QH2-cytochrome c reductase complex

After cytochrome c is reduced by the QH2-cytochrome c reductase complex, it is reoxidized by the cytochrome c oxidase complex, which transfers electrons to oxygen. As noted earlier, cytochrome c oxidase contains three copper ions and two heme groups (see Figure 8-18). The flow of electrons through these carriers is depicted in Figure 8-20. Four molecules of reduced cytochrome c bind, one at a time, to a site on subunit 11 of the oxidase. An electron is transferred from the heme of each cytochrome c, first to Cug bound to subunit 11, then to the heme a bound to subunit I, and finally to the Cub and heme that make up the oxygen reduction center. 

NADH and FADHg are produced as a result of substrate level dehydrogenations. Oxidation of these reduced coenzymes by oxygen is accomplished by the intervention of a series of electron carriers between the primary reductant and the terminal oxidant (Fig. 2). The electron-transport components represent redox couples of increasing redox potential and are therefore favored thermodynamically. The respiratory chain can be separated into four multienzyme complexes NADH-Q reductase (complex I), succinate-Q reductase (complex II), QH2"Cytochrome c reductase (complex III), and cytochrome c oxidase (complex IV). At each of these successive oxidation-reduction steps, a certain amount of free energy is available, the amount being determined by the difference in the oxidation-reduction potential of the two sequential components. The difference in the redox potential between... 

QH2-cytochrome c reductase (complex III), also termed the cytochrome b-Ci complex, has a molecular weight of approximately 300,000, with the protein components accoimting for 240,000 of this. It is comprised of 6-8 polypeptides including cytochrome b, cytochrome Ci, an iron-sulfur protein, an antimycin-binding protein, and a core protein (Rieske, 1976). 

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