Reduction of cytochrome c peroxidase compounds I and II by ferrocytochrome c. A stopped-flow kinetic investigation

The oxidation of yeast cytochrome c peroxidase by hydrogen peroxide produces a unique enzyme intermediate, cytochrome c peroxidase Compound I, in which the ferric heme iron has been oxidized to an oxyferryl state, Fe(IV), and an amino acid residue has been oxidized to a radical state. The reduction of cytochrome c peroxidase Compound I by horse heart ferrocytochrome c is biphasic in the presence of excess ferrocytochrome c as cytochrome c peroxidase Compound I is reduced to the native enzyme via a second enzyme intermediate, cytochrome c peroxidase Compound II. In the first phase of the reaction, the oxyferryl heme iron in Compound I is reduced to the ferric state producing Compound II which retains the amino acid free radical. The pseudo-first order rate constant for reduction of Compound I to Compound II increases with increasing cytochrome c concentration in a hyperbolic fashion. The limiting value at infinite cytochrome c concentration, which is attributed to the intracomplex electron transfer rate from ferrocytochrome c to the heme site in Compound I, is 450 +/- 20 s-1 at pH 7.5 and 25 degrees C. Ferricytochrome c inhibits the reaction in a competitive manner. The reduction of the free radical in Compound II is complex. At low cytochrome c peroxidase concentrations, the reduction rate is 5 +/- 3 s-1, independent of the ferrocytochrome c concentration. At higher peroxidase concentrations, a term proportional to the square of the Compound II concentration is involved in the reduction of the free radical. Reduction of Compound II is not inhibited by ferricytochrome c. The rates and equilibrium constant for the interconversion of the free radical and oxyferryl forms of Compound II have also been determined.