Modeling the sequence of electron transfer reactions in the single turnover of reduced, mammalian cytochrome c oxidase with oxygen

Abstract

Single-turnover studies of the reaction of reduced cytochrome oxidase with oxygen has led to a mechanistic model that specifies a linear sequence of electron transfer from cytochrome c to O2 via the four redox active metals of mitochondrial cytochrome oxidase. Fully reduced oxidase initially forms a dioxygen adduct within which two electrons are transferred at a rate of 6 x 10(4) s-1 to form a peroxy adduct. This two-electron step results in single-electron oxidation of cytochrome alpha 3 and cytochrome alpha. Cytochrome alpha is then re-reduced by CuA in an intramolecular reaction. Subsequent reoxidation of cytochrome alpha occurs at a rate nearly 100-fold slower than its initial oxidation. Oxidation of CuB in this sequence has been the most difficult to determine and here is specified as the third electron transfer step. This reaction sequence is unchanged in the presence of tightly bound cytochrome c, although cytochrome c is rapidly oxidized via CuA and cytochrome alpha. The linear model advanced here has CuA as the acceptor of electrons from cytochrome c, with cytochrome alpha serving as a bridge to deliver electrons from CuA (and cytochrome c) to the binuclear center, cytochrome alpha 3-CuB.