8885dc19690-7503/1/0411:32 AM Page698mac76mac76:385 Chapter 19 Oxidative Phosphorylation and Photophosphorylation Amytal (a barbiturate drug), rotenone (a plant product commonly used as an insecticide), and piericidin A(an Complex I Intermembrane antibiotic) inhibit electron flow from the Fe-s centers space(P side of Complex I to ubiquinone (table 19-4) and therefore block the overall process of oxidative phosphorylation Ubiquinol (QH2, the fully reduced form; Fig. 19-2) - QH diffuses in the inner mitochondrial membrane from Fe-S Complex I to Complex Ill, where it is oxidized to Q in a process that also involves the outward movement of H Matrix FMN Matrix(N side) Complex Il: Succinate to Ubiquinone We encountered Complex II in Chapter 16 as succinate dehydroge nase, the only membrane-bound enzyme in the citric acid cycle(p. 612). Although smaller and simpler than NAD++h+ Complex I, it contains five prosthetic groups of two types and four different protein subunits(Fig. 19-10) FIGURE 19-9 NADH: ubiquinone oxidoreductase(Complex D. Com- Subunits C and D are integral membrane proteins, each plex I catalyzes the transfer of a hydride ion from NADH to FMN, from which two electrons pass through a series of Fe-S centers to the iron- with three transmembrane helices. They contain a heme ulfur protein N-2 in the matrix arm of the complex. Electron transfer group, heme b, and a binding site for ubiquinone, the from N-2 to ubiquinone on the membrane arm forms QH2, which dif- final electron acceptor in the reaction catalyzed by fuses into the lipid bilayer. This electron transfer also drives the ex- Complex I Subunits A and B extend into the matrix(or pulsion from the matrix of four protons per pair of electrons. The de. the cytosol of a bacterium); they contain three 2Fe-2S tailed mechanism that couples electron and proton transfer in centers, bound FAD, and a binding site for the substrate I is not yet known, but probably involves a Q cycle similar succinate. The path of electron transfer from the Complex Ill in which QH2 participates twice per electron succinate-binding site to FAD, then through the Fe-s -12)Proton flux produces an electrochemical potential across centers to the Q-binding site, is more than 40 A long, nner mitochondrial membrane(N side negative, P side positive), but none of the individual electron-transfer distances conserves some of the energy released by the electron-transfer exceeds about 11 A-a reasonable distance for rapid reactions. This electrochemical pe electron transfer (Fig. 19-10) TABLE 19-4 Agents That Interfere with Oxidative Phosphorylation or Photophosphorylation type of interference Compound larget / mode of action Inhibition of electron transfer Cyanide Inhibit cytochrome oxidase from cytochrome b to cytochrome c1 Rotenone Amita Prevent electron transfer from Fe-s center to ubiquinone ompetes with QB for binding site in PSIl Inhibition of ATP synthase Aurovertin Inhibits F1 Inhibit Fo and CF Blocks proton flow through Fo and CFo Hydrophobic proton carrie In brown fat, forms proton-conducting pores in inner mitochondrial Inhibition of ATP-ADP exchange Atractyloside Inhibits adenine nucleotide translocaseAmytal (a barbiturate drug), rotenone (a plant product commonly used as an insecticide), and piericidin A (an antibiotic) inhibit electron flow from the Fe-S centers of Complex I to ubiquinone (Table 19–4) and therefore block the overall process of oxidative phosphorylation. Ubiquinol (QH2, the fully reduced form; Fig. 19–2) diffuses in the inner mitochondrial membrane from Complex I to Complex III, where it is oxidized to Q in a process that also involves the outward movement of H
. Complex II: Succinate to Ubiquinone We encountered Complex II in Chapter 16 as succinate dehydroge￾nase, the only membrane-bound enzyme in the citric acid cycle (p. 612). Although smaller and simpler than Complex I, it contains five prosthetic groups of two types and four different protein subunits (Fig. 19–10). Subunits C and D are integral membrane proteins, each with three transmembrane helices. They contain a heme group, heme b, and a binding site for ubiquinone, the final electron acceptor in the reaction catalyzed by Complex II. Subunits A and B extend into the matrix (or the cytosol of a bacterium); they contain three 2Fe-2S centers, bound FAD, and a binding site for the substrate, succinate. The path of electron transfer from the succinate-binding site to FAD, then through the Fe-S centers to the Q-binding site, is more than 40 Å long, but none of the individual electron-transfer distances exceeds about 11 Å—a reasonable distance for rapid electron transfer (Fig. 19–10). 698 Chapter 19 Oxidative Phosphorylation and Photophosphorylation Complex I Intermembrane space (P side) Matrix (N side) 2H+ 4H+ Fe-S FMN NADH NAD+
H+ 2e– 2e– N-2 Q QH2 Matrix arm Membrane arm FIGURE 19–9 NADH:ubiquinone oxidoreductase (Complex I). Com￾plex I catalyzes the transfer of a hydride ion from NADH to FMN, from which two electrons pass through a series of Fe-S centers to the iron￾sulfur protein N-2 in the matrix arm of the complex. Electron transfer from N-2 to ubiquinone on the membrane arm forms QH2, which dif￾fuses into the lipid bilayer. This electron transfer also drives the ex￾pulsion from the matrix of four protons per pair of electrons. The de￾tailed mechanism that couples electron and proton transfer in Complex I is not yet known, but probably involves a Q cycle similar to that in Complex III in which QH2 participates twice per electron pair (see Fig. 19–12). Proton flux produces an electrochemical potential across the inner mitochondrial membrane (N side negative, P side positive), which conserves some of the energy released by the electron-transfer reactions. This electrochemical potential drives ATP synthesis. TABLE 19–4 Agents That Interfere with Oxidative Phosphorylation or Photophosphorylation Type of interference Compound* Target/mode of action Inhibition of electron transfer Cyanide Carbon monoxide Antimycin A Blocks electron transfer from cytochrome b to cytochrome c1 Myxothiazol Rotenone Amytal Piericidin A DCMU Competes with QB for binding site in PSII Inhibition of ATP synthase Aurovertin Inhibits F1 Oligomycin Venturicidin DCCD Blocks proton flow through Fo and CFo Uncoupling of phosphorylation FCCP from electron transfer DNP Hydrophobic proton carriers Valinomycin K
ionophore Thermogenin In brown fat, forms proton-conducting pores in inner mitochondrial membrane Inhibition of ATP-ADP exchange Atractyloside Inhibits adenine nucleotide translocase * DCMU is 3-(3,4-dichlorophenyl)-1,1-dimethylurea; DCCD, dicyclohexylcarbodiimide; FCCP, cyanide-p-trifluoromethoxyphenylhydrazone; DNP, 2,4-dinitrophenol. Inhibit cytochrome oxidase Prevent electron transfer from Fe-S center to ubiquinone Inhibit Fo and CFo               8885d_c19_690-750 3/1/04 11:32 AM Page 698 mac76 mac76:385_reb: