Complex IV, known as cytochrome c oxidase, or cytochrome oxidase, exists as a dimer in the IMM. It oxidizes the mobile cytochrome c, and conducts electrons through cytochromes a and a3, finally reducing oxygen to water in a four-electron transfer reaction (Fig. 8.10). Copper is a common component of this and other oxidase enzymes. Small molecule poisons, such as azide, cyanide, and carbon monoxide, bind to the heme group of cytochrome a3 in cytochrome c oxidase and inhibit complex IV. In common with complexes I and III, the cytochrome oxidase complex pumps protons out of mitochondria, providing for the synthesis of about one mole of ATP per pair of electrons transferred to oxygen. The actual number of protons pumped is four. In addition, another four are required in the reduction of O2 to water. The overall reaction catalyzed by complex IV is:
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