Kinetics of yeast cytochrome c peroxidase compound I formation with modified substrates (peroxybenzoic acids)

The kinetics of formation of Compound I of yeast cytochrome c peroxidase (ferrocytochrome c:hydrogen-peroxide oxidoreductase, EC 1.11.1.5) with a series of peroxybenzoic acids were studied. Reactivity is affected not only by protein ionization, as in the reaction with H2O2, but also by substrate ionization. The reactivity of negatively charged substrates is markedly lower than that of uncharged species, implying that electrostatic factors profoundly influence substrate binding. The rate constants for neutral peroxybenzoic acids carrying electron-withdrawing substituents increase with increasing peroxy acid pKa. This behaviour suggests that, as previously discussed for reactions of turnip peroxidases, formation of peroxy anion by ionization of substrate within the active site is kinetically important. The results support the mechanism of cytochrome c peroxidase Compound I formation which has been proposed by Poulos and Kraut (J. Biol. Chem. 225 (1980), 8199-8205) on the basis of enzyme structural studies.