Abstract
In at least one component of the mitochondrial respiratory chain, cytochrome oxidase, exothermic electron transfer reactions are used to drive vectorial proton transport against an electrochemical hydrogen ion gradient across the mitochondrial inner membrane. The role of the gating of electrons (the regulation of the rates of electron transfer into and out of the proton transport site) in this coupling between electron transfer andproton pumping has been explored. The approach involves the solution of the steady-state rate equations pertinent to proton pump models which include, to various degrees, the uncoupled (i.e.,not linked to proton pumping) electron transfer processes which are likely to occur in any real electrontransfer-driven proton pump. This analysis furnishes a quantitative framework for examining the effects of variations in proton binding site pKas and metal center reduction potentials, the relationship between energy conservation efficiency and turnover rate, the conditions for maximum power out put or minimum heat production, and required eficiency of the gating of electrons. Some novel conclusions emerge from the analysis, including: (a) An efficient electron transfer-driven proton pump need not exhibit a pH-dependent reduction potential; (b) Very efficient gating of electrons is required for efficient electron transfer driven proton pumping, especially when a reasonable correlation of electrontransfer rateand electrontransfer exoergonicity is assumed; and (c)A consideration of the importanceand possible mechanisms of the gating of electrons suggests that efficient proton pumping by CUA in cytochrome oxidase could, in principle, take place with structural changes confined to the immediate vicinity of the copperion, while proton pumping by Fea would probably require conformational coupling between the iron and more remote structures in the enzyme. The conclusions are discussed with reference to proton pumping by cytochrome c oxidase, and some possible implications for oxidative phosphorylation are noted.