《生物化学原理》（英文版）BIOENERGETICS AND METABOLISM

13.3 Biological Oxidation-Reduction Reactions 511 (2)NAD++2H++2e一→NADH+H the values of e for both reductants are determined By convention, AE is expressed as E of the electron acceptor minus E of the electron donor. Because ac- Eetaldcu -Eo+Ri In lacetaldehyde metaldehyde is accepting electrons from NADH in our 0.026V,1.00 example,△E°=-0.197V-(-0.320V)=0.123V -0.197V+ 0.100=-0.167V and n is 2. Therefore △G°=-n7△E°=-2(96.5kJ/V·mol(0.123V) 0.026V1.00 -0.320V+-2 a.100=-0350V This is the free-energy change for the oxidation- Then AE is used to calculate AG(Eqn 13-5) eduction reaction at pH7, when acetaldehyde, ethanol, NAD, and NADH are all present at 1.00M concentra- △E=-0.167V-(-0.350)V=0.183V tions. If, instead, acetaldehyde and NAdh were present at 1.00 M but ethanol and NAD were present at 0.100 M, =-296.5kNV·mol0.183V the value for ag would be calculated as follows. first =-35 3 kJ/mol TABLE 13-7 Standard Reduction Potentials of Some Biologically Important Half-Reactions, at pH 7.0 and 25%C (298 K) Half-reaction 302+2H++2e 0816 Fe3++e--→Fe2 0.771 +2H+2e→NO2+H20 Cytochrome f(Fe)+e-cytochrome f(Fe f) 0.365 Fe(CN0e-( ferricyanide)+e→→Fe(CN0 Cytochrome a3(e3)+e→→ cytochrome a3(e2+) H202 Cytochrome a(Fe)+e- cytochrome a( Fef) Cytochrome c(Fe)+e 0.254 Cytochrome c1(e2+)+e→→ cytochrome C(e2+) b(Fe)+e- cytochrome b(Fez+) Ubiquinone+ 2HT 2e- ubiquinol H2 0045 Fumarate<+ 2H+ 2e- succinate. 0031 2H 2e"- H2 (at standard conditions, pH O Crotonyl-COA 2H 2e tyryl-Co 0.015 Oxaloacetate2-+2H++2e"-, malate2 Pyruvate 2H 2e - lactate Acetaldehyde 2H+ 2e- ethanol -0.197 FAD+2H+2e—→FADH2 0.219* Glutathione+2H++2e-—→2 reduced glutathione -0.23 Lipoic acid 2H+ 2e-dihydrolipoic acid -0.29 NAD+H +2e DH -0320 NADP++H+2e-— NADPH Acetoacetate 2H*+ 2e->.hydroxybutyrate -0.346 2H+2e→H2(atpH7) 0.414 Ferredoxin(Fe3+)+e- ferredoxin(Fe2+) -0.432 Source: Data mostly from Loach, PA (1976)h Handbook of Biochemistry and Molecular Biolgy, 3rd edn(Fasman, G.D., ed). Physical and Chemical Data, Vol. L pp 122-130, CRC Press, Boca Raton, FL This i the value for free FAD FAD bound to a sped fic flavoprotein(for example succinate dehydroganase) has a different

(2) NAD 2H 2e 88n NADH H E  0.320 V By convention, E is expressed as E of the electron acceptor minus E of the electron donor. Because ac￾etaldehyde is accepting electrons from NADH in our example, E  0.197 V  (0.320 V)  0.123 V, and n is 2. Therefore, G  n E  2(96.5 kJ/V  mol)(0.123 V)  23.7 kJ/mol This is the free-energy change for the oxidation￾reduction reaction at pH 7, when acetaldehyde, ethanol, NAD, and NADH are all present at 1.00 M concentra￾tions. If, instead, acetaldehyde and NADH were present at 1.00 M but ethanol and NAD were present at 0.100 M, the value for G would be calculated as follows. First, the values of E for both reductants are determined (Eqn 13–4): Eacetaldehyde  E  R nℑ T  ln  [ac [ e e t t a h ld a e n h o y l]  de]  0.197 V  0.02 2  6 V ln 0 1 .1 .0 0 0 0  0.167 V ENADH  E  R nℑ T  ln [ [ N N A A D D H ] ]  0.320 V  0.02 2  6 V ln 0 1 .1 .0 0 0 0  0.350 V Then E is used to calculate G (Eqn 13–5): E  0.167 V  (0.350) V  0.183 V G  n E  2(96.5 kJ/V
mol)(0.183 V)  35.3 kJ/mol 13.3 Biological Oxidation-Reduction Reactions 511 Half-reaction E (V)  1 2 O2 2H 2e 88n H2O 0.816 Fe3 e 88n Fe2 0.771 NO3  2H 2e 88n NO2  H2O 0.421 Cytochrome f (Fe3) e 88n cytochrome f (Fe2) 0.365 Fe(CN)6 3 (ferricyanide) e 88n Fe(CN)6 4 0.36 Cytochrome a3 (Fe3) e 88n cytochrome a3 (Fe2) 0.35 O2 2H 2e 88n H2O2 0.295 Cytochrome a (Fe3) e 88n cytochrome a (Fe2) 0.29 Cytochrome c (Fe3) e 88n cytochrome c (Fe2) 0.254 Cytochrome c1 (Fe3) e 88n cytochrome c1 (Fe2) 0.22 Cytochrome b (Fe3) e 88n cytochrome b (Fe2) 0.077 Ubiquinone 2H 2e 88n ubiquinol H2 0.045 Fumarate2 2H 2e 88n succinate2 0.031 2H 2e 88n H2 (at standard conditions, pH 0) 0.000 Crotonyl-CoA 2H 2e 88n butyryl-CoA 0.015 Oxaloacetate2 2H 2e 88n malate2 0.166 Pyruvate 2H 2e 88n lactate 0.185 Acetaldehyde 2H 2e 88n ethanol 0.197 FAD 2H 2e 88n FADH2 0.219* Glutathione 2H 2e 88n 2 reduced glutathione 0.23 S 2H 2e 88n H2S 0.243 Lipoic acid 2H 2e 88n dihydrolipoic acid 0.29 NAD H 2e 88n NADH 0.320 NADP H 2e 88n NADPH 0.324 Acetoacetate 2H 2e 88n -hydroxybutyrate 0.346 -Ketoglutarate CO2 2H 2e 88n isocitrate 0.38 2H 2e 88n H2 (at pH 7) 0.414 Ferredoxin (Fe3) e 88n ferredoxin (Fe2) 0.432 Standard Reduction Potentials of Some Biologically Important Half-Reactions, at pH 7.0 and 25 C (298 K) TABLE 13–7 Source: Data mostly from Loach, P.A. (1976) In Handbook of Biochemistry and Molecular Biology, 3rd edn (Fasman, G.D., ed.), Physical and Chemical Data, Vol. I, pp. 122–130, CRC Press, Boca Raton, FL. * This is the value for free FAD; FAD bound to a specific flavoprotein (for example succinate dehydrogenase) has a different E that depends on its protein environments.

Chapter 13 Principles of Bioenergetics It is thus possible to calculate the free-energy change NADH and NADPH Act with Dehydrogenases for any biological redox reaction at any concentrations as Soluble Electron Carriers of the redox pairs Nicotinamide adenine dinucleotide(NAd in its oxi Cellular oxidation of glucose to carbon dioxide dized form) and its close analog nicotinamide adenine Requires Specialized Electron Carriers dinucleotide phosphate(NADP T)are composed of two nucleotides joined through their phosphate groups by a The principles of oxidation-reduction energetics de- phosphoanhydride bond (Fig. 13-15a). Because the cribed above apply to the many metabolic reactions that nicotinamide ring resembles pyridine, these compounds involve electron transfers. For example, in many organ- are sometimes called pyridine nucleotides. The vita- isms, the oxidation of glucose supplies energy for the min niacin is the source of the nicotinamide moiety in production of ATP. The complete oxidation of glucose: nicotinamide nucleotides Both coenzymes undergo reversible reduction of CeH12O6+602→6C02+6H2O the nicotinamide ring(Fig. 13-15). As a substrate mol has a AG of-2, 840 kI/mol. This is a much larger re- ecule undergoes oxidation(dehydrogenation), giving up lease of free energy than is required for ATP synthesis two hydrogen atoms, the oxidized form of the nucleotide (50 to 60 kJ/mol; see Box 13-1) Cells convert glucose (NAD or NADP) accepts a hydride ion (H,the to CO2 not in a single, high-energy-releasing reaction, equivalent of a proton and two electrons) and is trans- but rather in a series of controlled reactions. some of formed into the reduced form(NADH or NADPH). The which are oxidations. The free energy released in these second proton removed from the substrate is released oxidation steps is of the same order of magnitude as that to the aqueous solvent. The half-reaction for each type required for ATP synthesis from ADP, with some energy of nucleotide is therefore to spare. Electrons removed in these oxidation steps are NAD++2e+2H→NADH+H+ transferred to coenzymes specialized for carrying elec- NADP++2e- 2H+- NADPH +H+ trons, such as Nad and Fad(described below) Reduction of nadt or nadp converts the benzenoid A Few Types of Coenzymes and Proteins Serve ring of the nicotinamide moiety(with a fixed positive as Universal electron carriers charge on the ring nitrogen) to the quinonoid form(with no charge on the nitrogen). Note that the reduced nu- The multitude of enzymes that catalyze cellular oxida- cleotides absorb light at 340 nm; the oxidized forms do tions channel electrons from their hundreds of different not(Fig. 13-15b). The plus sign in the abbreviations substrates into just a few types of universal electron car- NAD* and NadP does not indicate the net charge on riers. The reduction of these carriers in catabolic these molecules(they are both negatively charged) processes results in the conservation of free energy re- rather, it indicates that the nicotinamide ring is in its leased by substrate oxidation. NAD, NADP, FMN, and oxidized form, with a positive charge on the nitrogen FAD are water-soluble coenzymes that undergo atom In the abbreviations nadh and nadph. the "h versible oxidation and reduction in many of the electron- denotes the added hydride ion. To refer to these nu transfer reactions of metabolism. The nucleotides NAd cleotides without specifying their oxidation state, we and NADP move readily from one enzyme to another; use NAD and NADP. the flavin nucleotides FMn and FAD are usually very The total concentration of nAd*+ NAdh in most tightly bound to the enzymes, called flavoproteins, for tissues is about 10-m that of nadp+ nadph is which they serve as prosthetic groups. Lipid-soluble about 10-M. In many cells and tissues, the ratio of quinones such as ubiquinone and plastoquinone act as NAD(oxidized)to NADh (reduced) is high, favoring electron carriers and proton donors in the nonaqueous hydride transfer from a substrate to NAD to form environment of membranes. Iron-sulfur proteins and cy- NADH. By contrast, NADPH (reduced) is generally p tochromes, which have tightly bound prosthetic groups ent in greater amounts than its oxidized form, NADP that undergo reversible oxidation and reduction, also favoring hydride transfer from NADPH to a substrate serve as electron carriers in many oxidation-reduction This reflects the specialized metabolic roles of the two reactions. Some of these proteins are water-soluble, but coenzymes: nad generally functions in oxidations- others are peripheral or integral membrane proteins(see usually as part of a catabolic reaction; and NADPH is Fig11-6) the usual coenzyme in reductions-nearly always as We conclude this chapter by describing some chem. part of an anabolic reaction. A few enzymes can use ei- ical features of nucleotide coenzymes and some of the ther coenzyme, but most show a strong preference for enzymes(dehydrogenases and flavoproteins) that use one over the other. The processes in which these two them. The oxidation-reduction chemistry of quinones, cofactors function are also segregated in specific or iron-sulfur proteins, and cytochromes is discussed in ganelles of eukaryotic cells: oxidations of fuels such as Chapter 19 pyruvate, fatty acids, and a-keto acids derived from

It is thus possible to calculate the free-energy change for any biological redox reaction at any concentrations of the redox pairs. Cellular Oxidation of Glucose to Carbon Dioxide Requires Specialized Electron Carriers The principles of oxidation-reduction energetics de￾scribed above apply to the many metabolic reactions that involve electron transfers. For example, in many organ￾isms, the oxidation of glucose supplies energy for the production of ATP. The complete oxidation of glucose: C6H12O6 6O2 8n 6CO2 6H2O has a G of 2,840 kJ/mol. This is a much larger re￾lease of free energy than is required for ATP synthesis (50 to 60 kJ/mol; see Box 13–1). Cells convert glucose to CO2 not in a single, high-energy-releasing reaction, but rather in a series of controlled reactions, some of which are oxidations. The free energy released in these oxidation steps is of the same order of magnitude as that required for ATP synthesis from ADP, with some energy to spare. Electrons removed in these oxidation steps are transferred to coenzymes specialized for carrying elec￾trons, such as NAD and FAD (described below). A Few Types of Coenzymes and Proteins Serve as Universal Electron Carriers The multitude of enzymes that catalyze cellular oxida￾tions channel electrons from their hundreds of different substrates into just a few types of universal electron car￾riers. The reduction of these carriers in catabolic processes results in the conservation of free energy re￾leased by substrate oxidation. NAD, NADP, FMN, and FAD are water-soluble coenzymes that undergo re￾versible oxidation and reduction in many of the electron￾transfer reactions of metabolism. The nucleotides NAD and NADP move readily from one enzyme to another; the flavin nucleotides FMN and FAD are usually very tightly bound to the enzymes, called flavoproteins, for which they serve as prosthetic groups. Lipid-soluble quinones such as ubiquinone and plastoquinone act as electron carriers and proton donors in the nonaqueous environment of membranes. Iron-sulfur proteins and cy￾tochromes, which have tightly bound prosthetic groups that undergo reversible oxidation and reduction, also serve as electron carriers in many oxidation-reduction reactions. Some of these proteins are water-soluble, but others are peripheral or integral membrane proteins (see Fig. 11–6). We conclude this chapter by describing some chem￾ical features of nucleotide coenzymes and some of the enzymes (dehydrogenases and flavoproteins) that use them. The oxidation-reduction chemistry of quinones, iron-sulfur proteins, and cytochromes is discussed in Chapter 19. NADH and NADPH Act with Dehydrogenases as Soluble Electron Carriers Nicotinamide adenine dinucleotide (NAD in its oxi￾dized form) and its close analog nicotinamide adenine dinucleotide phosphate (NADP) are composed of two nucleotides joined through their phosphate groups by a phosphoanhydride bond (Fig. 13–15a). Because the nicotinamide ring resembles pyridine, these compounds are sometimes called pyridine nucleotides. The vita￾min niacin is the source of the nicotinamide moiety in nicotinamide nucleotides. Both coenzymes undergo reversible reduction of the nicotinamide ring (Fig. 13–15). As a substrate mol￾ecule undergoes oxidation (dehydrogenation), giving up two hydrogen atoms, the oxidized form of the nucleotide (NAD or NADP) accepts a hydride ion (:H, the equivalent of a proton and two electrons) and is trans￾formed into the reduced form (NADH or NADPH). The second proton removed from the substrate is released to the aqueous solvent. The half-reaction for each type of nucleotide is therefore NAD 2e 2H 8n NADH H NADP 2e 2H 8n NADPH H Reduction of NAD or NADP converts the benzenoid ring of the nicotinamide moiety (with a fixed positive charge on the ring nitrogen) to the quinonoid form (with no charge on the nitrogen). Note that the reduced nu￾cleotides absorb light at 340 nm; the oxidized forms do not (Fig. 13–15b). The plus sign in the abbreviations NAD and NADP does not indicate the net charge on these molecules (they are both negatively charged); rather, it indicates that the nicotinamide ring is in its oxidized form, with a positive charge on the nitrogen atom. In the abbreviations NADH and NADPH, the “H” denotes the added hydride ion. To refer to these nu￾cleotides without specifying their oxidation state, we use NAD and NADP. The total concentration of NAD NADH in most tissues is about 105 M; that of NADP NADPH is about 106 M. In many cells and tissues, the ratio of NAD (oxidized) to NADH (reduced) is high, favoring hydride transfer from a substrate to NAD to form NADH. By contrast, NADPH (reduced) is generally pres￾ent in greater amounts than its oxidized form, NADP, favoring hydride transfer from NADPH to a substrate. This reflects the specialized metabolic roles of the two coenzymes: NAD generally functions in oxidations— usually as part of a catabolic reaction; and NADPH is the usual coenzyme in reductions—nearly always as part of an anabolic reaction. A few enzymes can use ei￾ther coenzyme, but most show a strong preference for one over the other. The processes in which these two cofactors function are also segregated in specific or￾ganelles of eukaryotic cells: oxidations of fuels such as pyruvate, fatty acids, and -keto acids derived from 512 Chapter 13 Principles of Bioenergetics

3.3 Biological Oxidation-Reduction Reactions 513 NH NHg +H R Aside B side O=P-0 aDenine Oxidized (NAD) 0.6 NAD 0.4 In NADP* this hydroxyl group ed with phosphate 220240260280300320340360380 (b) FIGURE 13-15 NAD and NADP. (a)Nicotinamide adenine dinu. tra of NAD and NADH. Reduction of the nicotinamide ring produces cleotide,NAD+,and its phosphorylated analog NADP+ undergo re- a new, broad absorption band with a maximum at 340 nm. The pro- duction to NADH and NADPH, accepting a hydride ion (two elt duction of NADH during an enzyme-catalyzed reaction can be con- trons and one proton) from an oxidizable substrate. The hydride ion veniently followed by observing the appearance of the absorbance at is added to either the front(the A side)or the back(the B side) of the 340 nm(the molar extinction coefficient E340= 6, 200 M-cm-) lanar nicotinamide ring (see Table 13-8).(b)The UV absorption sp amino acids occur in the mitochondrial matrix whereas When NAD or NADP is reduced, the hydride ior eductive biosynthesis processes such as fatty acid syn- could in principle be transferred to either side of the thesis take place in the cytosol. This functional and spa- nicotinamide ring: the front(A side) or the back(B tial specialization allows a cell to maintain two distinct side), as represented in Figure 13-15a. Studies with iso pools of electron carriers, with two distinct functions. topically labeled substrates have shown that a given en More than 200 enzymes are known to catalyze re- zyme catalyzes either an A-type or a B-type transfer, but actions in which NAD(or NADP T )accepts a hydride not both. For example, yeast alcohol dehydrogenase and ion from a reduced substrate, or NADPH (or NADH) do- lactate dehydrogenase of vertebrate heart transfer a hy- nates a hydride ion to an oxidized substrate. The gen- dride ion to(or remove a hydride ion from) the a side eral reactions are of the nicotinamide ring; they are classed as type a de AH2+NAD+→→A+NADH+H hydrogenases to distinguish them from another group A+ NADPH +H+- AH,+ NADP+ of enzymes that transfer a hydride ion to(or remove a hydride ion from) the b side of the nicotinamide ring where AH2 is the reduced substrate and a the oxidized (Table 13-8). The specificity for one side or another can substrate. The general name for an enzyme of this type be very striking: lactate dehydrogenase, for example, is oxidoreductase; they are also commonly called de. prefers the a side over the B side by a factor of 5 X 10! hydrogenases. For example, alcohol dehydrogenase Most dehydrogenases that use NAD or NADP bind catalyzes the first step in the catabolism of ethanol, in the cofactor in a conserved protein domain called the which ethanol is oxidized to acetaldehyde Rossmann fold(named for Michael Rossmann, who de. CH3CH2OH+NAD→→CH3CHO+NADH+H duced the structure of lactate dehydrogenase and first described this structural motif). The Rossmann fold typ- ically consists of a six-stranded parallel B sheet and four Notice that one of the carbon atoms in ethanol has lost associated a helices(Fig. 13-16) a hydrogen; the compound has been oxidized from an The association between a dehydrogenase and NAd alcohol to an aldehyde(refer again to Fig 13-13 for the or NADP is relatively loose; the coenzyme readily diffuses oxidation states of carbon) from one enzyme to another, acting as a water-soluble

amino acids occur in the mitochondrial matrix, whereas reductive biosynthesis processes such as fatty acid syn￾thesis take place in the cytosol. This functional and spa￾tial specialization allows a cell to maintain two distinct pools of electron carriers, with two distinct functions. More than 200 enzymes are known to catalyze re￾actions in which NAD (or NADP) accepts a hydride ion from a reduced substrate, or NADPH (or NADH) do￾nates a hydride ion to an oxidized substrate. The gen￾eral reactions are AH2 NAD 8n A NADH H A NADPH H 8n AH2 NADP where AH2 is the reduced substrate and A the oxidized substrate. The general name for an enzyme of this type is oxidoreductase; they are also commonly called de￾hydrogenases. For example, alcohol dehydrogenase catalyzes the first step in the catabolism of ethanol, in which ethanol is oxidized to acetaldehyde: CH3CH2OH NAD 8n CH3CHO NADH H Ethanol Acetaldehyde Notice that one of the carbon atoms in ethanol has lost a hydrogen; the compound has been oxidized from an alcohol to an aldehyde (refer again to Fig. 13–13 for the oxidation states of carbon). When NAD or NADP is reduced, the hydride ion could in principle be transferred to either side of the nicotinamide ring: the front (A side) or the back (B side), as represented in Figure 13–15a. Studies with iso￾topically labeled substrates have shown that a given en￾zyme catalyzes either an A-type or a B-type transfer, but not both. For example, yeast alcohol dehydrogenase and lactate dehydrogenase of vertebrate heart transfer a hy￾dride ion to (or remove a hydride ion from) the A side of the nicotinamide ring; they are classed as type A de￾hydrogenases to distinguish them from another group of enzymes that transfer a hydride ion to (or remove a hydride ion from) the B side of the nicotinamide ring (Table 13–8). The specificity for one side or another can be very striking; lactate dehydrogenase, for example, prefers the A side over the B side by a factor of 5 107 ! Most dehydrogenases that use NAD or NADP bind the cofactor in a conserved protein domain called the Rossmann fold (named for Michael Rossmann, who de￾duced the structure of lactate dehydrogenase and first described this structural motif). The Rossmann fold typ￾ically consists of a six-stranded parallel sheet and four associated helices (Fig. 13–16). The association between a dehydrogenase and NAD or NADP is relatively loose; the coenzyme readily diffuses from one enzyme to another, acting as a water-soluble 13.3 Biological Oxidation-Reduction Reactions 513 C A CH2 OPP OH H O H H N OH R NH2 O P O O O O P O CH2 OH H H H H OH O O O  NH2 B side N (b) N N H H N or NADH ? N C NH2 B O H H C A R NH2 ? N B O H In NADP this hydroxyl group is esterified with phosphate. 2H 2e  H Absorbance NAD (reduced) A side H (oxidized) 1.0 Wavelength (nm) 0.0 220 240 260 280 300 320 340 360 380 Oxidized (NAD) Reduced (NADH) (a) 0.6 0.4 0.2 0.8 B H H H Adenine O FIGURE 13–15 NAD and NADP. (a) Nicotinamide adenine dinu￾cleotide, NAD, and its phosphorylated analog NADP undergo re￾duction to NADH and NADPH, accepting a hydride ion (two elec￾trons and one proton) from an oxidizable substrate. The hydride ion is added to either the front (the A side) or the back (the B side) of the planar nicotinamide ring (see Table 13–8). (b) The UV absorption spec￾tra of NAD and NADH. Reduction of the nicotinamide ring produces a new, broad absorption band with a maximum at 340 nm. The pro￾duction of NADH during an enzyme-catalyzed reaction can be con￾veniently followed by observing the appearance of the absorbance at 340 nm (the molar extinction coefficient 340  6,200 M1 cm1 ).

TABLE 13-8 Stereospecificity of Dehydrogenases That Employ NAD or NADP as Coenzymes Stereochemical specificity for nicotinamide Enzyme Coenzyme ing (A or B) Text page(s) Isocitrate dehydrogenase CX-XXX a-Ketoglutarate dehydrogenase XXX Glucose 6-phosphate dehydrogenase XXX Malate dehydrogenase Glutamate dehydrogenase nAd or NADP+ ABBABBAA X Lactate dehydrogenase Alcohol dehydrogenase XXX carrier of electrons from one metabolite to another. For (1)Glyceraldehyde 3-phosphate+NAD+-+ example, in the production of alcohol during fermenta- 3-phosphoglycerate NADH +H tion of glucose by yeast cells, a hydride ion is removed (2) Acetaldehyde NADH +H- ethanol+ NAD from glyceraldehyde 3-phosphate by one enzyme(glyc. eraldehyde 3-phosphate dehydrogenase, a type B en- Sum: Glyceraldehyde 3-phosphate +acetaldehyde zyme)and transferred to NAD. The NADH produced phosphoglycerate then leaves the enzyme surface and diffuses to another Notice that in the overall reaction there is no ne enzyme (alcohol dehydrogenase, a type A enzyme), duction or consumption of NAD or NADH; the coen which transfers a hydride ion to acetaldehyde, produc- zymes function catalytically and are recycled repeatedly without a net change in the concentration of NAD+ NADH Dietary Deficiency of Niacin, the Vitamin Form of NAD and NADP, Causes pellagra The pyridine-like rings of NAD and NADP are de- rived from the vitamin niacin(nicotinic acid; Fig 13-17), which is synthesized from tryptophan. Humans generally cannot synthesize niacin in sufficient quanti- s is especially so for tryptophan(maize, for example, has a low tryptophan content). Niacin deficiency, which affects all the NAD(P)-dependent dehydrogenases, causes the serious human disease pellagra(Italian for "rough skin) and characterized by the"three Ds: dermatitis, diarrhea, and dementia, followed in many cases by death. A century ago, pellagra was a common human disease; in the south ern United States, where maize was a dietary staple, about 100,000 people were afflicted and about 10,000 died between 1912 and 1916. In 1920 Joseph Goldberger showed pellagra to be caused by a dietary insufficiency, FIGURE 13-16 The nucleotide binding d and in 1937 Frank Strong, D. Wayne Wolley, and Conrad tate dehydrogenase (a) The Rossmann fold is a structural motif found Elvehjem identified niacin as the curative agent for in the NAD-binding site of many dehydrogenases. It consists of a blacktongue. Supplementation of the human diet with six-stranded parallel B sheet and four a helices: inspection reveals this inexpensive compound led to the eradication of pel the arrangement to be a pair of structurally similar B-a-B-a-B motifs. lagra in the populations of the developed world-with (b)The dinucleotide NAD binds in an extended conformation through one significant exception. Pellagra is still found among hydrogen bonds and salt bridges(derived from PDB ID 3LDH) alcoholics, whose intestinal absorption of niacin is much

carrier of electrons from one metabolite to another. For example, in the production of alcohol during fermenta￾tion of glucose by yeast cells, a hydride ion is removed from glyceraldehyde 3-phosphate by one enzyme (glyc￾eraldehyde 3-phosphate dehydrogenase, a type B en￾zyme) and transferred to NAD. The NADH produced then leaves the enzyme surface and diffuses to another enzyme (alcohol dehydrogenase, a type A enzyme), which transfers a hydride ion to acetaldehyde, produc￾ing ethanol: (1) Glyceraldehyde 3-phosphate NAD 8n 3-phosphoglycerate NADH H (2) Acetaldehyde NADH H 8n ethanol NAD Sum: Glyceraldehyde 3-phosphate acetaldehyde 8n 3-phosphoglycerate ethanol Notice that in the overall reaction there is no net pro￾duction or consumption of NAD or NADH; the coen￾zymes function catalytically and are recycled repeatedly without a net change in the concentration of NAD NADH. Dietary Deficiency of Niacin, the Vitamin Form of NAD and NADP, Causes Pellagra The pyridine-like rings of NAD and NADP are de￾rived from the vitamin niacin (nicotinic acid; Fig. 13–17), which is synthesized from tryptophan. Humans generally cannot synthesize niacin in sufficient quanti￾ties, and this is especially so for those with diets low in tryptophan (maize, for example, has a low tryptophan content). Niacin deficiency, which affects all the NAD(P)-dependent dehydrogenases, causes the serious human disease pellagra (Italian for “rough skin”) and a related disease in dogs, blacktongue. These diseases are characterized by the “three Ds”: dermatitis, diarrhea, and dementia, followed in many cases by death. A century ago, pellagra was a common human disease; in the south￾ern United States, where maize was a dietary staple, about 100,000 people were afflicted and about 10,000 died between 1912 and 1916. In 1920 Joseph Goldberger showed pellagra to be caused by a dietary insufficiency, and in 1937 Frank Strong, D. Wayne Wolley, and Conrad Elvehjem identified niacin as the curative agent for blacktongue. Supplementation of the human diet with this inexpensive compound led to the eradication of pel￾lagra in the populations of the developed world—with one significant exception. Pellagra is still found among alcoholics, whose intestinal absorption of niacin is much 514 Chapter 13 Principles of Bioenergetics Stereochemical specificity for nicotinamide Enzyme Coenzyme ring (A or B) Text page(s) Isocitrate dehydrogenase NAD A XXX–XXX -Ketoglutarate dehydrogenase NAD B XXX Glucose 6-phosphate dehydrogenase NADP B XXX Malate dehydrogenase NAD A XXX Glutamate dehydrogenase NAD or NADP B XXX Glyceraldehyde 3-phosphate dehydrogenase NAD B XXX Lactate dehydrogenase NAD A XXX Alcohol dehydrogenase NAD A XXX TABLE 13–8 Stereospecificity of Dehydrogenases That Employ NAD or NADP as Coenzymes FIGURE 13–16 The nucleotide binding domain of the enzyme lac￾tate dehydrogenase. (a) The Rossmann fold is a structural motif found in the NAD-binding site of many dehydrogenases. It consists of a six-stranded parallel sheet and four  helices; inspection reveals the arrangement to be a pair of structurally similar ---- motifs. (b) The dinucleotide NAD binds in an extended conformation through hydrogen bonds and salt bridges (derived from PDB ID 3LDH).

13.3 Biological Oxidation-Reduction Reactions 515 TABLE 13-9 Some Enzymes(Flavoproteins) That Employ Flavin Nucleotide Coenzymes Niacin Nicotine Enzyme nucleotide page(s) nicotinic acid) ACyl-CoA dehydrogenase FAD ydrolipayl dehydrogenase Succinate dehydrogenase FAD CH2-CH-Coo Glycerol 3-phosphate dehydrogenase FAD redoxin reductase NADH dehydrogenase(Complex D) FMN XXX FIGURE 13-17 Structures of niacin (nicotinic acid) and its deriva- tive nicotinamide. The biosynthetic precursor of these compounds is tryptophan In the laboratory, nicotinic acid was first produced by ox- loxazine ring is produced, abbreviated FADH and idation of the natural product nicotine-thus the name. Both nicotinic FMNH. Because flavoproteins can participate in either acid and nicotinamide cure pellagra, but nicotine(from cigarettes or one- or two-electron transfers, this class of proteins elsewhere) has no curative activity. is involved in a greater diversity of reactions than the NAD(P)-linked dehydrogenases Like the nicotinamide coenzymes( Fig. 13-15) reduced, and whose caloric needs are often met with dis. flavin nucleotides undergo a shift in a major absorption tilled spirits that are virtually devoid of vitamins, in- cluding niacin. In a few places, including the Deccan band on reduction. Flavoproteins that are fully reduced Plateau in India, pellagra still occurs, especially among (two electrons accepted) generally have an absorption the poor.■ electron), they acquire another absorption maximum at Flavin Nucleotides Are Tightly Bound in Flavoproteins about 450 nm; when fully oxidized, the flavin has max- ima at 370 and 440 nm. The intermediate radical form Flavoproteins(Table 13-9)are enzymes that catalyze reduced by one electron, has absorption maxima at oxidation-reduction reactions using either flavin 480, 580, and 625 nm. These changes can be used to as mononucleotide(FMN) or flavin adenine dinucleotide say reactions involving a flavoprotein (FAD) as coenzyme(Fig. 13-18). These coenzymes, the The flavin nucleotide in most flavoproteins is bound flavin nucleotides, are derived from the vitamin ri- rather tightly to the protein, and in some enzymes, such boflavin. The fused ring structure of flavin nucleotides as succinate dehydrogenase, it is bound covalently. Such ( the isoalloxazine ring) undergoes reversible reduction, tightly bound coenzymes are properly called prosthetic accepting either one or two electrons in the form of one groups. They do not transfer electrons by diffusing from or two hydrogen atoms(each atom an electron plus a one enzyme to another; rather, they provide a means by proton) from a reduced substrate. The fully reduced which the flavoprotein can temporarily hold electrons forms are abbreviated FADH2 and FMNH2. When a fully while it catalyzes electron transfer from a reduced sub oxidized flavin nucleotide accepts only one electron strate to an electron acceptor. One important feature of (one hydrogen atom), the semiquinone form of the isoal- the flavoproteins is the variability in the standard re- bound flavin nucleotide. Tight sociation between the enzyme and prosthetic group confers on the navin typical of that particular flavopro- tein, sometimes quite different rom the reduction potential of the free flavin nucleotide. fad bound fo example, has an E close to 0.0 V, compared with -0.219 V for free Conrad Elvehjem FAD: E for other flavoproteins 908-1993 1914-1966 1901-1962 ranges from -0.40 V to +0.06 V

reduced, and whose caloric needs are often met with dis￾tilled spirits that are virtually devoid of vitamins, in￾cluding niacin. In a few places, including the Deccan Plateau in India, pellagra still occurs, especially among the poor. ■ Flavin Nucleotides Are Tightly Bound in Flavoproteins Flavoproteins (Table 13–9) are enzymes that catalyze oxidation-reduction reactions using either flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD) as coenzyme (Fig. 13–18). These coenzymes, the flavin nucleotides, are derived from the vitamin ri￾boflavin. The fused ring structure of flavin nucleotides (the isoalloxazine ring) undergoes reversible reduction, accepting either one or two electrons in the form of one or two hydrogen atoms (each atom an electron plus a proton) from a reduced substrate. The fully reduced forms are abbreviated FADH2 and FMNH2. When a fully oxidized flavin nucleotide accepts only one electron (one hydrogen atom), the semiquinone form of the isoal￾loxazine ring is produced, abbreviated FADH• and FMNH• . Because flavoproteins can participate in either one- or two-electron transfers, this class of proteins is involved in a greater diversity of reactions than the NAD (P)-linked dehydrogenases. Like the nicotinamide coenzymes (Fig. 13–15), the flavin nucleotides undergo a shift in a major absorption band on reduction. Flavoproteins that are fully reduced (two electrons accepted) generally have an absorption maximum near 360 nm. When partially reduced (one electron), they acquire another absorption maximum at about 450 nm; when fully oxidized, the flavin has max￾ima at 370 and 440 nm. The intermediate radical form, reduced by one electron, has absorption maxima at 380, 480, 580, and 625 nm. These changes can be used to as￾say reactions involving a flavoprotein. The flavin nucleotide in most flavoproteins is bound rather tightly to the protein, and in some enzymes, such as succinate dehydrogenase, it is bound covalently. Such tightly bound coenzymes are properly called prosthetic groups. They do not transfer electrons by diffusing from one enzyme to another; rather, they provide a means by which the flavoprotein can temporarily hold electrons while it catalyzes electron transfer from a reduced sub￾strate to an electron acceptor. One important feature of the flavoproteins is the variability in the standard re￾duction potential (E ) of the bound flavin nucleotide. Tight as￾sociation between the enzyme and prosthetic group confers on the flavin ring a reduction potential typical of that particular flavopro￾tein, sometimes quite different from the reduction potential of the free flavin nucleotide. FAD bound to succinate dehydrogenase, for example, has an E close to 0.0 V, compared with 0.219 V for free FAD; E for other flavoproteins ranges from 0.40 V to 0.06 V. 13.3 Biological Oxidation-Reduction Reactions 515 O O  C O  NH2 NH3 CH3 CH2 CH COO C     C Niacin (nicotinic acid) Nicotine Nicotinamide Tryptophan FIGURE 13–17 Structures of niacin (nicotinic acid) and its deriva￾tive nicotinamide. The biosynthetic precursor of these compounds is tryptophan. In the laboratory, nicotinic acid was first produced by ox￾idation of the natural product nicotine—thus the name. Both nicotinic acid and nicotinamide cure pellagra, but nicotine (from cigarettes or elsewhere) has no curative activity. Frank Strong, 1908–1993 D. Wayne Woolley, 1914–1966 Conrad Elvehjem, 1901–1962 Flavin Text Enzyme nucleotide page(s) Acyl–CoA dehydrogenase FAD XXX Dihydrolipoyl dehydrogenase FAD XXX Succinate dehydrogenase FAD XXX Glycerol 3-phosphate dehydrogenase FAD XXX Thioredoxin reductase FAD XXX–XXX NADH dehydrogenase (Complex I) FMN XXX Glycolate oxidase FMN XXX TABLE 13–9 Some Enzymes (Flavoproteins) That Employ Flavin Nucleotide Coenzymes