Cytochrome oxidase: pathways for electron tunneling and proton transfer

Abstract

Electrons from cytochrome c, the substrate of cytochrome oxidase, a redox-linked proton pump, are accepted by CuA in subunit II. From there they are transferred to the proton pumping machinery in subunit I, cytochrome a and cytochrome a3–CuB. The reduction of the latter site, which is the dioxygen reducing unit, is coupled to proton uptake. Dioxygen reduction involves a peroxide and a ferryl ion intermediate, and it is the transition between these and back to the resting oxidized enzyme that are coupled to proton pumping. The X-ray structures suggest electron–transfer pathways that can account for the observed rates provided that the reorganization energies are small. They also reveal two proton-transfer pathways, and mutagenesis experiments have shown that one is used for proton uptake during the initial reduction of cytochrome a3–CuB, whereas the other mediates transfer of the pumped protons.