Water-gated mechanism of proton translocation by cytochrome c oxidase

摘要

Cytochrome c oxidase is essential for aerobic life as a membrane-bound energy transducer. O_2 reduction at the haem a_3-Cu_B centre consumes electrons transferred via haem a from cytochrome c outside the membrane. Protons are taken up from the inside, both to form water and to be pumped across the membrane (M.K.F. Wikstrom, Nature 266 (1977) 271 [1]; M. Wikstrom, K. Krab, M. Saraste, Cytochrome Oxidase, A Synthesis, Academic Press, London, 1981 [2]). The resulting electrochemical proton gradient drives ATP synthesis (P. Mitchell, Chemiosmotic Coupling in Oxidative and Photosynthetic Phosphorylation, Glynn Research, Bodmin, UK, 1966 [3]). Here we present a molecular mechanism for proton pumping coupled to oxygen reduction that is based on the unique properties of water in hydrophobic cavities. An array of water molecules conducts protons from a conserved glutamic acid, either to the Δ-propionate of haem a_3 (pumping), or to haem a_3-Cu_B (water formation). Switching between these pathways is controlled by the redox-state-dependent electric field between haem a and heam a_3-Cu_B, which determines the water-dipole orientation, and therefore the proton transfer direction. Proton transfer via the propionate provides a gate to O_2 reduction. This pumping mechanism explains the unique arrangement of the metal cofactors in the structure. It is consistent with the large body of biochemical data, and is shown to be plausible by molecular dynamics simulations.