CytcO (cytochrome c oxidase) is a membrane-bound multisubunit protein which catalyses the reduction of O2 to H2O. The reaction is arranged topographically so that the electrons and protons are taken from opposite sides of the membrane and, in addition, it is also linked to proton pumping across the membrane. Thus the CytcO moves an equivalent of two positive charges across the membrane per electron transferred to O2. Proton transfer through CytcO must be controlled by the protein to prevent leaks, which would dissipate the proton electrochemical gradient that is maintained across the membrane. The molecular mechanism by which the protein controls the unidirectionality of proton-transfer (cf. proton diode) reactions and energetically links electron transfer to proton translocation is not known. This short review summarizes selected results from studies aimed at understanding this mechanism, and we discuss a possible mechanistic principle utilized by the oxidase to pump protons.

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