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: