Cytochrome c oxidase is a molecular machine that transduces respiratory energy into an electrochemical proton gradient across the mitochondrial or bacterial membrane. Reduction of O2 to water takes place at a binuclear haem-copper site, to which four electrons are transferred from cytochrome c on the outer side of the membrane, and four protons from the inside. An additional and equal contribution to Δμ(H+) generation comes from so- called proton pumping, where four H+ ions are translocated, across the membrane per O2 reduced. Some recent progress in research on the mechanism of energy transduction by this class of enzymes is reviewed.
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