| Abstract: | Rates of yeast cytochrome c peroxidase (ferrocytochrome c:hydrogen-peroxide oxidoreductase, EC 1.11.1.5) catalyzed oxidation of bis(tripyridine)cobalt(II) ion, penta(amine)pyridineruthenium(II) ion and ferrocyanide ion by hydrogen peroxide have been found to obey the empirical equation: (formula; see text) in the pH range 5 to 8, and at saturating H2O2 concentrations. ( S] and CcP] are the concentrations of the reductant and the enzyme, respectively.) Values of k2 were found to be independent of the reductant. The term k0S] is only significant with the cobalt and ruthenium complexes at high pH. The mechanism proposed to account for this rate equation differs significantly from previous mechanistic proposals. In particular, the rate data require the assignment of the rate-limiting step at high substrate concentrations to a slow electron-transfer within the enzyme, and not, as previously suggested, to saturation of substrate binding to the enzyme. Also, the term k0S] implies that the reactive substrates, including the natural substrate (yeast cytochrome c), react with the hydrogen peroxide-heme complex and not with the radical species formed by reaction with hydrogen peroxide in the absence of reductants. |
