Proton and electrical charge translocation by cytochrome-c oxidase

O-2 is reduced to 2 H2O in the cytochrome-c oxidase reaction. The four protons consumed are taken from the inside (i phase) of the inner mitochondrial membrane, whereas the electrons are taken from cytochrome c on the opposite side to phase). These electron and proton transfers contribute 50% to the generation of Delta mu(H)+ across the membrane. The other 50% comes from the 4 H+ that are translocated ('pumped') across the membrane per O-2 reduced. In this paper we assess the number of electrical charges translocated in the different discrete steps of the catalytic cycle, by assessing the linkage of each step with electron transfer, proton uptake/release, and proton pumping. Delta mu(H)+ and pH(i) determine the number of protons bound to the enzyme from the i phase. At high Delta mu(H)+ these protons may dissociate, so that the number of protons bound to a discrete state of the enzyme may be lower than at Delta mu(H)+ = 0 This analysis may reconcile the difference between proton uptake stoichiometries previously determined for the isolated enzyme (Mitchell, R. and Rich, P.R. [1994] Biochim. Biophys. Acta 1186, 19-26) with those measured at high Delta mu(H)+ in intact mitochondria (Wikstrom, M. [1988] Chem. Scr. 28A, 71-74).