Proton and electrical charge translocation by cytochrome-c oxidase

AbstractO2 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 (o phase). These electron and proton transfers contribute 50% to the generation of ΔμH+ across the membrane. The other 50% comes from the 4 H+ that are translocated (`pumped') across the membrane per O2 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. ΔμH+ and pHi determine the number of protons bound to the enzyme from the i phase. At high ΔμH+ these protons may dissociate, so that the number of protons bound to a discrete state of the enzyme may be lower than at ΔμH+=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 ΔμH+ in intact mitochondria (Wikström, M. [1988] Chem. Scr. 28A, 71–74)