How Hydrogen Peroxide Is Metabolized by Oxidized Cytochrome c Oxidase

[Image: see text] In the absence of external electron donors, oxidized bovine cytochrome c oxidase (CcO) exhibits the ability to decompose excess H(2)O(2). Depending on the concentration of peroxide, two mechanisms of degradation were identified. At submillimolar peroxide concentrations, decomposition proceeds with virtually no production of superoxide and oxygen. In contrast, in the millimolar H(2)O(2) concentration range, CcO generates superoxide from peroxide. At submillimolar concentrations, the decomposition of H(2)O(2) occurs at least at two sites. One is the catalytic heme a(3)–Cu(B) center where H(2)O(2) is reduced to water. During the interaction of the enzyme with H(2)O(2), this center cycles back to oxidized CcO via the intermediate presence of two oxoferryl states. We show that at pH 8.0 two molecules of H(2)O(2) react with the catalytic center accomplishing one cycle. In addition, the reactions at the heme a(3)–Cu(B) center generate the surface-exposed lipid-based radical(s) that participates in the decomposition of peroxide. It is also found that the irreversible decline of the catalytic activity of the enzyme treated with submillimolar H(2)O(2) concentrations results specifically from the decrease in the rate of electron transfer from heme a to the heme a(3)–Cu(B) center during the reductive phase of the catalytic cycle. The rates of electron transfer from ferrocytochrome c to heme a and the kinetics of the oxidation of the fully reduced CcO with O(2) were not affected in the peroxide-modified CcO.