A Kinetic Study of the Cytochrome c-Hydrogen Peroxide Reaction

Abstract

The kinetics of the reaction of cytochromecwith H2O2was studied in aqueous phosphate media near physiological pH (5.7-7.2). The UV-VIS spectra indicated that the heme prosthetic group of the protein suffered an oxidative degradation during the reaction. The experimental rate law wasv=kexp[cytochromec] [H2O2], withkexp= (A+B[H+])/(1 +C[H+]). The apparent activation parameters associated withkexpwereEa= 46 ± 2 kJ mol-1, ∆Ho≠= 43 ± 2 kJ mol-1and ∆So≠= -111 ± 5 J K-1mol-1. The reaction showed base catalysis and was also catalyzed by both CrO42-and acrylamide. It was inhibited by the oxidants WO42-, MoO42-, VO3-and [Fe(CN)6]3-as well as by the reductants [Fe(CN)6]4-and D-mannitol, whereas Zn2+and superoxide dismutase had no appreciable effect. The results are consistent with initial reduction of iron from Fe(III) to Fe(II) (supported by inhibition caused by most oxidants), followed by a hydroxyl-radical mediated oxidative degradation of the heme prosthetic group (supported by the inhibition caused by both [Fe(CN)6]4-and D-mannitol).