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Cell_Biology_Alberts_310	Cell_Biology_Alberts	2. A–H + B–O–PO3 →A–B + Pi Net result: B–OH + ATP + A–H →A–B + ADP + Pi The condensation reaction, which by itself is energetically unfavorable, is forced to occur by being directly coupled to ATP hydrolysis in an enzyme-catalyzed reaction pathway (Figure 2–35A). A biosynthetic reaction of exactly this type synthesizes the amino acid glutamine (Figure 2–35B). We will see shortly that similar (but more complex) mechanisms are also used to produce nearly all of the large molecules of the cell.	Cell_Biology_Alberts. 2. A–H + B–O–PO3 →A–B + Pi Net result: B–OH + ATP + A–H →A–B + ADP + Pi The condensation reaction, which by itself is energetically unfavorable, is forced to occur by being directly coupled to ATP hydrolysis in an enzyme-catalyzed reaction pathway (Figure 2–35A). A biosynthetic reaction of exactly this type synthesizes the amino acid glutamine (Figure 2–35B). We will see shortly that similar (but more complex) mechanisms are also used to produce nearly all of the large molecules of the cell.
Cell_Biology_Alberts_311	Cell_Biology_Alberts	Figure 2–35 an example of an energetically unfavorable biosynthetic reaction driven by aTP hydrolysis. (a) schematic illustration of the formation of a–B in the condensation reaction described in the text. (B) The biosynthesis of the common amino acid glutamine from glutamic acid and ammonia. Glutamic acid is first converted to a high-energy phosphorylated intermediate (corresponding to the compound B–o–po3 described in the text), which then reacts with ammonia (corresponding to a–h) to form glutamine. in this example, both steps occur on the surface of the same enzyme, glutamine synthetase. The high-energy bonds are shaded red; here, as elsewhere throughout the book, the symbol pi = hpo42–, and a yellow “circled p” = po3 . naDh and naDph are important electron Carriers	Cell_Biology_Alberts. Figure 2–35 an example of an energetically unfavorable biosynthetic reaction driven by aTP hydrolysis. (a) schematic illustration of the formation of a–B in the condensation reaction described in the text. (B) The biosynthesis of the common amino acid glutamine from glutamic acid and ammonia. Glutamic acid is first converted to a high-energy phosphorylated intermediate (corresponding to the compound B–o–po3 described in the text), which then reacts with ammonia (corresponding to a–h) to form glutamine. in this example, both steps occur on the surface of the same enzyme, glutamine synthetase. The high-energy bonds are shaded red; here, as elsewhere throughout the book, the symbol pi = hpo42–, and a yellow “circled p” = po3 . naDh and naDph are important electron Carriers
Cell_Biology_Alberts_312	Cell_Biology_Alberts	naDh and naDph are important electron Carriers Other important activated carrier molecules participate in oxidation–reduction reactions and are commonly part of coupled reactions in cells. These activated carriers are specialized to carry electrons held at a high energy level (sometimes called “high-energy” electrons) and hydrogen atoms. The most important of these electron carriers are NAD+ (nicotinamide adenine dinucleotide) and the closely related molecule NADP+ (nicotinamide adenine dinucleotide phosphate). Each picks up a “packet of energy” corresponding to two electrons plus a proton (H+), and they are thereby converted to NADH (reduced nicotinamide adenine dinucleotide) and NADPH (reduced nicotinamide adenine dinucleotide phosphate), respectively (Figure 2–36). These molecules can therefore be regarded as carriers of hydride ions (the H+ plus two electrons, or H–).	Cell_Biology_Alberts. naDh and naDph are important electron Carriers Other important activated carrier molecules participate in oxidation–reduction reactions and are commonly part of coupled reactions in cells. These activated carriers are specialized to carry electrons held at a high energy level (sometimes called “high-energy” electrons) and hydrogen atoms. The most important of these electron carriers are NAD+ (nicotinamide adenine dinucleotide) and the closely related molecule NADP+ (nicotinamide adenine dinucleotide phosphate). Each picks up a “packet of energy” corresponding to two electrons plus a proton (H+), and they are thereby converted to NADH (reduced nicotinamide adenine dinucleotide) and NADPH (reduced nicotinamide adenine dinucleotide phosphate), respectively (Figure 2–36). These molecules can therefore be regarded as carriers of hydride ions (the H+ plus two electrons, or H–).
Cell_Biology_Alberts_313	Cell_Biology_Alberts	Like ATP, NADPH is an activated carrier that participates in many important biosynthetic reactions that would otherwise be energetically unfavorable. The NADPH is produced according to the general scheme shown in Figure 2–36A. During a special set of energy-yielding catabolic reactions, two hydrogen atoms are removed from a substrate molecule. Both electrons but just one proton (that is, a hydride ion, H–) are added to the nicotinamide ring of NADP+ to form NADPH; the second proton (H+) is released into solution. This is a typical oxidation–reduction reaction, in which the substrate is oxidized and NADP+ is reduced. NADPH readily gives up the hydride ion it carries in a subsequent oxidation–reduction reaction, because the nicotinamide ring can achieve a more stable arrangement of electrons without it. In this subsequent reaction, which Figure 2–36 NaDPH, an important carrier of electrons.	Cell_Biology_Alberts. Like ATP, NADPH is an activated carrier that participates in many important biosynthetic reactions that would otherwise be energetically unfavorable. The NADPH is produced according to the general scheme shown in Figure 2–36A. During a special set of energy-yielding catabolic reactions, two hydrogen atoms are removed from a substrate molecule. Both electrons but just one proton (that is, a hydride ion, H–) are added to the nicotinamide ring of NADP+ to form NADPH; the second proton (H+) is released into solution. This is a typical oxidation–reduction reaction, in which the substrate is oxidized and NADP+ is reduced. NADPH readily gives up the hydride ion it carries in a subsequent oxidation–reduction reaction, because the nicotinamide ring can achieve a more stable arrangement of electrons without it. In this subsequent reaction, which Figure 2–36 NaDPH, an important carrier of electrons.
Cell_Biology_Alberts_314	Cell_Biology_Alberts	Figure 2–36 NaDPH, an important carrier of electrons. HC (a) naDph is produced in reactions of the general type shown on the left, in which two hydrogen atoms are removed from a substrate. The oxidized form of the carrier molecule, naDp+, receives one hydrogen atom plus an electron (a hydride ion); the proton (h+) from the other h atom is released into solution. Because naDph holds its hydride ion in a high-energy linkage, the hydride ion can easily be transferred to other molecules, as shown on the right. (B) and (C) The structures of naDp+ and naDph. The part of the naDp+ molecule known as the nicotinamide ring accepts the hydride ion, h–, forming naDph. The molecules naD+ and naDh are identical in structure to naDp+ and naDph, respectively, except that they lack the indicated phosphate group.	Cell_Biology_Alberts. Figure 2–36 NaDPH, an important carrier of electrons. HC (a) naDph is produced in reactions of the general type shown on the left, in which two hydrogen atoms are removed from a substrate. The oxidized form of the carrier molecule, naDp+, receives one hydrogen atom plus an electron (a hydride ion); the proton (h+) from the other h atom is released into solution. Because naDph holds its hydride ion in a high-energy linkage, the hydride ion can easily be transferred to other molecules, as shown on the right. (B) and (C) The structures of naDp+ and naDph. The part of the naDp+ molecule known as the nicotinamide ring accepts the hydride ion, h–, forming naDph. The molecules naD+ and naDh are identical in structure to naDp+ and naDph, respectively, except that they lack the indicated phosphate group.
Cell_Biology_Alberts_315	Cell_Biology_Alberts	this phosphate group is missing in NAD+ and NADH regenerates NADP+, it is the NADPH that is oxidized and the substrate that is reduced. The NADPH is an effective donor of its hydride ion to other molecules for the same reason that ATP readily transfers a phosphate: in both cases the transfer is accompanied by a large negative free-energy change. One example of the use of NADPH in biosynthesis is shown in Figure 2–37. The extra phosphate group on NADPH has no effect on the electron-transfer properties of NADPH compared with NADH, being far away from the region involved in electron transfer (see Figure 2–36C). It does, however, give a molecule of NADPH a slightly different shape from that of NADH, making it possible for NADPH and NADH to bind as substrates to completely different sets of enzymes. Thus, the two types of carriers are used to transfer electrons (or hydride ions) between two different sets of molecules.	Cell_Biology_Alberts. this phosphate group is missing in NAD+ and NADH regenerates NADP+, it is the NADPH that is oxidized and the substrate that is reduced. The NADPH is an effective donor of its hydride ion to other molecules for the same reason that ATP readily transfers a phosphate: in both cases the transfer is accompanied by a large negative free-energy change. One example of the use of NADPH in biosynthesis is shown in Figure 2–37. The extra phosphate group on NADPH has no effect on the electron-transfer properties of NADPH compared with NADH, being far away from the region involved in electron transfer (see Figure 2–36C). It does, however, give a molecule of NADPH a slightly different shape from that of NADH, making it possible for NADPH and NADH to bind as substrates to completely different sets of enzymes. Thus, the two types of carriers are used to transfer electrons (or hydride ions) between two different sets of molecules.
Cell_Biology_Alberts_316	Cell_Biology_Alberts	Why should there be this division of labor? The answer lies in the need to regulate two sets of electron-transfer reactions independently. NADPH operates chiefly with enzymes that catalyze anabolic reactions, supplying the high-energy electrons needed to synthesize energy-rich biological molecules. NADH, by contrast, has a special role as an intermediate in the catabolic system of reactions that generate ATP through the oxidation of food molecules, as we will discuss shortly. The genesis of NADH from NAD+, and of NADPH from NADP+, occur by different pathways and are independently regulated, so that the cell can adjust the supply of electrons for these two contrasting purposes. Inside the cell the ratio of NAD+ to NADH is kept high, whereas the ratio of NADP+ to NADPH is kept low. This provides plenty of NAD+ to act as an oxidizing agent and plenty of NADPH to act as a reducing agent (Figure 2–37B)—as required for their special roles in catabolism and anabolism, respectively.	Cell_Biology_Alberts. Why should there be this division of labor? The answer lies in the need to regulate two sets of electron-transfer reactions independently. NADPH operates chiefly with enzymes that catalyze anabolic reactions, supplying the high-energy electrons needed to synthesize energy-rich biological molecules. NADH, by contrast, has a special role as an intermediate in the catabolic system of reactions that generate ATP through the oxidation of food molecules, as we will discuss shortly. The genesis of NADH from NAD+, and of NADPH from NADP+, occur by different pathways and are independently regulated, so that the cell can adjust the supply of electrons for these two contrasting purposes. Inside the cell the ratio of NAD+ to NADH is kept high, whereas the ratio of NADP+ to NADPH is kept low. This provides plenty of NAD+ to act as an oxidizing agent and plenty of NADPH to act as a reducing agent (Figure 2–37B)—as required for their special roles in catabolism and anabolism, respectively.
Cell_Biology_Alberts_317	Cell_Biology_Alberts	There are many other activated Carrier molecules in Cells Other activated carriers also pick up and carry a chemical group in an easily transferred, high-energy linkage. For example, coenzyme A carries a readily transferable Figure 2–37 NaDPH as a reducing agent. (a) The final stage in the biosynthetic route leading to cholesterol. as in many other biosynthetic reactions, the reduction of the C=C bond is achieved by the transfer of a hydride ion from the carrier molecule naDph, plus a proton (h+) from the solution. (B) Keeping naDph levels high and naDh levels low alters their affinities for electrons (see panel 14–1, p. 765). This causes naDph to be a much stronger electron donor (reducing agent) than naDh, and naD+ therefore to be a much better electron acceptor (oxidizing agent) than naDp+, as indicated.	Cell_Biology_Alberts. There are many other activated Carrier molecules in Cells Other activated carriers also pick up and carry a chemical group in an easily transferred, high-energy linkage. For example, coenzyme A carries a readily transferable Figure 2–37 NaDPH as a reducing agent. (a) The final stage in the biosynthetic route leading to cholesterol. as in many other biosynthetic reactions, the reduction of the C=C bond is achieved by the transfer of a hydride ion from the carrier molecule naDph, plus a proton (h+) from the solution. (B) Keeping naDph levels high and naDh levels low alters their affinities for electrons (see panel 14–1, p. 765). This causes naDph to be a much stronger electron donor (reducing agent) than naDh, and naD+ therefore to be a much better electron acceptor (oxidizing agent) than naDp+, as indicated.
Cell_Biology_Alberts_318	Cell_Biology_Alberts	acetyl group in a thioester linkage, and in this activated form is known as acetyl CoA (acetyl coenzyme A). Acetyl CoA (Figure 2–38) is used to add two carbon units in the biosynthesis of larger molecules.	Cell_Biology_Alberts. acetyl group in a thioester linkage, and in this activated form is known as acetyl CoA (acetyl coenzyme A). Acetyl CoA (Figure 2–38) is used to add two carbon units in the biosynthesis of larger molecules.
Cell_Biology_Alberts_319	Cell_Biology_Alberts	In acetyl CoA, as in other carrier molecules, the transferable group makes up only a small part of the molecule. The rest consists of a large organic portion that serves as a convenient “handle,” facilitating the recognition of the carrier molecule by specific enzymes. As with acetyl CoA, this handle portion very often contains a nucleotide (usually adenosine), a curious fact that may be a relic from an early stage of evolution. It is currently thought that the main catalysts for early life-forms—before DNA or proteins—were RNA molecules (or their close relatives), as described in Chapter 6. It is tempting to speculate that many of the carrier molecules that we find today originated in this earlier RNA world, where their nucleotide portions could have been useful for binding them to RNA enzymes (ribozymes).	Cell_Biology_Alberts. In acetyl CoA, as in other carrier molecules, the transferable group makes up only a small part of the molecule. The rest consists of a large organic portion that serves as a convenient “handle,” facilitating the recognition of the carrier molecule by specific enzymes. As with acetyl CoA, this handle portion very often contains a nucleotide (usually adenosine), a curious fact that may be a relic from an early stage of evolution. It is currently thought that the main catalysts for early life-forms—before DNA or proteins—were RNA molecules (or their close relatives), as described in Chapter 6. It is tempting to speculate that many of the carrier molecules that we find today originated in this earlier RNA world, where their nucleotide portions could have been useful for binding them to RNA enzymes (ribozymes).
Cell_Biology_Alberts_320	Cell_Biology_Alberts	Thus, ATP transfers phosphate, NADPH transfers electrons and hydrogen, and acetyl CoA transfers two-carbon acetyl groups. FADH2 (reduced flavin adenine dinucleotide) is used like NADH in electron and proton transfers (Figure 2–39). The reactions of other activated carrier molecules involve the transfer of a methyl, carboxyl, or glucose group for biosyntheses (Table 2–3). These activated carriers Figure 2–38 The structure of the important activated carrier molecule acetyl Coa. a ball-and-stick model is shown above the structure. The sulfur atom (yellow) forms a thioester bond to acetate. Because this is a high-energy linkage, releasing a large amount of free energy when it is hydrolyzed, the acetate molecule can be readily transferred to other molecules. Figure 2–39 FaDH2 is a carrier of hydrogens and high-energy electrons, like NaDH and NaDPH. (a) structure of faDh2, with its hydrogen-carrying atoms highlighted in yellow. (B) The formation of faDh2 from faD.	Cell_Biology_Alberts. Thus, ATP transfers phosphate, NADPH transfers electrons and hydrogen, and acetyl CoA transfers two-carbon acetyl groups. FADH2 (reduced flavin adenine dinucleotide) is used like NADH in electron and proton transfers (Figure 2–39). The reactions of other activated carrier molecules involve the transfer of a methyl, carboxyl, or glucose group for biosyntheses (Table 2–3). These activated carriers Figure 2–38 The structure of the important activated carrier molecule acetyl Coa. a ball-and-stick model is shown above the structure. The sulfur atom (yellow) forms a thioester bond to acetate. Because this is a high-energy linkage, releasing a large amount of free energy when it is hydrolyzed, the acetate molecule can be readily transferred to other molecules. Figure 2–39 FaDH2 is a carrier of hydrogens and high-energy electrons, like NaDH and NaDPH. (a) structure of faDh2, with its hydrogen-carrying atoms highlighted in yellow. (B) The formation of faDh2 from faD.
Cell_Biology_Alberts_321	Cell_Biology_Alberts	are generated in reactions that are coupled to ATP hydrolysis, as in the example in Figure 2–40. Therefore, the energy that enables their groups to be used for biosynthesis ultimately comes from the catabolic reactions that generate ATP. Similar processes occur in the synthesis of the very large molecules of the cell—the nucleic acids, proteins, and polysaccharides—that we discuss next. The synthesis of Biological polymers is Driven by aTp hydrolysis	Cell_Biology_Alberts. are generated in reactions that are coupled to ATP hydrolysis, as in the example in Figure 2–40. Therefore, the energy that enables their groups to be used for biosynthesis ultimately comes from the catabolic reactions that generate ATP. Similar processes occur in the synthesis of the very large molecules of the cell—the nucleic acids, proteins, and polysaccharides—that we discuss next. The synthesis of Biological polymers is Driven by aTp hydrolysis
Cell_Biology_Alberts_322	Cell_Biology_Alberts	The synthesis of Biological polymers is Driven by aTp hydrolysis As discussed previously, the macromolecules of the cell constitute most of its dry mass (see Figure 2–7). These molecules are made from subunits (or monomers) that are linked together in a condensation reaction, in which the constituents of a water molecule (OH plus H) are removed from the two reactants. Consequently, the reverse reaction—the breakdown of all three types of polymers—occurs by the enzyme-catalyzed addition of water (hydrolysis). This hydrolysis reaction is energetically favorable, whereas the biosynthetic reactions require an energy input (see Figure 2–9).	Cell_Biology_Alberts. The synthesis of Biological polymers is Driven by aTp hydrolysis As discussed previously, the macromolecules of the cell constitute most of its dry mass (see Figure 2–7). These molecules are made from subunits (or monomers) that are linked together in a condensation reaction, in which the constituents of a water molecule (OH plus H) are removed from the two reactants. Consequently, the reverse reaction—the breakdown of all three types of polymers—occurs by the enzyme-catalyzed addition of water (hydrolysis). This hydrolysis reaction is energetically favorable, whereas the biosynthetic reactions require an energy input (see Figure 2–9).
Cell_Biology_Alberts_323	Cell_Biology_Alberts	The nucleic acids (DNA and RNA), proteins, and polysaccharides are all polymers that are produced by the repeated addition of a monomer onto one end of a growing chain. The synthesis reactions for these three types of macromolecules are outlined in Figure 2–41. As indicated, the condensation step in each case depends on energy from nucleoside triphosphate hydrolysis. And yet, except for the nucleic acids, there are no phosphate groups left in the final product molecules. How are the reactions that release the energy of ATP hydrolysis coupled to polymer synthesis? For each type of macromolecule, an enzyme-catalyzed pathway exists which resembles that discussed previously for the synthesis of the amino acid glutamine (see Figure 2–35). The principle is exactly the same, in that the –OH group that will	Cell_Biology_Alberts. The nucleic acids (DNA and RNA), proteins, and polysaccharides are all polymers that are produced by the repeated addition of a monomer onto one end of a growing chain. The synthesis reactions for these three types of macromolecules are outlined in Figure 2–41. As indicated, the condensation step in each case depends on energy from nucleoside triphosphate hydrolysis. And yet, except for the nucleic acids, there are no phosphate groups left in the final product molecules. How are the reactions that release the energy of ATP hydrolysis coupled to polymer synthesis? For each type of macromolecule, an enzyme-catalyzed pathway exists which resembles that discussed previously for the synthesis of the amino acid glutamine (see Figure 2–35). The principle is exactly the same, in that the –OH group that will
Cell_Biology_Alberts_324	Cell_Biology_Alberts	Figure 2–40 a carboxyl group-transfer reaction using an activated carrier molecule. Carboxylated biotin is used by the enzyme pyruvate carboxylase to transfer a carboxyl group in the production of oxaloacetate, a molecule needed for the citric acid cycle. The acceptor molecule for this group-transfer reaction is pyruvate. other enzymes use biotin, a B-complex vitamin, to transfer carboxyl groups to other acceptor molecules. note that synthesis of carboxylated biotin requires energy that is derived from aTp—a general feature of many activated carriers.	Cell_Biology_Alberts. Figure 2–40 a carboxyl group-transfer reaction using an activated carrier molecule. Carboxylated biotin is used by the enzyme pyruvate carboxylase to transfer a carboxyl group in the production of oxaloacetate, a molecule needed for the citric acid cycle. The acceptor molecule for this group-transfer reaction is pyruvate. other enzymes use biotin, a B-complex vitamin, to transfer carboxyl groups to other acceptor molecules. note that synthesis of carboxylated biotin requires energy that is derived from aTp—a general feature of many activated carriers.
Cell_Biology_Alberts_325	Cell_Biology_Alberts	Figure 2–41 The synthesis of polysaccharides, proteins, and nucleic acids. synthesis of each kind of biological polymer involves the loss of water in a condensation reaction. not shown is the consumption of high-energy nucleoside triphosphates that is required to activate each monomer before its addition. in contrast, the reverse reaction—the breakdown CH2OHOHOOHOHCH2OHOHOOHOHCH2OHOOHOHOOCH2OHOOHOHCH2OHOOHOHOOCH2OHOOHOHOHOOH(A)POLYSACCHARIDESglucoseglycogenglycogenH2O energy from nucleoside triphosphate hydrolysis HCROCNHHCRCOOHHHRHCCNOOHHCROCNHHCRCRHCCNOOHHOprotein amino acid (C) PROTEINS A OCH2OHOOOO_POC OCH2OHOHOHOPOG OOHOHCH2O_A OCH2OHOOOO_POC OCH2OHOOPOG OOHOHCH2O_H2O (B) NUCLEIC ACIDS RNA nucleotide H2O energy from nucleoside triphosphate hydrolysis energy from nucleoside triphosphate hydrolysis RNA protein of all three types of polymers—occurs by the simple addition of water (hydrolysis).	Cell_Biology_Alberts. Figure 2–41 The synthesis of polysaccharides, proteins, and nucleic acids. synthesis of each kind of biological polymer involves the loss of water in a condensation reaction. not shown is the consumption of high-energy nucleoside triphosphates that is required to activate each monomer before its addition. in contrast, the reverse reaction—the breakdown CH2OHOHOOHOHCH2OHOHOOHOHCH2OHOOHOHOOCH2OHOOHOHCH2OHOOHOHOOCH2OHOOHOHOHOOH(A)POLYSACCHARIDESglucoseglycogenglycogenH2O energy from nucleoside triphosphate hydrolysis HCROCNHHCRCOOHHHRHCCNOOHHCROCNHHCRCRHCCNOOHHOprotein amino acid (C) PROTEINS A OCH2OHOOOO_POC OCH2OHOHOHOPOG OOHOHCH2O_A OCH2OHOOOO_POC OCH2OHOOPOG OOHOHCH2O_H2O (B) NUCLEIC ACIDS RNA nucleotide H2O energy from nucleoside triphosphate hydrolysis energy from nucleoside triphosphate hydrolysis RNA protein of all three types of polymers—occurs by the simple addition of water (hydrolysis).
Cell_Biology_Alberts_326	Cell_Biology_Alberts	be removed in the condensation reaction is first activated by becoming involved in a high-energy linkage to a second molecule. However, the actual mechanisms used to link ATP hydrolysis to the synthesis of proteins and polysaccharides are more complex than that used for glutamine synthesis, since a series of high-energy intermediates is required to generate the final high-energy bond that is broken during the condensation step (discussed in Chapter 6 for protein synthesis).	Cell_Biology_Alberts. be removed in the condensation reaction is first activated by becoming involved in a high-energy linkage to a second molecule. However, the actual mechanisms used to link ATP hydrolysis to the synthesis of proteins and polysaccharides are more complex than that used for glutamine synthesis, since a series of high-energy intermediates is required to generate the final high-energy bond that is broken during the condensation step (discussed in Chapter 6 for protein synthesis).
Cell_Biology_Alberts_327	Cell_Biology_Alberts	Each activated carrier has limits in its ability to drive a biosynthetic reaction. The ∆G for the hydrolysis of ATP to ADP and inorganic phosphate (Pi) depends on the concentrations of all of the reactants, but under the usual conditions in a cell it is between –46 and –54 kJ/mole. In principle, this hydrolysis reaction could drive an unfavorable reaction with a ∆G of, perhaps, +40 kJ/mole, provided that a suitable reaction path is available. For some biosynthetic reactions, however, even –50 kJ/mole does not provide enough of a driving force. In these cases, the path of ATP hydrolysis can be altered so that it initially produces AMP and pyrophosphate (PPi), which is itself then hydrolyzed in a subsequent step (Figure 2–42). The whole process makes available a total free-energy change of about –100 kJ/ mole. An important type of biosynthetic reaction that is driven in this way is the synthesis of nucleic acids (polynucleotides) from nucleoside triphosphates, as illustrated on the	Cell_Biology_Alberts. Each activated carrier has limits in its ability to drive a biosynthetic reaction. The ∆G for the hydrolysis of ATP to ADP and inorganic phosphate (Pi) depends on the concentrations of all of the reactants, but under the usual conditions in a cell it is between –46 and –54 kJ/mole. In principle, this hydrolysis reaction could drive an unfavorable reaction with a ∆G of, perhaps, +40 kJ/mole, provided that a suitable reaction path is available. For some biosynthetic reactions, however, even –50 kJ/mole does not provide enough of a driving force. In these cases, the path of ATP hydrolysis can be altered so that it initially produces AMP and pyrophosphate (PPi), which is itself then hydrolyzed in a subsequent step (Figure 2–42). The whole process makes available a total free-energy change of about –100 kJ/ mole. An important type of biosynthetic reaction that is driven in this way is the synthesis of nucleic acids (polynucleotides) from nucleoside triphosphates, as illustrated on the
Cell_Biology_Alberts_328	Cell_Biology_Alberts	of about –100 kJ/ mole. An important type of biosynthetic reaction that is driven in this way is the synthesis of nucleic acids (polynucleotides) from nucleoside triphosphates, as illustrated on the right side of Figure 2–43.	Cell_Biology_Alberts. of about –100 kJ/ mole. An important type of biosynthetic reaction that is driven in this way is the synthesis of nucleic acids (polynucleotides) from nucleoside triphosphates, as illustrated on the right side of Figure 2–43.
Cell_Biology_Alberts_329	Cell_Biology_Alberts	Note that the repetitive condensation reactions that produce macromolecules can be oriented in one of two ways, giving rise to either the head polymerization or the tail polymerization of monomers. In so-called head polymerization, the reactive bond required for the condensation reaction is carried on the end of the Figure 2–43 Synthesis of a polynucleotide, RNa or DNa, is a multistep process driven by aTP hydrolysis. in the first step, a nucleoside monophosphate is activated by the sequential transfer of the terminal phosphate groups from two aTp molecules. The high-energy intermediate formed—a nucleoside triphosphate—exists free in solution until it reacts with the growing end of an Rna or a Dna chain with release of pyrophosphate. hydrolysis of the latter to inorganic phosphate is highly favorable and helps to drive the overall reaction in the direction of polynucleotide synthesis. for details, see Chapter 5.	Cell_Biology_Alberts. Note that the repetitive condensation reactions that produce macromolecules can be oriented in one of two ways, giving rise to either the head polymerization or the tail polymerization of monomers. In so-called head polymerization, the reactive bond required for the condensation reaction is carried on the end of the Figure 2–43 Synthesis of a polynucleotide, RNa or DNa, is a multistep process driven by aTP hydrolysis. in the first step, a nucleoside monophosphate is activated by the sequential transfer of the terminal phosphate groups from two aTp molecules. The high-energy intermediate formed—a nucleoside triphosphate—exists free in solution until it reacts with the growing end of an Rna or a Dna chain with release of pyrophosphate. hydrolysis of the latter to inorganic phosphate is highly favorable and helps to drive the overall reaction in the direction of polynucleotide synthesis. for details, see Chapter 5.
Cell_Biology_Alberts_330	Cell_Biology_Alberts	Figure 2–42 an alternative pathway of aTP hydrolysis, in which pyrophosphate is first formed and then hydrolyzed. This route releases about twice as much free energy (approximately –100 kJ/mole) as the reaction shown earlier in figure 2–33, and it forms amp instead of aDp. (a) in the two successive hydrolysis reactions, oxygen atoms from the participating water molecules are retained in the products, as indicated, whereas the hydrogen atoms dissociate to form free hydrogen ions (h+, not shown). (B) summary of overall reaction. HEAD POLYMERIZATION (e.g., PROTEINS, FATTY ACIDS) TAIL POLYMERIZATION (e.g., DNA, RNA, POLYSACCHARIDES) growing polymer, and it must therefore be regenerated each time that a monomer is added. In this case, each monomer brings with it the reactive bond that will be used in adding the next monomer in the series. In tail polymerization, the reactive bond carried by each monomer is instead used immediately for its own addition (Figure 2–44).	Cell_Biology_Alberts. Figure 2–42 an alternative pathway of aTP hydrolysis, in which pyrophosphate is first formed and then hydrolyzed. This route releases about twice as much free energy (approximately –100 kJ/mole) as the reaction shown earlier in figure 2–33, and it forms amp instead of aDp. (a) in the two successive hydrolysis reactions, oxygen atoms from the participating water molecules are retained in the products, as indicated, whereas the hydrogen atoms dissociate to form free hydrogen ions (h+, not shown). (B) summary of overall reaction. HEAD POLYMERIZATION (e.g., PROTEINS, FATTY ACIDS) TAIL POLYMERIZATION (e.g., DNA, RNA, POLYSACCHARIDES) growing polymer, and it must therefore be regenerated each time that a monomer is added. In this case, each monomer brings with it the reactive bond that will be used in adding the next monomer in the series. In tail polymerization, the reactive bond carried by each monomer is instead used immediately for its own addition (Figure 2–44).
Cell_Biology_Alberts_331	Cell_Biology_Alberts	We shall see in later chapters that both of these types of polymerization are used. The synthesis of polynucleotides and some simple polysaccharides occurs by tail polymerization, for example, whereas the synthesis of proteins occurs by a head polymerization process.	Cell_Biology_Alberts. We shall see in later chapters that both of these types of polymerization are used. The synthesis of polynucleotides and some simple polysaccharides occurs by tail polymerization, for example, whereas the synthesis of proteins occurs by a head polymerization process.
Cell_Biology_Alberts_332	Cell_Biology_Alberts	Living cells need to create and maintain order within themselves to survive and grow. This is thermodynamically possible only because of a continual input of energy, part of which must be released from the cells to their environment as heat that disorders the surroundings. The only chemical reactions possible are those that increase the total amount of disorder in the universe. The free-energy change for a reaction, ∆G, measures this disorder, and it must be less than zero for a reaction to proceed spontaneously. This ∆G depends both on the intrinsic properties of the reacting molecules and their concentrations, and it can be calculated from these concentrations if either the equilibrium constant (K) for the reaction or its standard free-energy change, ∆G°, is known.	Cell_Biology_Alberts. Living cells need to create and maintain order within themselves to survive and grow. This is thermodynamically possible only because of a continual input of energy, part of which must be released from the cells to their environment as heat that disorders the surroundings. The only chemical reactions possible are those that increase the total amount of disorder in the universe. The free-energy change for a reaction, ∆G, measures this disorder, and it must be less than zero for a reaction to proceed spontaneously. This ∆G depends both on the intrinsic properties of the reacting molecules and their concentrations, and it can be calculated from these concentrations if either the equilibrium constant (K) for the reaction or its standard free-energy change, ∆G°, is known.
Cell_Biology_Alberts_333	Cell_Biology_Alberts	The energy needed for life comes ultimately from the electromagnetic radiation of the sun, which drives the formation of organic molecules in photosynthetic organisms such as green plants. Animals obtain their energy by eating organic molecules and oxidizing them in a series of enzyme-catalyzed reactions that are coupled to the formation of ATP—a common currency of energy in all cells.	Cell_Biology_Alberts. The energy needed for life comes ultimately from the electromagnetic radiation of the sun, which drives the formation of organic molecules in photosynthetic organisms such as green plants. Animals obtain their energy by eating organic molecules and oxidizing them in a series of enzyme-catalyzed reactions that are coupled to the formation of ATP—a common currency of energy in all cells.
Cell_Biology_Alberts_334	Cell_Biology_Alberts	To make possible the continual generation of order in cells, energetically favorable reactions, such as the hydrolysis of ATP, are coupled to energetically unfavorable reactions. In the biosynthesis of macromolecules, ATP is used to form reactive phosphorylated intermediates. Because the energetically unfavorable reaction of biosynthesis now becomes energetically favorable, ATP hydrolysis is said to drive the reaction. Polymeric molecules such as proteins, nucleic acids, and polysaccharides are assembled from small activated precursor molecules by repetitive condensation reactions that are driven in this way. Other reactive molecules, called either activated carriers or coenzymes, transfer other chemical groups in the course of biosynthesis: NADPH transfers hydrogen as a proton plus two electrons (a hydride ion), for example, whereas acetyl CoA transfers an acetyl group.	Cell_Biology_Alberts. To make possible the continual generation of order in cells, energetically favorable reactions, such as the hydrolysis of ATP, are coupled to energetically unfavorable reactions. In the biosynthesis of macromolecules, ATP is used to form reactive phosphorylated intermediates. Because the energetically unfavorable reaction of biosynthesis now becomes energetically favorable, ATP hydrolysis is said to drive the reaction. Polymeric molecules such as proteins, nucleic acids, and polysaccharides are assembled from small activated precursor molecules by repetitive condensation reactions that are driven in this way. Other reactive molecules, called either activated carriers or coenzymes, transfer other chemical groups in the course of biosynthesis: NADPH transfers hydrogen as a proton plus two electrons (a hydride ion), for example, whereas acetyl CoA transfers an acetyl group.
Cell_Biology_Alberts_335	Cell_Biology_Alberts	The constant supply of energy that cells need to generate and maintain the biological order that keeps them alive comes from the chemical-bond energy in food molecules. The proteins, lipids, and polysaccharides that make up most of the food we eat must be broken down into smaller molecules before our cells can use them—either Figure 2–44 The orientation of the active intermediates in the repetitive condensation reactions that form biological polymers. The head growth of polymers is compared with its alternative, tail growth. as indicated, these two mechanisms are used to produce different types of biological macromolecules.	Cell_Biology_Alberts. The constant supply of energy that cells need to generate and maintain the biological order that keeps them alive comes from the chemical-bond energy in food molecules. The proteins, lipids, and polysaccharides that make up most of the food we eat must be broken down into smaller molecules before our cells can use them—either Figure 2–44 The orientation of the active intermediates in the repetitive condensation reactions that form biological polymers. The head growth of polymers is compared with its alternative, tail growth. as indicated, these two mechanisms are used to produce different types of biological macromolecules.
Cell_Biology_Alberts_336	Cell_Biology_Alberts	as a source of energy or as building blocks for other molecules. Enzymatic digestion breaks down the large polymeric molecules in food into their monomer subunits—proteins into amino acids, polysaccharides into sugars, and fats into fatty acids and glycerol. After digestion, the small organic molecules derived from food enter the cytosol of cells, where their gradual oxidation begins. Sugars are particularly important fuel molecules, and they are oxidized in small controlled steps to carbon dioxide (CO2) and water (Figure 2–45). In this section, we trace the major steps in the breakdown, or catabolism, of sugars and show how they produce ATP, NADH, and other activated carrier molecules in animal cells. A very similar pathway also operates in plants, fungi, and many bacteria. As we shall see, the oxidation of fatty acids is equally important for cells. Other molecules, such as proteins, can also serve as energy sources when they are funneled through appropriate enzymatic pathways.	Cell_Biology_Alberts. as a source of energy or as building blocks for other molecules. Enzymatic digestion breaks down the large polymeric molecules in food into their monomer subunits—proteins into amino acids, polysaccharides into sugars, and fats into fatty acids and glycerol. After digestion, the small organic molecules derived from food enter the cytosol of cells, where their gradual oxidation begins. Sugars are particularly important fuel molecules, and they are oxidized in small controlled steps to carbon dioxide (CO2) and water (Figure 2–45). In this section, we trace the major steps in the breakdown, or catabolism, of sugars and show how they produce ATP, NADH, and other activated carrier molecules in animal cells. A very similar pathway also operates in plants, fungi, and many bacteria. As we shall see, the oxidation of fatty acids is equally important for cells. Other molecules, such as proteins, can also serve as energy sources when they are funneled through appropriate enzymatic pathways.
Cell_Biology_Alberts_337	Cell_Biology_Alberts	Glycolysis is a Central aTp-producing pathway The major process for oxidizing sugars is the sequence of reactions known as glycolysis—from the Greek glukus, “sweet,” and lusis, “rupture.” Glycolysis produces ATP without the involvement of molecular oxygen (O2 gas). It occurs in the cytosol of most cells, including many anaerobic microorganisms. Glycolysis probably evolved early in the history of life, before photosynthetic organisms introduced oxygen into the atmosphere. During glycolysis, a glucose molecule with six carbon atoms is converted into two molecules of pyruvate, each of which contains three carbon atoms. For each glucose molecule, two molecules of ATP are hydrolyzed to provide energy to drive the early steps, but four molecules of ATP are produced in the later steps. At the end of glycolysis, there is consequently a net gain of two molecules of ATP for each glucose molecule broken down. Two molecules of the activated carrier NADH are also produced.	Cell_Biology_Alberts. Glycolysis is a Central aTp-producing pathway The major process for oxidizing sugars is the sequence of reactions known as glycolysis—from the Greek glukus, “sweet,” and lusis, “rupture.” Glycolysis produces ATP without the involvement of molecular oxygen (O2 gas). It occurs in the cytosol of most cells, including many anaerobic microorganisms. Glycolysis probably evolved early in the history of life, before photosynthetic organisms introduced oxygen into the atmosphere. During glycolysis, a glucose molecule with six carbon atoms is converted into two molecules of pyruvate, each of which contains three carbon atoms. For each glucose molecule, two molecules of ATP are hydrolyzed to provide energy to drive the early steps, but four molecules of ATP are produced in the later steps. At the end of glycolysis, there is consequently a net gain of two molecules of ATP for each glucose molecule broken down. Two molecules of the activated carrier NADH are also produced.
Cell_Biology_Alberts_338	Cell_Biology_Alberts	The glycolytic pathway is outlined in Figure 2–46 and shown in more detail in Panel 2–8 (pp. 104–105) and Movie 2.5. Glycolysis involves a sequence of 10 separate reactions, each producing a different sugar intermediate and each catalyzed by a different enzyme. Like most enzymes, these have names ending in ase—such as isomerase and dehydrogenase—to indicate the type of reaction they catalyze. Although no molecular oxygen is used in glycolysis, oxidation occurs, in that electrons are removed by NAD+ (producing NADH) from some of the carbons derived from the glucose molecule. The stepwise nature of the process releases the energy of oxidation in small packets, so that much of it can be stored in activated carrier molecules rather than all of it being released as heat (see Figure 2–45). Thus, some of the energy released by oxidation drives the direct synthesis of ATP molecules from ADP and Pi, and some remains with the electrons in the electron carrier NADH.	Cell_Biology_Alberts. The glycolytic pathway is outlined in Figure 2–46 and shown in more detail in Panel 2–8 (pp. 104–105) and Movie 2.5. Glycolysis involves a sequence of 10 separate reactions, each producing a different sugar intermediate and each catalyzed by a different enzyme. Like most enzymes, these have names ending in ase—such as isomerase and dehydrogenase—to indicate the type of reaction they catalyze. Although no molecular oxygen is used in glycolysis, oxidation occurs, in that electrons are removed by NAD+ (producing NADH) from some of the carbons derived from the glucose molecule. The stepwise nature of the process releases the energy of oxidation in small packets, so that much of it can be stored in activated carrier molecules rather than all of it being released as heat (see Figure 2–45). Thus, some of the energy released by oxidation drives the direct synthesis of ATP molecules from ADP and Pi, and some remains with the electrons in the electron carrier NADH.
Cell_Biology_Alberts_339	Cell_Biology_Alberts	Figure 2–45 Schematic representation of the controlled stepwise oxidation of sugar in a cell, compared with ordinary burning. (a) if the sugar were oxidized to Co2 and h2o in a single step, it would release an amount of energy much larger than could be captured for useful purposes. (B) in the cell, enzymes catalyze oxidation via a series of small steps in which free energy is transferred in conveniently sized packets to carrier molecules—most often aTp and naDh. at each step, an enzyme controls the reaction by reducing the activation-energy barrier that has to be surmounted before the specific reaction can occur. The total free energy released is exactly the same in (a) and (B). large activation energy overcome by the heat from a fre all free energy is released as heat; none is stored	Cell_Biology_Alberts. Figure 2–45 Schematic representation of the controlled stepwise oxidation of sugar in a cell, compared with ordinary burning. (a) if the sugar were oxidized to Co2 and h2o in a single step, it would release an amount of energy much larger than could be captured for useful purposes. (B) in the cell, enzymes catalyze oxidation via a series of small steps in which free energy is transferred in conveniently sized packets to carrier molecules—most often aTp and naDh. at each step, an enzyme controls the reaction by reducing the activation-energy barrier that has to be surmounted before the specific reaction can occur. The total free energy released is exactly the same in (a) and (B). large activation energy overcome by the heat from a fre all free energy is released as heat; none is stored
Cell_Biology_Alberts_340	Cell_Biology_Alberts	large activation energy overcome by the heat from a fre all free energy is released as heat; none is stored Two molecules of NADH are formed per molecule of glucose in the course of glycolysis. In aerobic organisms, these NADH molecules donate their electrons to the electron-transport chain described in Chapter 14, and the NAD+ formed from the NADH is used again for glycolysis (see step 6 in Panel 2–8, pp. 104–105). fermentations produce aTp in the absence of oxygen For most animal and plant cells, glycolysis is only a prelude to the final stage of the breakdown of food molecules. In these cells, the pyruvate formed by glycolysis is rapidly transported into the mitochondria, where it is converted into CO2 plus acetyl CoA, whose acetyl group is then completely oxidized to CO2 and H2O.	Cell_Biology_Alberts. large activation energy overcome by the heat from a fre all free energy is released as heat; none is stored Two molecules of NADH are formed per molecule of glucose in the course of glycolysis. In aerobic organisms, these NADH molecules donate their electrons to the electron-transport chain described in Chapter 14, and the NAD+ formed from the NADH is used again for glycolysis (see step 6 in Panel 2–8, pp. 104–105). fermentations produce aTp in the absence of oxygen For most animal and plant cells, glycolysis is only a prelude to the final stage of the breakdown of food molecules. In these cells, the pyruvate formed by glycolysis is rapidly transported into the mitochondria, where it is converted into CO2 plus acetyl CoA, whose acetyl group is then completely oxidized to CO2 and H2O.
Cell_Biology_Alberts_341	Cell_Biology_Alberts	In contrast, for many anaerobic organisms—which do not utilize molecular oxygen and can grow and divide without it—glycolysis is the principal source of the cell’s ATP. Certain animal tissues, such as skeletal muscle, can also continue to function when molecular oxygen is limited. In these anaerobic conditions, the pyruvate and the NADH electrons stay in the cytosol. The pyruvate is converted into products excreted from the cell—for example, into ethanol and CO2 in the yeasts used in brewing and breadmaking, or into lactate in muscle. In this process, the NADH gives up its electrons and is converted back into NAD+. This regeneration of NAD+ is required to maintain the reactions of glycolysis (Figure 2–47). Energy-yielding pathways like these, in which organic molecules both donate and accept electrons (and which are often, as in these cases, anaerobic), are called one molecule of glucose	Cell_Biology_Alberts. In contrast, for many anaerobic organisms—which do not utilize molecular oxygen and can grow and divide without it—glycolysis is the principal source of the cell’s ATP. Certain animal tissues, such as skeletal muscle, can also continue to function when molecular oxygen is limited. In these anaerobic conditions, the pyruvate and the NADH electrons stay in the cytosol. The pyruvate is converted into products excreted from the cell—for example, into ethanol and CO2 in the yeasts used in brewing and breadmaking, or into lactate in muscle. In this process, the NADH gives up its electrons and is converted back into NAD+. This regeneration of NAD+ is required to maintain the reactions of glycolysis (Figure 2–47). Energy-yielding pathways like these, in which organic molecules both donate and accept electrons (and which are often, as in these cases, anaerobic), are called one molecule of glucose
Cell_Biology_Alberts_342	Cell_Biology_Alberts	Energy-yielding pathways like these, in which organic molecules both donate and accept electrons (and which are often, as in these cases, anaerobic), are called one molecule of glucose OH to be fructose 1,6 six-carbon sugar to two three-carbon sugarsSTEP 5 two molecules of glyceraldehyde Figure 2–46 an outline of glycolysis. each of the 10 steps shown is catalyzed by a different enzyme. note that step 4 cleaves a six-carbon sugar into two three-carbon sugars, so that the number of molecules at every stage after this doubles. as indicated, step 6 begins the energy-generation phase of glycolysis. Because two molecules of aTp are hydrolyzed in the early, energy-investment phase, glycolysis results in the net synthesis of 2 aTp and 2 naDh molecules per molecule of glucose (see also panel 2–8). Figure 2–47 Two pathways for the anaerobic breakdown of pyruvate.	Cell_Biology_Alberts. Energy-yielding pathways like these, in which organic molecules both donate and accept electrons (and which are often, as in these cases, anaerobic), are called one molecule of glucose OH to be fructose 1,6 six-carbon sugar to two three-carbon sugarsSTEP 5 two molecules of glyceraldehyde Figure 2–46 an outline of glycolysis. each of the 10 steps shown is catalyzed by a different enzyme. note that step 4 cleaves a six-carbon sugar into two three-carbon sugars, so that the number of molecules at every stage after this doubles. as indicated, step 6 begins the energy-generation phase of glycolysis. Because two molecules of aTp are hydrolyzed in the early, energy-investment phase, glycolysis results in the net synthesis of 2 aTp and 2 naDh molecules per molecule of glucose (see also panel 2–8). Figure 2–47 Two pathways for the anaerobic breakdown of pyruvate.
Cell_Biology_Alberts_343	Cell_Biology_Alberts	Figure 2–47 Two pathways for the anaerobic breakdown of pyruvate. (a) When there is inadequate oxygen, for example, in a muscle cell undergoing vigorous contraction, the pyruvate produced by glycolysis is converted to lactate as shown. This reaction regenerates the naD+ consumed in step 6 of glycolysis, but the whole pathway yields much less energy overall than complete oxidation. (B) in some organisms that can grow anaerobically, such as yeasts, pyruvate is C regeneration C converted via acetaldehyde into carbon dioxide and ethanol. again, this pathway CO HCOH regenerates naD+ from naDh, as required to enable glycolysis to continue. Both (a) and (B) are examples of fermentations.	Cell_Biology_Alberts. Figure 2–47 Two pathways for the anaerobic breakdown of pyruvate. (a) When there is inadequate oxygen, for example, in a muscle cell undergoing vigorous contraction, the pyruvate produced by glycolysis is converted to lactate as shown. This reaction regenerates the naD+ consumed in step 6 of glycolysis, but the whole pathway yields much less energy overall than complete oxidation. (B) in some organisms that can grow anaerobically, such as yeasts, pyruvate is C regeneration C converted via acetaldehyde into carbon dioxide and ethanol. again, this pathway CO HCOH regenerates naD+ from naDh, as required to enable glycolysis to continue. Both (a) and (B) are examples of fermentations.
Cell_Biology_Alberts_344	Cell_Biology_Alberts	fermentations. Studies of the commercially important fermentations carried out by yeasts inspired much of early biochemistry. Work in the nineteenth century led in 1896 to the then startling recognition that these processes could be studied outside living organisms, in cell extracts. This revolutionary discovery eventually made it possible to dissect out and study each of the individual reactions in the fermentation process. The piecing together of the complete glycolytic pathway in the 1930s was a major triumph of biochemistry, and it was quickly followed by the recognition of the central role of ATP in cell processes. Glycolysis illustrates how enzymes Couple oxidation to energy storage	Cell_Biology_Alberts. fermentations. Studies of the commercially important fermentations carried out by yeasts inspired much of early biochemistry. Work in the nineteenth century led in 1896 to the then startling recognition that these processes could be studied outside living organisms, in cell extracts. This revolutionary discovery eventually made it possible to dissect out and study each of the individual reactions in the fermentation process. The piecing together of the complete glycolytic pathway in the 1930s was a major triumph of biochemistry, and it was quickly followed by the recognition of the central role of ATP in cell processes. Glycolysis illustrates how enzymes Couple oxidation to energy storage
Cell_Biology_Alberts_345	Cell_Biology_Alberts	Glycolysis illustrates how enzymes Couple oxidation to energy storage The formation of ATP during glycolysis provides a particularly clear demonstration of how enzymes couple energetically unfavorable reactions with favorable ones, thereby driving the many chemical reactions that make life possible. Two central reactions in glycolysis (steps 6 and 7) convert the three-carbon sugar intermediate glyceraldehyde 3-phosphate (an aldehyde) into 3-phosphoglycerate (a carboxylic acid; see Panel 2–8, pp. 104–105), thus oxidizing an aldehyde group to a carboxylic acid group. The overall reaction releases enough free energy to convert a molecule of ADP to ATP and to transfer two electrons (and a proton) from the aldehyde to NAD+ to form NADH, while still liberating enough heat to the environment to make the overall reaction energetically favorable (∆G° for the overall reaction is –12.5 kJ/mole).	Cell_Biology_Alberts. Glycolysis illustrates how enzymes Couple oxidation to energy storage The formation of ATP during glycolysis provides a particularly clear demonstration of how enzymes couple energetically unfavorable reactions with favorable ones, thereby driving the many chemical reactions that make life possible. Two central reactions in glycolysis (steps 6 and 7) convert the three-carbon sugar intermediate glyceraldehyde 3-phosphate (an aldehyde) into 3-phosphoglycerate (a carboxylic acid; see Panel 2–8, pp. 104–105), thus oxidizing an aldehyde group to a carboxylic acid group. The overall reaction releases enough free energy to convert a molecule of ADP to ATP and to transfer two electrons (and a proton) from the aldehyde to NAD+ to form NADH, while still liberating enough heat to the environment to make the overall reaction energetically favorable (∆G° for the overall reaction is –12.5 kJ/mole).
Cell_Biology_Alberts_346	Cell_Biology_Alberts	Figure 2–48 outlines this remarkable feat of energy harvesting. The chemical reactions are precisely guided by two enzymes to which the sugar intermediates A short-lived covalent bond is formed between glyceraldehyde 3-phosphate and the –SH group of a cysteine side chain of the enzyme glyceraldehyde 3-phosphate dehydrogenase. The enzyme also binds noncovalently to NAD+. Glyceraldehyde 3-phosphate is oxidized as the enzyme removes a hydrogen atom (yellow) and transfers it, along with an electron, to NAD+, forming NADH (see Figure 2–37). Part of the energy + H+ released by the oxidation of the aldehyde is thus stored in NADH, and part is stored in the high- glyceraldehyde 3-phosphate to the CO enzyme. A molecule of inorganic phosphatehigh-energy inorganic displaces the high-energy thioesterphosphate phosphate bond to create 1,3-bisphospho-glycerate, which contains a high-energy phosphate bond. 1,3-bisphosphoglycerate	Cell_Biology_Alberts. Figure 2–48 outlines this remarkable feat of energy harvesting. The chemical reactions are precisely guided by two enzymes to which the sugar intermediates A short-lived covalent bond is formed between glyceraldehyde 3-phosphate and the –SH group of a cysteine side chain of the enzyme glyceraldehyde 3-phosphate dehydrogenase. The enzyme also binds noncovalently to NAD+. Glyceraldehyde 3-phosphate is oxidized as the enzyme removes a hydrogen atom (yellow) and transfers it, along with an electron, to NAD+, forming NADH (see Figure 2–37). Part of the energy + H+ released by the oxidation of the aldehyde is thus stored in NADH, and part is stored in the high- glyceraldehyde 3-phosphate to the CO enzyme. A molecule of inorganic phosphatehigh-energy inorganic displaces the high-energy thioesterphosphate phosphate bond to create 1,3-bisphospho-glycerate, which contains a high-energy phosphate bond. 1,3-bisphosphoglycerate
Cell_Biology_Alberts_347	Cell_Biology_Alberts	1,3-bisphosphoglycerate The high-energy phosphate group is transferred to ADP to form ATP. The oxidation of an aldehyde to a carboxylic acid releases energy, much of which is captured in the activated carriers ATP and NADH.	Cell_Biology_Alberts. 1,3-bisphosphoglycerate The high-energy phosphate group is transferred to ADP to form ATP. The oxidation of an aldehyde to a carboxylic acid releases energy, much of which is captured in the activated carriers ATP and NADH.
Cell_Biology_Alberts_348	Cell_Biology_Alberts	The oxidation of an aldehyde to a carboxylic acid releases energy, much of which is captured in the activated carriers ATP and NADH. Figure 2–48 energy storage in steps 6 and 7 of glycolysis. (a) in step 6, the enzyme glyceraldehyde 3-phosphate dehydrogenase couples the energetically favorable oxidation of an aldehyde to the energetically unfavorable formation of a high-energy phosphate bond. at the same time, it enables energy to be stored in naDh. The formation of the high-energy phosphate bond is driven by the oxidation reaction, and the enzyme thereby acts like the “paddle wheel” coupler in figure 2–32B. in step 7, the newly formed high-energy phosphate bond in 1,3-bisphosphoglycerate is transferred to aDp, forming a molecule of aTp and leaving a free carboxylic acid group on the oxidized sugar. The part of the molecule that undergoes a change is shaded in blue; the rest of the molecule remains unchanged throughout all these reactions.	Cell_Biology_Alberts. The oxidation of an aldehyde to a carboxylic acid releases energy, much of which is captured in the activated carriers ATP and NADH. Figure 2–48 energy storage in steps 6 and 7 of glycolysis. (a) in step 6, the enzyme glyceraldehyde 3-phosphate dehydrogenase couples the energetically favorable oxidation of an aldehyde to the energetically unfavorable formation of a high-energy phosphate bond. at the same time, it enables energy to be stored in naDh. The formation of the high-energy phosphate bond is driven by the oxidation reaction, and the enzyme thereby acts like the “paddle wheel” coupler in figure 2–32B. in step 7, the newly formed high-energy phosphate bond in 1,3-bisphosphoglycerate is transferred to aDp, forming a molecule of aTp and leaving a free carboxylic acid group on the oxidized sugar. The part of the molecule that undergoes a change is shaded in blue; the rest of the molecule remains unchanged throughout all these reactions.
Cell_Biology_Alberts_349	Cell_Biology_Alberts	(B) summary of the overall chemical change produced by reactions 6 and 7. are tightly bound. As detailed in Figure 2–48, the first enzyme (glyceraldehyde 3-phosphate dehydrogenase) forms a short-lived covalent bond to the aldehyde through a reactive –SH group on the enzyme, and catalyzes its oxidation by NAD+ in this attached state. The reactive enzyme–substrate bond is then displaced by an inorganic phosphate ion to produce a high-energy phosphate intermediate, which is released from the enzyme. This intermediate binds to the second enzyme (phosphoglycerate kinase), which catalyzes the energetically favorable transfer of the high-energy phosphate just created to ADP, forming ATP and completing the process of oxidizing an aldehyde to a carboxylic acid. Note that the C–H bond oxidation energy in step 6 drives the formation of both NADH and a high-energy phosphate bond. The breakage of the high-energy bond then drives ATP formation.	Cell_Biology_Alberts. (B) summary of the overall chemical change produced by reactions 6 and 7. are tightly bound. As detailed in Figure 2–48, the first enzyme (glyceraldehyde 3-phosphate dehydrogenase) forms a short-lived covalent bond to the aldehyde through a reactive –SH group on the enzyme, and catalyzes its oxidation by NAD+ in this attached state. The reactive enzyme–substrate bond is then displaced by an inorganic phosphate ion to produce a high-energy phosphate intermediate, which is released from the enzyme. This intermediate binds to the second enzyme (phosphoglycerate kinase), which catalyzes the energetically favorable transfer of the high-energy phosphate just created to ADP, forming ATP and completing the process of oxidizing an aldehyde to a carboxylic acid. Note that the C–H bond oxidation energy in step 6 drives the formation of both NADH and a high-energy phosphate bond. The breakage of the high-energy bond then drives ATP formation.