Horseradish peroxidase C

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Abstract
Horseradish peroxidase C (HRP; ferric) reacts with H2O2 to form Compound I, with an equilibrium constant of about 1014 M−1. Two-step reduction of Compound I to Compound II and further to the ferric enzyme occurs reversibly at Eo′ values of 0.90 and 0.93 V (pH 7.0), respectively. The pH dependence of Eo′ values for each one-electron step, ferrous → ferric → Compound II → Compound I indicates the presence of redox-linked ionization at pKa values of 7.3 in the ferrous state, 11.0 in the ferric and 8.6 in Compound II. Zinc-substituted HRP C is oxidized to its free-radical form at an Eo′ value of 0.74 (pH 6.0). Comparison of oxidized zinc HRP C with Compound I shows that Compound I contains a porphyrin π-cation radical. The flash photolysis study on the NO-ferric HRP C complex clearly indicates that the iron is pentacoordinated in HRP C while it is hexacoordinated in metmyoglobin. From the kinetic analysis of the acid-alkaline conversion of HRP C, the second-order rate constants of the reactions with H+ and HO− are estimated to be 1.5 × 1010 and 6.7 × 104 M−1s−1, respectively. The latter rate constant greatly varies with the kind of hemoproteins. In the presence of HRP C and O2, indole-3-acetate is oxidized to its hydroperoxide form, which reacts effectively with HRP C to form Compound I and further converts Compound I to a verdohemoprotein.