《生物化学原理》（英文版）chapter 19 Oxidative phosphorylation

8885ac19690-7503/1/0411:32 AM Page700mac76mac76:385 Chapter 19 Oxidative Phosphorylation and Photophosphorylation 2Fe-2S Rieske iron. Intermembrane sulfur protein Cytochrome c1 pace(P side) Cytochrome b center rome b FIGURE 19-11 Cytochrome bC1 complex(Complex lID. The com- QH2 to the Rieske iron-sulfur protein, binds at QP, near the 2Fe-2S plex is a dimer of identical monomers, each with 11 different sub. center and heme b on the P side. The dimeric structure is essential units.(a)Structure of a monomer. The functional core is three sub- to the function of Complex ll. The interface between monomers forms units: cytochrome b(green)with its two hemes(bH and bu, light red); two pockets, each containing a Qp site from one monomer and a QN the Rieske iron-sulfur protein (purple)with its 2Fe-2S centers (yellow); site from the other. The ubiquinone intermediates move within these and cytochrome c1(blue)with its heme(red)(PDB ID 1BGY.(b)The sheltered pocket dimeric functional unit. Cytochrome C1 and the Rieske iron-sulfur pro- Complex Ill crystallizes in two distinct conformations(not shown) tein project from the P surface and can interact with cytochrome c In one, the Rieske Fe-s center is close to its electron acceptor, the (not part of the functional complex)in the intermembrane space. The heme of cytochrome cu, but relatively distant from cytochrome b and complex has two distinct binding sites for ubiquinone, QN and Q the QHa-binding site at which the Rieske Fe-S center receives elec which correspond to the sites of inhibition by two drugs that block trons. In the other, the Fe-S center has moved away from cytochrome xidative phosphorylation. Antimycin A, which blocks electron flow C1 and toward cytochrome b. The Rieske protein is thought to oscil- from heme bH to Q, binds at QN, close to heme bH on the n (matrix) late between these two conformations as it is reduced, then oxidized de of the membrane Myxothiazol, which prevents electron flow from and protons through the complex. The net equation for Complex /V: Cytochrome c to o2 In the final step of the the redox reactions of this Q cycle(Fig. 19-12)is spiratory chain, Complex I, also called cytochrome QH2+ 2 cyt cr(oxidized)+ 2HN- oxidase, carries electrons from cytochrome c to mo- Q+2 cyt c,(reduced )+ 4Hp (19-3) lecular oxygen, reducing it to H.O. Complex Iv is a large enzyme(13 subunits; Mr 204,000)of the inner mito- Theq cycle accommodates the switch between the two- chondrial membrane. Bacteria contain a form that is electron carrier ubiquinone and the one-electron carri- much simpler, with only three or four subunits, but sti ers-cytochromes b562, b566, C1, and c-and explains the capable of catalyzing both electron transfer and proton measured stoichiometry of four protons translocated pumping. Comparison of the mitochondrial and bacter- per pair of electrons passing through the Complex IlI to ial complexes suggests that three subunits are critical cytochrome c. Although the path of electrons through to the function(Fig. 19-13) this segment of the respiratory chain is complicated, the Mitochondrial subunit ii contains two cu ior net effect of the transfer is simple: QH2 is oxidized to Q plexed with the -sh groups of two Cys residues in a and two molecules of cytochrome c are reduced binuclear center (CuA; Fig. 19-13b) that resembles the Cytochrome c(see Fig 4-18) is a soluble protein of 2Fe-2S centers of iron-sulfur proteins. Subunit I con- the intermembrane space. After its single heme accepts tains two heme groups, designated a and a3, and an- an electron from Complex Ill, cytochrome c moves to other copper ion(CuB). Heme aa and Cug form a sec- Complex Iv to donate the electron to a binuclear cop- ond binuclear center that accepts electrons from heme per center. a and transfers them to Oe bound to heme a3

and protons through the complex. The net equation for the redox reactions of this Q cycle (Fig. 19–12) is QH2
2 cyt c1(oxidized)
2H
N On Q
2 cyt c1(reduced)
4H
P (19–3) The Q cycle accommodates the switch between the two￾electron carrier ubiquinone and the one-electron carri￾ers—cytochromes b562, b566, c1, and c—and explains the measured stoichiometry of four protons translocated per pair of electrons passing through the Complex III to cytochrome c. Although the path of electrons through this segment of the respiratory chain is complicated, the net effect of the transfer is simple: QH2 is oxidized to Q and two molecules of cytochrome c are reduced. Cytochrome c (see Fig. 4–18) is a soluble protein of the intermembrane space. After its single heme accepts an electron from Complex III, cytochrome c moves to Complex IV to donate the electron to a binuclear cop￾per center. Complex IV: Cytochrome c to O2 In the final step of the respiratory chain, Complex IV, also called cytochrome oxidase, carries electrons from cytochrome c to mo￾lecular oxygen, reducing it to H2O. Complex IV is a large enzyme (13 subunits; Mr 204,000) of the inner mito￾chondrial membrane. Bacteria contain a form that is much simpler, with only three or four subunits, but still capable of catalyzing both electron transfer and proton pumping. Comparison of the mitochondrial and bacter￾ial complexes suggests that three subunits are critical to the function (Fig. 19–13). Mitochondrial subunit II contains two Cu ions com￾plexed with the OSH groups of two Cys residues in a binuclear center (CuA; Fig. 19–13b) that resembles the 2Fe-2S centers of iron-sulfur proteins. Subunit I con￾tains two heme groups, designated a and a3, and an￾other copper ion (CuB). Heme a3 and CuB form a sec￾ond binuclear center that accepts electrons from heme a and transfers them to O2 bound to heme a3. 700 Chapter 19 Oxidative Phosphorylation and Photophosphorylation FIGURE 19–11 Cytochrome bc1 complex (Complex III). The com￾plex is a dimer of identical monomers, each with 11 different sub￾units. (a) Structure of a monomer. The functional core is three sub￾units: cytochrome b (green) with its two hemes (bH and bL, light red); the Rieske iron-sulfur protein (purple) with its 2Fe-2S centers (yellow); and cytochrome c1 (blue) with its heme (red) (PDB ID 1BGY). (b) The dimeric functional unit. Cytochrome c1 and the Rieske iron-sulfur pro￾tein project from the P surface and can interact with cytochrome c (not part of the functional complex) in the intermembrane space. The complex has two distinct binding sites for ubiquinone, QN and QP, which correspond to the sites of inhibition by two drugs that block oxidative phosphorylation. Antimycin A, which blocks electron flow from heme bH to Q, binds at QN, close to heme bH on the N (matrix) side of the membrane. Myxothiazol, which prevents electron flow from QH2 to the Rieske iron-sulfur protein, binds at QP, near the 2Fe-2S center and heme bL on the P side. The dimeric structure is essential to the function of Complex III. The interface between monomers forms two pockets, each containing a QP site from one monomer and a QN site from the other. The ubiquinone intermediates move within these sheltered pockets. Complex III crystallizes in two distinct conformations (not shown). In one, the Rieske Fe-S center is close to its electron acceptor, the heme of cytochrome c1, but relatively distant from cytochrome b and the QH2-binding site at which the Rieske Fe-S center receives elec￾trons. In the other, the Fe-S center has moved away from cytochrome c1 and toward cytochrome b. The Rieske protein is thought to oscil￾late between these two conformations as it is reduced, then oxidized. (a) Intermembrane space (P side) Matrix (N side) Cytochrome c1 Cytochrome b Rieske iron￾sulfur protein 2Fe-2S (b) Cytochrome c1 Cytochrome c Rieske iron￾sulfur protein 2Fe-2S center Cytochrome b (P side) (N side) bL QP QN bH c1 Heme 8885d_c19_690-750 3/1/04 11:32 AM Page 700 mac76 mac76:385_reb:

8885dc19690-7503/1/0411:32 AM Page701mac76mac76:385 19.1 Electron-Transfer Reactions in Mitochondria dation of Oxidation of Intermembrane Cyt Q QH2. trix (N side) H。 Q+ 2Hp cyt c,(reduced) QH2+ 2Hp+Q+ cyt ci(reduced Net equation: QH2+ 2 cyt cn(oxidized )+ 2HN -)Q+2 cyt ci (red FIGURE 19-12 The Q cycle. The path of electrons through Complex QH2 donates one electron(via the Rieske Fe-S center)to cytochrome Ill is shown by blue arrows. On the P side of the membrane, two mol- C1, and one electron(via cytochrome b) to a molecule of Q near the ecules of QH2 are oxidized to Q near the P side, releasing two pro- N side, reducing it in two steps to QH2. This reduction also uses two tons per Q(four protons in all) into the intermembrane space. Eac Electron transfer through Complex Iv is from cyto- bound to the complex until completely converted to chrome c to the Cua center, to heme a, to the heme water. a3-CuB center, and finally to O2(Fig. 19-14. For every ur electrons passing through this complex, the enzyme The Energy of Electron Transfer Is Efficiently consumes four"substrate"H from the matrix( side) Conserved in a Proton Gradient in converting O2 to 2H,O. It also uses the energy of this redox reaction to pump one proton outward into the in- The transfer of two electrons from NADH through the termembrane space (P side) for each electron that respiratory chain to molecular oxygen can be written as passes through, adding to the electrochemical potential NADH+H++O2→→NAD++H2O roduced by redox-driven proton transport through Complexes I and III. The overall reaction catalyzed by This net reaction is highly exergonic edox pair NAD NADH, E is-0.320 V, and for the pair O2/H2O E°is0.816VThe△E"° for this reaction is therefo 4 Cyt c(reduced)+8H+O2—→ 1. 14 V, and the standard free-energy change(see eqn 4 cyt c(oxidized)+ 4Hp+ 2H20 (19-4) 13-6, p 510)is This four-electron reduction of o involves redox cen- △G°=-n△E° (196 ters that carry only one electron at a time, and it must occur without the release of incompletely reduced =-2965kJN·mol)(1.14V) intermediates such as hydrogen peroxide or hydroxyl -220 k/mol (of NADH) free radicals--very reactive species that would damage This standard free-energy change is based on the as- cellular components. The intermediates remain tightly sumption of equal concentrations (1 M) of NADH and

Electron transfer through Complex IV is from cyto￾chrome c to the CuA center, to heme a, to the heme a3–CuB center, and finally to O2 (Fig. 19–14). For every four electrons passing through this complex, the enzyme consumes four “substrate” H
from the matrix (N side) in converting O2 to 2H2O. It also uses the energy of this redox reaction to pump one proton outward into the in￾termembrane space (P side) for each electron that passes through, adding to the electrochemical potential produced by redox-driven proton transport through Complexes I and III. The overall reaction catalyzed by Complex IV is 4 Cyt c (reduced)
8H
N
O2 On 4 cyt c (oxidized)
4H
P
2H2O (19–4) This four-electron reduction of O2 involves redox cen￾ters that carry only one electron at a time, and it must occur without the release of incompletely reduced intermediates such as hydrogen peroxide or hydroxyl free radicals—very reactive species that would damage cellular components. The intermediates remain tightly bound to the complex until completely converted to water. The Energy of Electron Transfer Is Efficiently Conserved in a Proton Gradient The transfer of two electrons from NADH through the respiratory chain to molecular oxygen can be written as NADH
H

1 2 O2 On NAD

H2O (19–5) This net reaction is highly exergonic. For the redox pair NAD
/NADH, E is 0.320 V, and for the pair O2/H2O, E is 0.816 V. The E for this reaction is therefore 1.14 V, and the standard free-energy change (see Eqn 13–6, p. 510) is G  n E (19–6)  2(96.5 kJ/V  mol)(1.14 V)  220 kJ/mol (of NADH) This standard free-energy change is based on the as￾sumption of equal concentrations (1 M) of NADH and 19.1 Electron-Transfer Reactions in Mitochondria 701 bH bL Cyt c1 Cyt c Oxidation of first QH2 Fe-S Q 2H+ •Q – QH2 Q bH bL Cyt c1 Cyt c Fe-S 2H+ Q Oxidation of second QH2 Matrix (N side) Intermembrane space (P side) QH2 2H+ •Q – QH2 QH2 cyt c1
(oxidized) QH2 2 cyt c1
(oxidized)
2HP
cyt c1
(reduced) Q 2 cyt c1

(reduced)
•Q  •Q 
4HP
2HP
2HN

2HN
QH2 QH2 cyt c1
(oxidized)

Q cyt c1
(reduced) Net equation: FIGURE 19–12 The Q cycle. The path of electrons through Complex III is shown by blue arrows. On the P side of the membrane, two mol￾ecules of QH2 are oxidized to Q near the P side, releasing two pro￾tons per Q (four protons in all) into the intermembrane space. Each QH2 donates one electron (via the Rieske Fe-S center) to cytochrome c1, and one electron (via cytochrome b) to a molecule of Q near the N side, reducing it in two steps to QH2. This reduction also uses two protons per Q, which are taken up from the matrix. 8885d_c19_690-750 3/1/04 11:32 AM Page 701 mac76 mac76:385_reb:

8885ac19690-7503/1/0411:32 AM Page702 6mac76:385 702 Chapter 19 Oxidative Phosphorylation and Photophosphorylation b FIGURE 19-13 Critical subunits of cytochrome oxidase( Complex c-binding site are located in a domain of subunit ll that protrudes Iv). The bovine complex is shown here(PDB ID 10CC).(a) The core from the P side of the inner membrane(into the intermembrane space) of Complex IV, with three subunits. Subunit I (yellow) has two heme Subunit Ill (green)seems to be essential for Complex IV function, but groups, a and a] (red), and a copper ion, CuB (green sphere). Heme its role is not well understood. (b) The binuclear center of CuA.The a3 and Cug form a binuclear Fe-Cu center. Subunit ll (blue)contains Cu ions(green spheres)share electrons equally. When the center is two Cu ions (green spheres)complexed with the--SH groups of two reduced they have the formal charges CuCu+; when oxidized, Cys residues in a binuclear center, CuA that resembles the 2Fe-2S cen- Cu*Cu+ Ligands around the Cu ions include two His(dark blue ters of iron-sulfur proteins. This binuclear center and the cytochrome two Cys (yellow), an Asp(red), and Met(orange) residues. NAD. In actively respiring mitochondria, the actions of many dehydrogenases keep the actual [NADHVNAD ratio well above unity, and the real free-energy change for the reaction shown in Equation 19-5 is therefore substantially greater(more negative) than -220 kJ/mol Intermembrane 4H+ a similar calculation for the oxidation of succinate hows that electron transfer from succinate(E fo (P side) '4Cytc fumarate/succinate=0.031 V to Oe has a smaller, but still negative, standard free-energy change of about 150 k/mol FIGURE 19-14 Path of electrons through Complex IV. The three pro- teins critical to electron flow are subunits L, ll, and ll. The larger green structure includes the other ten proteins in the complex Electron trans- fer through Complex IV begins when two molecules of reduced cy- tochrome c( top)each donate an electron to the binuclear center Cua- From here electrons pass through heme a to the Fe-Cu center(cy tochrome a] and CuB). Oxygen now binds to heme a3 and is reduced to its peroxy derivative(o2 by two electrons from the Fe-Cu center. Delivery of two more electrons from cytochrome c(making four elec in all) converts the O2- molecules of water. with umption of four"substrate"protons from the matrix. At the same time, 4H+ 4H 2H,O four more protons are pumped from the matrix by an as yet unknown (substrate)(pumped) (N side)

NAD
. In actively respiring mitochondria, the actions of many dehydrogenases keep the actual [NADH]/[NAD
] ratio well above unity, and the real free-energy change for the reaction shown in Equation 19–5 is therefore substantially greater (more negative) than 220 kJ/mol. A similar calculation for the oxidation of succinate shows that electron transfer from succinate (E for fumarate/succinate  0.031 V) to O2 has a smaller, but still negative, standard free-energy change of about 150 kJ/mol. 702 Chapter 19 Oxidative Phosphorylation and Photophosphorylation (a) (b) FIGURE 19–13 Critical subunits of cytochrome oxidase (Complex IV). The bovine complex is shown here (PDB ID 1OCC). (a) The core of Complex IV, with three subunits. Subunit I (yellow) has two heme groups, a and a3 (red), and a copper ion, CuB (green sphere). Heme a3 and CuB form a binuclear Fe-Cu center. Subunit II (blue) contains two Cu ions (green spheres) complexed with the OSH groups of two Cys residues in a binuclear center, CuA, that resembles the 2Fe-2S cen￾ters of iron-sulfur proteins. This binuclear center and the cytochrome c–binding site are located in a domain of subunit II that protrudes from the P side of the inner membrane (into the intermembrane space). Subunit III (green) seems to be essential for Complex IV function, but its role is not well understood. (b) The binuclear center of CuA. The Cu ions (green spheres) share electrons equally. When the center is reduced they have the formal charges Cu1
Cu1
; when oxidized, Cu1.5
Cu1.5
. Ligands around the Cu ions include two His (dark blue), two Cys (yellow), an Asp (red), and Met (orange) residues. Subunit II Subunit I 4H+ 4H+ (pumped) Subunit III 4H+ (substrate) 4e– 2H2O O2 4Cyt c CuB CuA Fe-Cu center a3 a Intermembrane space (P side) Matrix (N side) FIGURE 19–14 Path of electrons through Complex IV. The three pro￾teins critical to electron flow are subunits I, II, and III. The larger green structure includes the other ten proteins in the complex. Electron trans￾fer through Complex IV begins when two molecules of reduced cy￾tochrome c (top) each donate an electron to the binuclear center CuA. From here electrons pass through heme a to the Fe-Cu center (cy￾tochrome a3 and CuB). Oxygen now binds to heme a3 and is reduced to its peroxy derivative (O2 2) by two electrons from the Fe-Cu center. Delivery of two more electrons from cytochrome c (making four elec￾trons in all) converts the O2 2 to two molecules of water, with con￾sumption of four “substrate” protons from the matrix. At the same time, four more protons are pumped from the matrix by an as yet unknown mechanism. 8885d_c19_690-750 3/1/04 11:32 AM Page 702 mac76 mac76:385_reb:

8885ac19690-7503/1/0411:32 AM Page703mac76mac76:385 19.1 Electron-Transfer Reactions in Mitochondria Intermembrane space(P side C q NADH H+ NAD+ Succinate Fumarate 2O2+ 2H H2o Matrix(N side) FIGURE 19-15 Summary of the flow of electrons and protons then transfers electrons from reduced cytochrome c to O2 Electron through the four complexes of the respiratory chain. Electrons reach flow through Complexes L, l, and IV is accompanied by proton flow Q through Complexes I and Il QH2 serves as a mobile carrier of elec. from the matrix to the intermembrane space. Recall that electrons from trons and protons. It passes electrons to Complex Ill, which passes B oxidation of fatty acids can also enter the respiratory chain through them to another mobile connecting link, cytochrome c Q(see Fig. 19-8) Much of this energy is used to pump protons out of units more alkaline than that of the intermembrane the matrix. For each pair of electrons transferred to O2, space, so the calculated free-energy change for pump four protons are pumped out by Complex I, four by Com- ing protons outward is about 20 k/mol (of H), most plex Ill, and two by Complex IV (Fig. 19-15). The vec- of which is contributed by the electrical portion of the torial equation for the process is therefore electrochemical potential. Because the transfer of two NADH 1lHN +5O2-NAD+ 10HP+H20(19-7 electrons from NADH to O, is accompanied by the out- ward pumping of 10 H(Egn 19-7, roughly 200 kJ of The electrochemical energy inherent in this difference the 220 kj released by oxidation of a mole of NADH is in proton concentration and separation of charge rep conserved in the proton gradient resents a temporary conservation of much of the energy When protons flow spontaneously down their elec of electron transfer. The energy stored in such a gradi- trochemical gradient, energy is made available to do it, termed the proton-motive force, has two com- work. In mitochondria, chloroplasts, and aerobic bacte- ponents: (1) the chemical potential energy due to the ria, the electrochemical energy in the proton gradient difference in concentration of a chemical species(h) drives the synthesis of ATP from ADP and Pi. We return in the two regions separated by the membrane, and (2) to the energetics and stoichiometry of ATP synthesis the electrical potential energy that results from the driven by the electrochemical potential of the proton separation of charge when a proton moves across the gradient in Section 19.2 membrane without a counterion(Fig. 19-16) As we showed in Chapter ll, the free-energy change for the creation of an electrochemical gradient by an ion pump is +Z Ay (19-8) [HTI =C [HTJ=C1 where C2 and Ci are the concentrations of an ion in two HT OH regions, and C2>Cl; Z is the absolute value of its elec H+ OH trical charge(1 for a proton), and Ay is the transmem- H+ OH brane difference in electrical potential, measured in volts HOHˉ For protons at25°C H+ 2.3(log [H]P- log [H IN) OH =23(pHN-pHp)=23△pH AG=RThn(C2C1)+Z子M and Equation 19-8 reduces to =23 RT ApH+3△ψ FIGURE 19-16 Proton-motive force. The inner mitochondrial mem. △G=23RT△pH+Mψ brane separates two compartments of different [H"I, resulting in dif- =(5.70kJ/mol)△pH+(96.5kN·mol) ferences in chemical concentration(ApH) and charge distribution (AM) across the membrane. The net effect is the proton-motive force In actively respiring mitochondria, the measured A is (AC), which can be calculated as shown here. This is explained more 0. 15 to 0.20 V and the ph of the matrix is about 0. 75 fully

Much of this energy is used to pump protons out of the matrix. For each pair of electrons transferred to O2, four protons are pumped out by Complex I, four by Com￾plex III, and two by Complex IV (Fig. 19–15). The vec￾torial equation for the process is therefore NADH
11H
N
1 2 O2 On NAD

10H
P
H2O (19–7) The electrochemical energy inherent in this difference in proton concentration and separation of charge rep￾resents a temporary conservation of much of the energy of electron transfer. The energy stored in such a gradi￾ent, termed the proton-motive force, has two com￾ponents: (1) the chemical potential energy due to the difference in concentration of a chemical species (H
) in the two regions separated by the membrane, and (2) the electrical potential energy that results from the separation of charge when a proton moves across the membrane without a counterion (Fig. 19–16). As we showed in Chapter 11, the free-energy change for the creation of an electrochemical gradient by an ion pump is G  RT ln

Z  (19–8) where C2 and C1 are the concentrations of an ion in two regions, and C2 C1; Z is the absolute value of its elec￾trical charge (1 for a proton), and  is the transmem￾brane difference in electrical potential, measured in volts. For protons at 25 C, ln
 2.3(log [H
]P  log [H
]N)  2.3(pHN  pHP)  2.3 pH and Equation 19–8 reduces to G  2.3RT pH
 (19–9)  (5.70 kJ/mol)pH
(96.5 kJ/V  mol)∆ In actively respiring mitochondria, the measured ∆ is 0.15 to 0.20 V and the pH of the matrix is about 0.75 C2 C1 C2 C1 units more alkaline than that of the intermembrane space, so the calculated free-energy change for pump￾ing protons outward is about 20 kJ/mol (of H
), most of which is contributed by the electrical portion of the electrochemical potential. Because the transfer of two electrons from NADH to O2 is accompanied by the out￾ward pumping of 10 H
(Eqn 19–7), roughly 200 kJ of the 220 kJ released by oxidation of a mole of NADH is conserved in the proton gradient. When protons flow spontaneously down their elec￾trochemical gradient, energy is made available to do work. In mitochondria, chloroplasts, and aerobic bacte￾ria, the electrochemical energy in the proton gradient drives the synthesis of ATP from ADP and Pi. We return to the energetics and stoichiometry of ATP synthesis driven by the electrochemical potential of the proton gradient in Section 19.2. 19.1 Electron-Transfer Reactions in Mitochondria 703 Intermembrane space (P side) Matrix (N side) 4H+ 1 –2 O2 + 2H+ H2O Succinate Fumarate 4H+ II NADH + H+ NAD+ Cyt c IV 2H+ III I Q FIGURE 19–15 Summary of the flow of electrons and protons through the four complexes of the respiratory chain. Electrons reach Q through Complexes I and II. QH2 serves as a mobile carrier of elec￾trons and protons. It passes electrons to Complex III, which passes them to another mobile connecting link, cytochrome c. Complex IV then transfers electrons from reduced cytochrome c to O2. Electron flow through Complexes I, III, and IV is accompanied by proton flow from the matrix to the intermembrane space. Recall that electrons from
oxidation of fatty acids can also enter the respiratory chain through Q (see Fig. 19–8). N side [H
]N  C1 OH OH OH OH OH OH OH H
H
H
H
H
H
H
P side [H
]P  C2 G  RT ln (C2/C1)
Zℑ   2.3RT pH
ℑ ∆ H
Proton pump   FIGURE 19–16 Proton-motive force. The inner mitochondrial mem￾brane separates two compartments of different [H
], resulting in dif￾ferences in chemical concentration (pH) and charge distribution () across the membrane. The net effect is the proton-motive force (G), which can be calculated as shown here. This is explained more fully in the text. 8885d_c19_690-750 3/1/04 11:32 AM Page 703 mac76 mac76:385_reb:

8885ac19690-7503/1/0411:32 AM Page704mac76mac76:385 704 Chapter 19 Oxidative Phosphorylation and Photophosphorylation Plant mitochondria have Alternative mechanisms Complex Iv, cytochrome oxidase. This for Oxidizing NADH copper-containing enzyme, which also contains cytochromes a and a3, accumulates electrons Plant mitochondria supply the cell with ATP during then passes them to O2, reducing it to H2O periods of low illumination or darkness by mechanisms entirely analogous to those used by nonphotosynthetic a Some electrons enter this chain of carriers organisms. In the light, the principal source of mito- through alternative paths. Succinate is oxidized chondrial NADH is a reaction in which glycine, produced by succinate dehydrogenase(Complex D) by a process known as photorespiration, is converted to which contains a flavoprotein that passes serine(see Fig 20-21) electrons through several Fe-s centers to ubiquinone. Electrons derived from the 2 Glycine+ NAD- serine CO2+ NHs NADH +H oxidation of fatty acids pass to ubiquinone via For reasons discussed in Chapter 20, plants must carry he electron-transferring flavoprotein out this reaction even when they do not need NADH for a Plants also have an alternative, cyanide-resistant ATP production. To regenerate NAD from unneeded NADH oxidation pathway. NADH, plant mitochondria transfer electrons from NADH directly to ubiquinone and from ubiquinone directly to O2, bypassing Complexes Ill and Iv and their proton 19.2 ATP Synthesis pumps. In this process the energy in NADH is dissipated How is a concentration gradient of protons transformed as heat, which can sometimes be of value to the plant into atp? We have seen that electron transfer releases, (Box 19-1). Unlike cytochrome oxidase(Complex Iv) and the proton-motive force conserves, more than the alternative QH2 oxidase is not inhibited by cyanide. enough free energy(about 200 kJ)per"mole"of Cyanide-resistant NADH oxidation is therefore the hall- tron pairs to drive the formation of a mole of ATP, which requires about 50 kJ(see Box 13-1). Mi- SUMMARY 19. 1 Electron-Transfer reactions tochondrial oxidative phospho- in mitochondria rylation therefore poses no thermodynamic problem. But a Chemiosmotic theory provides the intellectual what is the chemical mechanism framework for understanding many biological that couples proton flux with energy transductions, including oxidative phosphorylation? phosphorylation and photophosphorylation. The chemiosmotic model The mechanism of energy coupling is similar in proposed by Peter Mitchell, is both cases: the energy of electron flow is the paradigm for this mecha- conserved by the concomitant pumping of nism. According to the model Peter Mitchell, protons across the membrane, producing an (Fig. 19-17, the electrochemi 1920-1992 electrochemical gradient, the proton-motive cal energy inherent in the difference in proton concen- force tration and separation of charge across the inner mito- a In mitochondria, hydride ions removed from chondrial membrane-the proton- motive force--drives substrates by NAD-linked dehydrogenases the synthesis of ATP as protons flow passively back into donate electrons to the respiratory the matrix through a proton pore associated with ATP (electron-transfer) chain, which transfers the synthase. To emphasize this crucial role of the proton electrons to molecular O2, reducing it to HO motive force, the equation for ATP synthesis is some- a Shuttle systems convey reducing equivalents times written from cytosolic NADH to mitochondrial NADH. ADP+P1+nH→→ATP+H2O+nH(19-10) Reducing equivalents from all NAD-linked Mitchell used"chemiosmotic to describe enzymatic re- dehydrogenations are transferred to mito- actions that involve, simultaneously, a chemical reaction chondrial NAdh dehydrogenase(Complex D) and a transport process. The operational definition of a Reducing equivalents are then passed through coupling "is shown in Figure 19-18. When isolated mi- a series of Fe-s centers to ubiquinone, which tochondria are suspended in a buffer containing ADP transfers the electrons to cytochrome b, the Pi, and an oxidizable substrate such as succinate, three first carrier in Complex I. In this complex, easily measured processes occur: (1)the substrate is electrons take two separate paths through two oxidized ( succinate yields fumarate), (2)O2 is consumed, b-type cytochromes and cytochrome ci to an and (3) ATP is synthesized. Oxygen consumption and Fe-S center. The Fe-S center passes electrons ATP synthesis depend on the presence of an oxidizable one at a time through cytochrome c and into substrate(succinate in this case) as well as ADP and P

Plant Mitochondria Have Alternative Mechanisms for Oxidizing NADH Plant mitochondria supply the cell with ATP during periods of low illumination or darkness by mechanisms entirely analogous to those used by nonphotosynthetic organisms. In the light, the principal source of mito￾chondrial NADH is a reaction in which glycine, produced by a process known as photorespiration, is converted to serine (see Fig. 20–21): 2 Glycine
NAD
88n serine
CO2
NH3
NADH
H
For reasons discussed in Chapter 20, plants must carry out this reaction even when they do not need NADH for ATP production. To regenerate NAD
from unneeded NADH, plant mitochondria transfer electrons from NADH directly to ubiquinone and from ubiquinone directly to O2, bypassing Complexes III and IV and their proton pumps. In this process the energy in NADH is dissipated as heat, which can sometimes be of value to the plant (Box 19–1). Unlike cytochrome oxidase (Complex IV), the alternative QH2 oxidase is not inhibited by cyanide. Cyanide-resistant NADH oxidation is therefore the hall￾mark of this unique plant electron-transfer pathway. SUMMARY 19.1 Electron-Transfer Reactions in Mitochondria ■ Chemiosmotic theory provides the intellectual framework for understanding many biological energy transductions, including oxidative phosphorylation and photophosphorylation. The mechanism of energy coupling is similar in both cases: the energy of electron flow is conserved by the concomitant pumping of protons across the membrane, producing an electrochemical gradient, the proton-motive force. ■ In mitochondria, hydride ions removed from substrates by NAD-linked dehydrogenases donate electrons to the respiratory (electron-transfer) chain, which transfers the electrons to molecular O2, reducing it to H2O. ■ Shuttle systems convey reducing equivalents from cytosolic NADH to mitochondrial NADH. Reducing equivalents from all NAD-linked dehydrogenations are transferred to mito￾chondrial NADH dehydrogenase (Complex I). ■ Reducing equivalents are then passed through a series of Fe-S centers to ubiquinone, which transfers the electrons to cytochrome b, the first carrier in Complex III. In this complex, electrons take two separate paths through two b-type cytochromes and cytochrome c1 to an Fe-S center. The Fe-S center passes electrons, one at a time, through cytochrome c and into Complex IV, cytochrome oxidase. This copper-containing enzyme, which also contains cytochromes a and a3, accumulates electrons, then passes them to O2, reducing it to H2O. ■ Some electrons enter this chain of carriers through alternative paths. Succinate is oxidized by succinate dehydrogenase (Complex II), which contains a flavoprotein that passes electrons through several Fe-S centers to ubiquinone. Electrons derived from the oxidation of fatty acids pass to ubiquinone via the electron-transferring flavoprotein. ■ Plants also have an alternative, cyanide-resistant NADH oxidation pathway. 19.2 ATP Synthesis How is a concentration gradient of protons transformed into ATP? We have seen that electron transfer releases, and the proton-motive force conserves, more than enough free energy (about 200 kJ) per “mole” of elec￾tron pairs to drive the formation of a mole of ATP, which requires about 50 kJ (see Box 13–1). Mi￾tochondrial oxidative phospho￾rylation therefore poses no thermodynamic problem. But what is the chemical mechanism that couples proton flux with phosphorylation? The chemiosmotic model, proposed by Peter Mitchell, is the paradigm for this mecha￾nism. According to the model (Fig. 19–17), the electrochemi￾cal energy inherent in the difference in proton concen￾tration and separation of charge across the inner mito￾chondrial membrane—the proton-motive force—drives the synthesis of ATP as protons flow passively back into the matrix through a proton pore associated with ATP synthase. To emphasize this crucial role of the proton￾motive force, the equation for ATP synthesis is some￾times written ADP
Pi
nH
P On ATP
H2O
nH
N (19–10) Mitchell used “chemiosmotic” to describe enzymatic re￾actions that involve, simultaneously, a chemical reaction and a transport process. The operational definition of “coupling” is shown in Figure 19–18. When isolated mi￾tochondria are suspended in a buffer containing ADP, Pi , and an oxidizable substrate such as succinate, three easily measured processes occur: (1) the substrate is oxidized (succinate yields fumarate), (2) O2 is consumed, and (3) ATP is synthesized. Oxygen consumption and ATP synthesis depend on the presence of an oxidizable substrate (succinate in this case) as well as ADP and Pi . 704 Chapter 19 Oxidative Phosphorylation and Photophosphorylation Peter Mitchell, 1920–1992 8885d_c19_690-750 3/1/04 11:32 AM Page 704 mac76 mac76:385_reb: