary D-xylulose can enter the central metabolic pathways. | Biochemistry_Lippinco. B. Acidic sugar synthesis D-Glucuronic acid, whose structure is that of glucose with an oxidized carbon 6 (−CH2OH → −COOH), and its C-5 epimer, L-iduronic acid, are essential components of GAG. Glucuronic acid is also required for the detoxification of lipophilic compounds, such as bilirubin (see p. 282), steroids (see p. 240), and many drugs, including the statins (see p. 224), because conjugation with glucuronate (glucuronidation) increases water solubility. In plants and mammals (other than guinea pigs and primates, including humans), glucuronic acid is a precursor of ascorbic acid (vitamin C) as shown in Figure 14.9. This uronic acid pathway also provides a mechanism by which dietary D-xylulose can enter the central metabolic pathways. |
Biochemistry_Lippincott_557 | Biochemistry_Lippinco | C) as shown in Figure 14.9. This uronic acid pathway also provides a mechanism by which dietary D-xylulose can enter the central metabolic pathways. 1. Glucuronic acid: Glucuronic acid can be obtained in small amounts from the diet and from the lysosomal degradation of GAG. It also can be synthesized by the uronic acid pathway, in which glucose 1-phosphate reacts with uridine triphosphate (UTP) and is converted to UDP-glucose. Oxidation of UDP-glucose produces UDP-glucuronic acid, the form that supplies glucuronic acid for GAG synthesis and glucuronidation (Fig. 14.10). The end product of glucuronic acid metabolism in humans is Dxylulose 5-phosphate, which can enter the pentose phosphate pathway and produce the glycolytic intermediates glyceraldehyde 3-phosphate and fructose 6-phosphate (see Fig. 14.9; see also Fig. 13.2, p. 146). | Biochemistry_Lippinco. C) as shown in Figure 14.9. This uronic acid pathway also provides a mechanism by which dietary D-xylulose can enter the central metabolic pathways. 1. Glucuronic acid: Glucuronic acid can be obtained in small amounts from the diet and from the lysosomal degradation of GAG. It also can be synthesized by the uronic acid pathway, in which glucose 1-phosphate reacts with uridine triphosphate (UTP) and is converted to UDP-glucose. Oxidation of UDP-glucose produces UDP-glucuronic acid, the form that supplies glucuronic acid for GAG synthesis and glucuronidation (Fig. 14.10). The end product of glucuronic acid metabolism in humans is Dxylulose 5-phosphate, which can enter the pentose phosphate pathway and produce the glycolytic intermediates glyceraldehyde 3-phosphate and fructose 6-phosphate (see Fig. 14.9; see also Fig. 13.2, p. 146). |
Biochemistry_Lippincott_558 | Biochemistry_Lippinco | 2. l-Iduronic acid: Synthesis of L-iduronic acid occurs after D-glucuronic acid has been incorporated into the carbohydrate chain. Uronosyl 5epimerase causes epimerization of the D-to the L-sugar. C. Core protein synthesis The core protein is made by ribosomes on the rough endoplasmic reticulum (RER), enters the RER lumen, and then moves to the Golgi, where it is glycosylated by membrane-bound glycosyltransferases. D. Carbohydrate chain synthesis Carbohydrate chain formation is initiated by synthesis of a short linker on the core protein on which carbohydrate chain synthesis will occur. The most common linker is a trihexoside formed by the transfer of a xylose from UDP-xylose to the hydroxyl group of a serine (or threonine) catalyzed by xylosyltransferase. Two galactose molecules are then added, completing the trihexoside. This is followed by sequential addition of alternating acidic and amino sugars (Fig. 14.11) and epimerization of some D-glucuronyl to Liduronyl residues. | Biochemistry_Lippinco. 2. l-Iduronic acid: Synthesis of L-iduronic acid occurs after D-glucuronic acid has been incorporated into the carbohydrate chain. Uronosyl 5epimerase causes epimerization of the D-to the L-sugar. C. Core protein synthesis The core protein is made by ribosomes on the rough endoplasmic reticulum (RER), enters the RER lumen, and then moves to the Golgi, where it is glycosylated by membrane-bound glycosyltransferases. D. Carbohydrate chain synthesis Carbohydrate chain formation is initiated by synthesis of a short linker on the core protein on which carbohydrate chain synthesis will occur. The most common linker is a trihexoside formed by the transfer of a xylose from UDP-xylose to the hydroxyl group of a serine (or threonine) catalyzed by xylosyltransferase. Two galactose molecules are then added, completing the trihexoside. This is followed by sequential addition of alternating acidic and amino sugars (Fig. 14.11) and epimerization of some D-glucuronyl to Liduronyl residues. |
Biochemistry_Lippincott_559 | Biochemistry_Lippinco | E. Sulfate group addition Sulfation of a GAG occurs after the monosaccharide to be sulfated has been incorporated into the growing carbohydrate chain. The source of the sulfate is 3´-phosphoadenosyl-5´-phosphosulfate ([PAPS] a molecule of adenosine monophosphate with a sulfate group attached to the 5´-phosphate; see Fig. 17.16, p. 210). The sulfation reaction is catalyzed by sulfotransferases. Synthesis of the sulfated GAG chondroitin sulfate is shown in Figure 14.11. [Note: PAPS is also the sulfur donor in glycosphingolipid synthesis (see p. 210).] A defect in the sulfation of the growing GAG chains results in one of several autosomal-recessive disorders, the chondrodystrophies, which affect the proper development and maintenance of the skeletal system. IV. DEGRADATION | Biochemistry_Lippinco. E. Sulfate group addition Sulfation of a GAG occurs after the monosaccharide to be sulfated has been incorporated into the growing carbohydrate chain. The source of the sulfate is 3´-phosphoadenosyl-5´-phosphosulfate ([PAPS] a molecule of adenosine monophosphate with a sulfate group attached to the 5´-phosphate; see Fig. 17.16, p. 210). The sulfation reaction is catalyzed by sulfotransferases. Synthesis of the sulfated GAG chondroitin sulfate is shown in Figure 14.11. [Note: PAPS is also the sulfur donor in glycosphingolipid synthesis (see p. 210).] A defect in the sulfation of the growing GAG chains results in one of several autosomal-recessive disorders, the chondrodystrophies, which affect the proper development and maintenance of the skeletal system. IV. DEGRADATION |
Biochemistry_Lippincott_560 | Biochemistry_Lippinco | IV. DEGRADATION GAG are degraded in lysosomes, which contain hydrolytic enzymes that are most active at a pH of ~5. Therefore, as a group, these enzymes are called acid hydrolases. [Note: The low pH optimum is a protective mechanism that prevents the enzymes from destroying the cell should leakage occur into the cytosol where the pH is neutral.] The half-lives of GAG vary from minutes to months and are influenced by the type of GAG and its location in the body. A. GAG phagocytosis Because GAG are extracellular or cell-surface compounds, they must first be engulfed by invagination of the cell membrane (phagocytosis), forming a vesicle inside of which are the GAG to be degraded. This vesicle then fuses with a lysosome, forming a single digestive vesicle in which the GAG are efficiently degraded (see p. 150 for a discussion of phagocytosis). B. Lysosomal degradation | Biochemistry_Lippinco. IV. DEGRADATION GAG are degraded in lysosomes, which contain hydrolytic enzymes that are most active at a pH of ~5. Therefore, as a group, these enzymes are called acid hydrolases. [Note: The low pH optimum is a protective mechanism that prevents the enzymes from destroying the cell should leakage occur into the cytosol where the pH is neutral.] The half-lives of GAG vary from minutes to months and are influenced by the type of GAG and its location in the body. A. GAG phagocytosis Because GAG are extracellular or cell-surface compounds, they must first be engulfed by invagination of the cell membrane (phagocytosis), forming a vesicle inside of which are the GAG to be degraded. This vesicle then fuses with a lysosome, forming a single digestive vesicle in which the GAG are efficiently degraded (see p. 150 for a discussion of phagocytosis). B. Lysosomal degradation |
Biochemistry_Lippincott_561 | Biochemistry_Lippinco | B. Lysosomal degradation The lysosomal degradation of GAG requires a large number of acid hydrolases for complete digestion. First, the polysaccharide chains are cleaved by endoglycosidases, producing oligosaccharides. Further degradation of the oligosaccharides occurs sequentially from the nonreducing end (see p. 127) of each chain, the last group (sulfate or sugar) added during synthesis being the first group removed (by sulfatases or exoglycosidases). Examples of some of these enzymes and the bonds they hydrolyze are shown in Figure 14.12. [Note: Endo-and exoglycosidases are also involved in the lysosomal degradation of glycoproteins (see p. 170) and glycolipids (see p. 210). Deficiencies in these enzymes result in the accumulation of partially degraded carbohydrates, causing tissue damage.] glucosamine; S = sulfate. | Biochemistry_Lippinco. B. Lysosomal degradation The lysosomal degradation of GAG requires a large number of acid hydrolases for complete digestion. First, the polysaccharide chains are cleaved by endoglycosidases, producing oligosaccharides. Further degradation of the oligosaccharides occurs sequentially from the nonreducing end (see p. 127) of each chain, the last group (sulfate or sugar) added during synthesis being the first group removed (by sulfatases or exoglycosidases). Examples of some of these enzymes and the bonds they hydrolyze are shown in Figure 14.12. [Note: Endo-and exoglycosidases are also involved in the lysosomal degradation of glycoproteins (see p. 170) and glycolipids (see p. 210). Deficiencies in these enzymes result in the accumulation of partially degraded carbohydrates, causing tissue damage.] glucosamine; S = sulfate. |
Biochemistry_Lippincott_562 | Biochemistry_Lippinco | Multiple sulfatase deficiency (Austin disease) is a rare lysosomal storage disease in which all sulfatases are nonfunctional because of a defect in the formation of formylglycine, an amino acid derivative required at the active site for enzymic activity to occur. V. MUCOPOLYSACCHARIDOSES | Biochemistry_Lippinco. Multiple sulfatase deficiency (Austin disease) is a rare lysosomal storage disease in which all sulfatases are nonfunctional because of a defect in the formation of formylglycine, an amino acid derivative required at the active site for enzymic activity to occur. V. MUCOPOLYSACCHARIDOSES |
Biochemistry_Lippincott_563 | Biochemistry_Lippinco | The mucopolysaccharidoses are hereditary diseases (approximately 1:25,000 live births) caused by a deficiency of any one of the lysosomal hydrolases normally involved in the degradation of heparan sulfate and/or dermatan sulfate (see Fig. 14.12). They are progressive disorders characterized by lysosomal accumulation of GAG in various tissues, causing a range of symptoms, such as skeletal and ECM deformities, and intellectual disability. All are autosomal-recessive disorders except Hunter syndrome, which is X linked. Children who are homozygous for any one of these diseases are apparently normal at birth and then gradually deteriorate. In severe deficiencies, death occurs in childhood. There currently is no cure. Incomplete lysosomal degradation of GAG results in the presence of oligosaccharides in the urine. These fragments can be used to diagnose the specific mucopolysaccharidosis by identifying the structure present on the nonreducing end of the oligosaccharide, because that residue | Biochemistry_Lippinco. The mucopolysaccharidoses are hereditary diseases (approximately 1:25,000 live births) caused by a deficiency of any one of the lysosomal hydrolases normally involved in the degradation of heparan sulfate and/or dermatan sulfate (see Fig. 14.12). They are progressive disorders characterized by lysosomal accumulation of GAG in various tissues, causing a range of symptoms, such as skeletal and ECM deformities, and intellectual disability. All are autosomal-recessive disorders except Hunter syndrome, which is X linked. Children who are homozygous for any one of these diseases are apparently normal at birth and then gradually deteriorate. In severe deficiencies, death occurs in childhood. There currently is no cure. Incomplete lysosomal degradation of GAG results in the presence of oligosaccharides in the urine. These fragments can be used to diagnose the specific mucopolysaccharidosis by identifying the structure present on the nonreducing end of the oligosaccharide, because that residue |
Biochemistry_Lippincott_564 | Biochemistry_Lippinco | in the urine. These fragments can be used to diagnose the specific mucopolysaccharidosis by identifying the structure present on the nonreducing end of the oligosaccharide, because that residue would have been the substrate for the missing enzyme. Diagnosis is confirmed by measuring the patient’s cellular level of the lysosomal hydrolases. Bone marrow and cord blood transplants, in which transplanted macrophages produce the enzymes that degrade GAG, have been used to treat Hurler and Hunter syndromes, with limited success. Enzyme replacement therapy is available for both syndromes but does not prevent neurologic damage. | Biochemistry_Lippinco. in the urine. These fragments can be used to diagnose the specific mucopolysaccharidosis by identifying the structure present on the nonreducing end of the oligosaccharide, because that residue would have been the substrate for the missing enzyme. Diagnosis is confirmed by measuring the patient’s cellular level of the lysosomal hydrolases. Bone marrow and cord blood transplants, in which transplanted macrophages produce the enzymes that degrade GAG, have been used to treat Hurler and Hunter syndromes, with limited success. Enzyme replacement therapy is available for both syndromes but does not prevent neurologic damage. |
Biochemistry_Lippincott_565 | Biochemistry_Lippinco | VI. GLYCOPROTEIN OVERVIEW | Biochemistry_Lippinco. VI. GLYCOPROTEIN OVERVIEW |
Biochemistry_Lippincott_566 | Biochemistry_Lippinco | Glycoproteins are proteins to which oligosaccharides (glycans) are covalently attached. [Note: Glycosylation is the most common posttranslational modification of proteins.] They differ from the proteoglycans in several important ways. Glycoproteins contain highly variable amounts of carbohydrate but typically less than that of proteoglycans. For example, the glycoprotein immunoglobulin G (IgG) contains <4% of its mass as carbohydrate, whereas the proteoglycan aggrecan contains >80%. In glycoproteins, the glycan is relatively short (usually two to ten sugar residues in length, although it can be longer); does not contain repeating disaccharide units and, consequently, is structurally diverse; is often branched instead of linear; and may or may not be negatively charged. Membrane-bound glycoproteins participate in a broad range of cellular phenomena, including cell-surface recognition (by other cells, hormones, and viruses), cell-surface antigenicity (such as the blood group antigens), | Biochemistry_Lippinco. Glycoproteins are proteins to which oligosaccharides (glycans) are covalently attached. [Note: Glycosylation is the most common posttranslational modification of proteins.] They differ from the proteoglycans in several important ways. Glycoproteins contain highly variable amounts of carbohydrate but typically less than that of proteoglycans. For example, the glycoprotein immunoglobulin G (IgG) contains <4% of its mass as carbohydrate, whereas the proteoglycan aggrecan contains >80%. In glycoproteins, the glycan is relatively short (usually two to ten sugar residues in length, although it can be longer); does not contain repeating disaccharide units and, consequently, is structurally diverse; is often branched instead of linear; and may or may not be negatively charged. Membrane-bound glycoproteins participate in a broad range of cellular phenomena, including cell-surface recognition (by other cells, hormones, and viruses), cell-surface antigenicity (such as the blood group antigens), |
Biochemistry_Lippincott_567 | Biochemistry_Lippinco | participate in a broad range of cellular phenomena, including cell-surface recognition (by other cells, hormones, and viruses), cell-surface antigenicity (such as the blood group antigens), and as components of the ECM and of the mucins of the gastrointestinal and urogenital tracts, where they act as protective biologic lubricants. In addition, almost all of the globular proteins present in human plasma are glycoproteins, although albumin is an exception. Figure 14.13 summarizes some glycoprotein functions. | Biochemistry_Lippinco. participate in a broad range of cellular phenomena, including cell-surface recognition (by other cells, hormones, and viruses), cell-surface antigenicity (such as the blood group antigens), and as components of the ECM and of the mucins of the gastrointestinal and urogenital tracts, where they act as protective biologic lubricants. In addition, almost all of the globular proteins present in human plasma are glycoproteins, although albumin is an exception. Figure 14.13 summarizes some glycoprotein functions. |
Biochemistry_Lippincott_568 | Biochemistry_Lippinco | VII. OLIGOSACCHARIDE STRUCTURE The oligosaccharide (glycan) components of glycoproteins are generally branched heteropolymers composed primarily of D-hexoses, with the addition in some cases of neuraminic acid (a nonose) and of L-fucose, a 6-deoxyhexose. A. Carbohydrate–protein linkage The glycan may be attached to the protein through an N-or an O-glycosidic link (see p. 86). In the former case, the sugar chain is attached to the amide group of an asparagine side chain and, in the latter case, to the hydroxyl group of either a serine or threonine side chain. [Note: In the case of collagen, there is an O-glycosidic linkage between galactose or glucose and the hydroxyl group of hydroxylysine (see p. 47).] B. N-and O-Linked oligosaccharides A glycoprotein may contain only one type of glycosidic linkage (N or O linked) or may have both types within the same molecule. 1. | Biochemistry_Lippinco. VII. OLIGOSACCHARIDE STRUCTURE The oligosaccharide (glycan) components of glycoproteins are generally branched heteropolymers composed primarily of D-hexoses, with the addition in some cases of neuraminic acid (a nonose) and of L-fucose, a 6-deoxyhexose. A. Carbohydrate–protein linkage The glycan may be attached to the protein through an N-or an O-glycosidic link (see p. 86). In the former case, the sugar chain is attached to the amide group of an asparagine side chain and, in the latter case, to the hydroxyl group of either a serine or threonine side chain. [Note: In the case of collagen, there is an O-glycosidic linkage between galactose or glucose and the hydroxyl group of hydroxylysine (see p. 47).] B. N-and O-Linked oligosaccharides A glycoprotein may contain only one type of glycosidic linkage (N or O linked) or may have both types within the same molecule. 1. |
Biochemistry_Lippincott_569 | Biochemistry_Lippinco | B. N-and O-Linked oligosaccharides A glycoprotein may contain only one type of glycosidic linkage (N or O linked) or may have both types within the same molecule. 1. O-Linked: The O-linked glycans may have one or more of a wide variety of sugars arranged in either a linear or a branched pattern. Many are found in extracellular glycoproteins or as membrane glycoprotein components. For example, O-linked oligosaccharides on the surface of red blood cells help provide the ABO blood group determinants. If the terminal sugar on the glycan is GalNAc, the blood group is A. If it is galactose, the blood group is B. If neither GalNAc nor galactose is present, the blood group is O. 2. | Biochemistry_Lippinco. B. N-and O-Linked oligosaccharides A glycoprotein may contain only one type of glycosidic linkage (N or O linked) or may have both types within the same molecule. 1. O-Linked: The O-linked glycans may have one or more of a wide variety of sugars arranged in either a linear or a branched pattern. Many are found in extracellular glycoproteins or as membrane glycoprotein components. For example, O-linked oligosaccharides on the surface of red blood cells help provide the ABO blood group determinants. If the terminal sugar on the glycan is GalNAc, the blood group is A. If it is galactose, the blood group is B. If neither GalNAc nor galactose is present, the blood group is O. 2. |
Biochemistry_Lippincott_570 | Biochemistry_Lippinco | 2. N-Linked: The N-linked glycans fall into two broad classes: complex oligosaccharides and high-mannose oligosaccharides. Both contain the same pentasaccharide core shown in Figure 14.14, but the complex oligosaccharides contain a diverse group of additional sugars, for example, GlcNAc, GalNAc, L-fucose, and NANA, whereas the high mannose oligosaccharides contain primarily mannose. VIII. GLYCOPROTEIN SYNTHESIS Proteins destined to function in the cytoplasm are synthesized on free cytosolic ribosomes. However, proteins, including glycoproteins, that are destined for cellular membranes, lysosomes, or to be exported from the cell, are synthesized on ribosomes attached to the RER. These proteins contain specific signal sequences that act as molecular addresses, targeting the proteins to their proper destinations. An N-terminal hydrophobic sequence initially directs these proteins to the RER, allowing the growing polypeptide to be extruded into the lumen (see p. | Biochemistry_Lippinco. 2. N-Linked: The N-linked glycans fall into two broad classes: complex oligosaccharides and high-mannose oligosaccharides. Both contain the same pentasaccharide core shown in Figure 14.14, but the complex oligosaccharides contain a diverse group of additional sugars, for example, GlcNAc, GalNAc, L-fucose, and NANA, whereas the high mannose oligosaccharides contain primarily mannose. VIII. GLYCOPROTEIN SYNTHESIS Proteins destined to function in the cytoplasm are synthesized on free cytosolic ribosomes. However, proteins, including glycoproteins, that are destined for cellular membranes, lysosomes, or to be exported from the cell, are synthesized on ribosomes attached to the RER. These proteins contain specific signal sequences that act as molecular addresses, targeting the proteins to their proper destinations. An N-terminal hydrophobic sequence initially directs these proteins to the RER, allowing the growing polypeptide to be extruded into the lumen (see p. |
Biochemistry_Lippincott_571 | Biochemistry_Lippinco | 459). The proteins are then transported via secretory vesicles to the Golgi, which acts as a sorting center (Fig. 14.15). In the Golgi, those glycoproteins that are to be secreted from the cell (or are targeted for lysosomes) are packaged into vesicles that fuse with the cell (or lysosomal) membrane and release their contents. Those that are destined to become components of the cell membrane are integrated into the Golgi membrane, which buds off, forming vesicles that add their membrane-bound glycoproteins to the cell membrane. [Note: Therefore, the membrane glycoproteins are oriented with the carbohydrate portion on the outside of the cell (see Fig. 14.15).] A. | Biochemistry_Lippinco. 459). The proteins are then transported via secretory vesicles to the Golgi, which acts as a sorting center (Fig. 14.15). In the Golgi, those glycoproteins that are to be secreted from the cell (or are targeted for lysosomes) are packaged into vesicles that fuse with the cell (or lysosomal) membrane and release their contents. Those that are destined to become components of the cell membrane are integrated into the Golgi membrane, which buds off, forming vesicles that add their membrane-bound glycoproteins to the cell membrane. [Note: Therefore, the membrane glycoproteins are oriented with the carbohydrate portion on the outside of the cell (see Fig. 14.15).] A. |
Biochemistry_Lippincott_572 | Biochemistry_Lippinco | A. The precursors of the carbohydrate components of glycoproteins are nucleotide sugars, which include UDP-glucose, UDP-galactose, UDP-GlcNAc, and UDP-GalNAc. In addition, guanosine diphosphate (GDP)mannose, GDP-L-fucose (which is synthesized from GDP-mannose), and CMP-NANA may donate sugars to the growing chain. [Note: When the acidic NANA is present, the oligosaccharide has a negative charge at physiologic pH.] The oligosaccharides are covalently attached to the side chains of specific amino acids in the protein, where the three-dimensional structure of the protein determines whether or not a specific amino acid is glycosylated. B. O-Linked glycoprotein synthesis | Biochemistry_Lippinco. A. The precursors of the carbohydrate components of glycoproteins are nucleotide sugars, which include UDP-glucose, UDP-galactose, UDP-GlcNAc, and UDP-GalNAc. In addition, guanosine diphosphate (GDP)mannose, GDP-L-fucose (which is synthesized from GDP-mannose), and CMP-NANA may donate sugars to the growing chain. [Note: When the acidic NANA is present, the oligosaccharide has a negative charge at physiologic pH.] The oligosaccharides are covalently attached to the side chains of specific amino acids in the protein, where the three-dimensional structure of the protein determines whether or not a specific amino acid is glycosylated. B. O-Linked glycoprotein synthesis |
Biochemistry_Lippincott_573 | Biochemistry_Lippinco | B. O-Linked glycoprotein synthesis Synthesis of the O-linked glycoproteins is very similar to that of the GAG (see p. 158). First, the protein to which sugars are to be attached is synthesized on the RER and extruded into its lumen. Glycosylation begins with the transfer of GalNAc (from UDP-GalNAc) to the hydroxyl group of a specific serine or threonine residues. The glycosyltransferases responsible for the stepwise synthesis (from individual sugars) of the oligosaccharides are bound to the membranes of the Golgi. They act in a specific order, without using a template as is required for DNA, ribonucleic acid (RNA), and protein synthesis (see Unit VII), but instead by recognizing the actual structure of the growing oligosaccharide as the appropriate substrate. C. N-Linked glycoprotein synthesis | Biochemistry_Lippinco. B. O-Linked glycoprotein synthesis Synthesis of the O-linked glycoproteins is very similar to that of the GAG (see p. 158). First, the protein to which sugars are to be attached is synthesized on the RER and extruded into its lumen. Glycosylation begins with the transfer of GalNAc (from UDP-GalNAc) to the hydroxyl group of a specific serine or threonine residues. The glycosyltransferases responsible for the stepwise synthesis (from individual sugars) of the oligosaccharides are bound to the membranes of the Golgi. They act in a specific order, without using a template as is required for DNA, ribonucleic acid (RNA), and protein synthesis (see Unit VII), but instead by recognizing the actual structure of the growing oligosaccharide as the appropriate substrate. C. N-Linked glycoprotein synthesis |
Biochemistry_Lippincott_574 | Biochemistry_Lippinco | C. N-Linked glycoprotein synthesis Synthesis of N-linked glycoproteins occurs in the lumen of the RER and requires the participation of the phosphorylated form of dolichol (dolichol pyrophosphate), a lipid of the RER membrane (Fig. 14.16). The initial product is processed in the RER and Golgi. = terminal group (fucose or Nacetylneuraminic acid); mRNA = messenger RNA; Asn = asparagine. | Biochemistry_Lippinco. C. N-Linked glycoprotein synthesis Synthesis of N-linked glycoproteins occurs in the lumen of the RER and requires the participation of the phosphorylated form of dolichol (dolichol pyrophosphate), a lipid of the RER membrane (Fig. 14.16). The initial product is processed in the RER and Golgi. = terminal group (fucose or Nacetylneuraminic acid); mRNA = messenger RNA; Asn = asparagine. |
Biochemistry_Lippincott_575 | Biochemistry_Lippinco | 1. Dolichol-linked oligosaccharide synthesis: As with the O-linked glycoproteins, the protein is synthesized on the RER and enters its lumen. However, it does not become glycosylated with individual sugars. Instead, a lipid-linked oligosaccharide is first constructed. This consists of dolichol (an RER membrane lipid made from an intermediate of cholesterol synthesis; see p. 221) attached through a pyrophosphate linkage to an oligosaccharide containing GlcNAc, mannose, and glucose. The sugars to be added sequentially to the dolichol by membrane-bound glycosyltransferases are first GlcNAc, followed by mannose and glucose (see Fig. 14.16). The entire 14-sugar oligosaccharide is then transferred from dolichol to the amide nitrogen of an asparagine residue in the protein to be glycosylated by a protein–oligosaccharide transferase present in the RER. [Note: Tunicamycin inhibits N-linked glycosylation.] | Biochemistry_Lippinco. 1. Dolichol-linked oligosaccharide synthesis: As with the O-linked glycoproteins, the protein is synthesized on the RER and enters its lumen. However, it does not become glycosylated with individual sugars. Instead, a lipid-linked oligosaccharide is first constructed. This consists of dolichol (an RER membrane lipid made from an intermediate of cholesterol synthesis; see p. 221) attached through a pyrophosphate linkage to an oligosaccharide containing GlcNAc, mannose, and glucose. The sugars to be added sequentially to the dolichol by membrane-bound glycosyltransferases are first GlcNAc, followed by mannose and glucose (see Fig. 14.16). The entire 14-sugar oligosaccharide is then transferred from dolichol to the amide nitrogen of an asparagine residue in the protein to be glycosylated by a protein–oligosaccharide transferase present in the RER. [Note: Tunicamycin inhibits N-linked glycosylation.] |
Biochemistry_Lippincott_576 | Biochemistry_Lippinco | Congenital disorders of glycosylation (CDG) are syndromes caused primarily by defects in the N-linked glycosylation of proteins, either oligosaccharide assembly (type I) or processing (type II). 2. | Biochemistry_Lippinco. Congenital disorders of glycosylation (CDG) are syndromes caused primarily by defects in the N-linked glycosylation of proteins, either oligosaccharide assembly (type I) or processing (type II). 2. |
Biochemistry_Lippincott_577 | Biochemistry_Lippinco | 2. N-Linked oligosaccharide processing: After addition to the protein, the N-linked oligosaccharide is processed by the removal of specific mannosyl and glucosyl residues as the glycoprotein moves through the RER. Finally, the oligosaccharide chains are completed in the Golgi by addition of a variety of sugars (for example, GlcNAc, GalNAc, and additional mannoses and then fucose or NANA as terminal groups) to produce a complex glycoprotein. Alternatively, they are not processed further, leaving branched, mannose-containing chains in a high-mannose glycoprotein (see Fig. 14.16). The ultimate fate of N-linked glycoproteins is the same as that of the O-linked glycoproteins (for example, they can be released by the cell or become part of a cell membrane). In addition, N-linked glycoproteins can be targeted to the lysosomes. [Note: Nonenzymatic glycosylation of proteins is known as glycation (see p. 33).] 3. | Biochemistry_Lippinco. 2. N-Linked oligosaccharide processing: After addition to the protein, the N-linked oligosaccharide is processed by the removal of specific mannosyl and glucosyl residues as the glycoprotein moves through the RER. Finally, the oligosaccharide chains are completed in the Golgi by addition of a variety of sugars (for example, GlcNAc, GalNAc, and additional mannoses and then fucose or NANA as terminal groups) to produce a complex glycoprotein. Alternatively, they are not processed further, leaving branched, mannose-containing chains in a high-mannose glycoprotein (see Fig. 14.16). The ultimate fate of N-linked glycoproteins is the same as that of the O-linked glycoproteins (for example, they can be released by the cell or become part of a cell membrane). In addition, N-linked glycoproteins can be targeted to the lysosomes. [Note: Nonenzymatic glycosylation of proteins is known as glycation (see p. 33).] 3. |
Biochemistry_Lippincott_578 | Biochemistry_Lippinco | Lysosomal enzymes: N-Linked glycoproteins being processed in the Golgi can be phosphorylated on carbon 6 of one or more mannosyl residues. UDP-GlcNAc provides the phosphate in a reaction catalyzed by a phosphotransferase. Receptors, located in the Golgi membrane, bind the mannose 6-phosphate residues of these proteins, which are then packaged into vesicles and sent to the lysosomes (Fig. 14.17). I-Cell disease is a rare lysosomal storage disease in which the phosphotransferase is deficient. This causes the proteins to be secreted, rather than being targeted to lysosomes. Consequently, the acid hydrolases normally found in the lysosomal matrix are absent, resulting in an accumulation of the substrates for these missing enzymes. [Note: I-Cell disease is so named because of the large inclusion bodies seen in cells of patients with this disease.] In addition, high amounts of lysosomal enzymes are found in the patient’s plasma and urine, indicating that the targeting process to lysosomes | Biochemistry_Lippinco. Lysosomal enzymes: N-Linked glycoproteins being processed in the Golgi can be phosphorylated on carbon 6 of one or more mannosyl residues. UDP-GlcNAc provides the phosphate in a reaction catalyzed by a phosphotransferase. Receptors, located in the Golgi membrane, bind the mannose 6-phosphate residues of these proteins, which are then packaged into vesicles and sent to the lysosomes (Fig. 14.17). I-Cell disease is a rare lysosomal storage disease in which the phosphotransferase is deficient. This causes the proteins to be secreted, rather than being targeted to lysosomes. Consequently, the acid hydrolases normally found in the lysosomal matrix are absent, resulting in an accumulation of the substrates for these missing enzymes. [Note: I-Cell disease is so named because of the large inclusion bodies seen in cells of patients with this disease.] In addition, high amounts of lysosomal enzymes are found in the patient’s plasma and urine, indicating that the targeting process to lysosomes |
Biochemistry_Lippincott_579 | Biochemistry_Lippinco | bodies seen in cells of patients with this disease.] In addition, high amounts of lysosomal enzymes are found in the patient’s plasma and urine, indicating that the targeting process to lysosomes (rather than the synthetic pathway of these enzymes) is deficient. I-Cell disease is characterized by skeletal abnormalities, restricted joint movement, coarse (dysmorphic) facial features, and severe psychomotor impairment. [Note: | Biochemistry_Lippinco. bodies seen in cells of patients with this disease.] In addition, high amounts of lysosomal enzymes are found in the patient’s plasma and urine, indicating that the targeting process to lysosomes (rather than the synthetic pathway of these enzymes) is deficient. I-Cell disease is characterized by skeletal abnormalities, restricted joint movement, coarse (dysmorphic) facial features, and severe psychomotor impairment. [Note: |
Biochemistry_Lippincott_580 | Biochemistry_Lippinco | Because I-cell disease has features in common with the mucopolysaccharidoses and sphingolipidoses (see p. 210), it is termed a mucolipidosis (type II).] Currently, there is no cure, and death from cardiopulmonary complications usually occurs in early childhood. IX. LYSOSOMAL GLYCOPROTEIN DEGRADATION | Biochemistry_Lippinco. Because I-cell disease has features in common with the mucopolysaccharidoses and sphingolipidoses (see p. 210), it is termed a mucolipidosis (type II).] Currently, there is no cure, and death from cardiopulmonary complications usually occurs in early childhood. IX. LYSOSOMAL GLYCOPROTEIN DEGRADATION |
Biochemistry_Lippincott_581 | Biochemistry_Lippinco | IX. LYSOSOMAL GLYCOPROTEIN DEGRADATION Degradation of glycoproteins is similar to that of the GAG (see p. 163). The lysosomal acid hydrolases are each generally specific for the removal of one component of the glycoprotein. They are primarily exoenzymes that remove their respective groups in the reverse order of their incorporation (last on, first off). If any one degradative enzyme is missing, degradation by the other exoenzymes cannot continue. A group of very rare autosomal-recessive diseases called the glycoprotein storage diseases (oligosaccharidoses), caused by a deficiency of any one of the degradative enzymes, results in accumulation of partially degraded structures in the lysosomes. For example, α-mannosidosis type 3 is a severe, progressive, fatal deficiency of the enzyme α-mannosidase. Presentation is similar to Hurler syndrome, but immune deficiency is also seen. Mannose-rich oligosaccharide fragments appear in the urine. Diagnosis is by enzyme assay. X. CHAPTER SUMMARY | Biochemistry_Lippinco. IX. LYSOSOMAL GLYCOPROTEIN DEGRADATION Degradation of glycoproteins is similar to that of the GAG (see p. 163). The lysosomal acid hydrolases are each generally specific for the removal of one component of the glycoprotein. They are primarily exoenzymes that remove their respective groups in the reverse order of their incorporation (last on, first off). If any one degradative enzyme is missing, degradation by the other exoenzymes cannot continue. A group of very rare autosomal-recessive diseases called the glycoprotein storage diseases (oligosaccharidoses), caused by a deficiency of any one of the degradative enzymes, results in accumulation of partially degraded structures in the lysosomes. For example, α-mannosidosis type 3 is a severe, progressive, fatal deficiency of the enzyme α-mannosidase. Presentation is similar to Hurler syndrome, but immune deficiency is also seen. Mannose-rich oligosaccharide fragments appear in the urine. Diagnosis is by enzyme assay. X. CHAPTER SUMMARY |
Biochemistry_Lippincott_582 | Biochemistry_Lippinco | Glycosaminoglycans (GAG) are long, negatively charged, unbranched, heteropolysaccharide chains generally composed of a repeating disaccharide unit [acidic sugar–amino sugar]n (Fig. 14.18). The amino sugar is either D-glucosamine or D-galactosamine in which the amino group is usually acetylated, thus eliminating its positive charge. The amino sugar may also be sulfated on carbon 4 or 6 or on a nonacetylated nitrogen. The acidic sugar is either D-glucuronic acid or its C-5 epimer L-iduronic acid. GAG bind large amounts of water, thereby producing the gel-like matrix that forms the basis of the body’s ground substance. The viscous, lubricating properties of mucous secretions are also caused by the presence of GAG, which led to the original naming of these compounds as mucopolysaccharides. There are six major types of GAG, including chondroitin 4-and 6-sulfates, keratan sulfate, dermatan sulfate, heparin, heparan sulfate, and hyaluronic acid. All GAG, except hyaluronic acid, are found | Biochemistry_Lippinco. Glycosaminoglycans (GAG) are long, negatively charged, unbranched, heteropolysaccharide chains generally composed of a repeating disaccharide unit [acidic sugar–amino sugar]n (Fig. 14.18). The amino sugar is either D-glucosamine or D-galactosamine in which the amino group is usually acetylated, thus eliminating its positive charge. The amino sugar may also be sulfated on carbon 4 or 6 or on a nonacetylated nitrogen. The acidic sugar is either D-glucuronic acid or its C-5 epimer L-iduronic acid. GAG bind large amounts of water, thereby producing the gel-like matrix that forms the basis of the body’s ground substance. The viscous, lubricating properties of mucous secretions are also caused by the presence of GAG, which led to the original naming of these compounds as mucopolysaccharides. There are six major types of GAG, including chondroitin 4-and 6-sulfates, keratan sulfate, dermatan sulfate, heparin, heparan sulfate, and hyaluronic acid. All GAG, except hyaluronic acid, are found |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.