Reaction of hydrogen peroxide and peroxidase activity in carboxymethylated cytochrome c: spectroscopic and kinetic studies

Abstract

The peroxidase activity of carboxymethylated cytochrome c (Cmcytc) has been investigated by spectroscopic and kinetic techniques to examine the effect of carboxymethylation on the peroxidase activity of native cytochrome c (cytc). The optical spectrum suggests that the reaction of Cmcytc with H₂O₂ proceeds through only one intermediate, compound I. The apparent rate constant (kapp) for the reaction was found to be 17, 72 and 210 M⁻¹ s⁻¹ at pH 7.0, 5.0 and 3.5 respectively. These values are about 60 times larger than those reported for native cytc (0.236 M⁻¹ s⁻¹ at pH 7.0), and about five orders of magnitude lower than those for classical peroxidases. Cmcytc was found to catalyse oxidation of organic and inorganic substrates. The second order rate constant for the oxidation of 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS) by Cmcytc (205 [H₂O₂] s⁻¹) is found to be larger than the corresponding value for native cytc (50 [H₂O₂] s⁻¹) at pH 6.0. The carboxymethylation of cytc ruptures the Fe-S (Met 80) bond and increases the rate of its reaction with H₂O₂, and its catalytic activity. The specific activity of Cmcytc was measured spectrophotometrically by the reported method using ABTS as substrate, and was found to be 288, 473 and 872 μM min⁻¹ mg⁻¹ at pH 7.0, 5.0 and 3.5 respectively. Resonance Raman studies indicated the presence of a bis-histidine coordinated form of Cmcytc at neutral pH, and the existence of a population distribution of different ligation states such as bis-histidine (HH), histidine-water (HW) and five coordinate (5C) forms at lower pH. The relative population of different species in Cmcytc was found to be HH (~100%, ~50%, ~44%), HW (~0%, ~44%, 41%) and 5C (~0%, ~6%,15%) at pH 7.0, 4.7 and 3.1 respectively. We have attempted to correlate the pH dependence of the reaction of Cmcytc with hydrogen peroxide and its peroxidase activity with the haem stereochemical structures observed for Cmcytc. Steady-state and time-resolved tryptophan fluorescence studies on Cmcytc were done to probe the conformational changes around the haem pocket of Cmcytc.