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: