The oxidation-reduction potentials of compound I/compound II and compound II/ferric couples of horseradish peroxidases A2 and C.

The reversibility of the stepwise reduction of Compound I to the ferric state via Compound II was confirmed in horseradish peroxidases A2 and C. The values of E'o (compound I/Compound II) and E'O (Compound II/ferric) were measured from equilibrium data coupled with the K2IrCl6-K3IrCl6 system in a narrow region of pH near 6.3. The ferric enzymes were also oxidized by ferricyanide to Compound II at alkaline pH and the values of E'O (Compound II/ferric) were measured from the equilibrium data. The pH dependence of E'O (Compound II/ferric) was in accord with the equation: E'O = EO + 0.058 log (Kr[H+] + [H+]2)/(KO + [H+]), where Kr and KO are proton dissociation constants in the ferric enzyme and Compound II, respectively. The pH-E'O (Compound I/Compound II) curves were likewise obtained from the equation, E'O = EO + 0.058 log (Kr + [H+]), where Kr is the proton dissociation constant in Compound II. The forward and backward rate constants were measured in each of one-electron transfer reactions of the peroxidases with the K2IrCl6-K3IrCl6 system at various pH values. The E'O values calculated on the assumption that the ratio of the rate constants equals the equilibrium constant were compared with those obtained from the equilibrium data.