Mechanisms of inter- and intra-molecular electron transfer in cytochromes

Abstract

The reduction of cyt(III)-c by a variety of reducing free radicals in aqueous solutions was investigated to gain an insight into the pathways involved in electron transfer. Most of the free radicals investigated reduce cyt(III)-c with high rate constants (10⁷–10⁹ dm³ mol^–1s^–1). Several radicals (e.g., ethylene glycol, glycerol and serine), ordinarily considered to be good reductants, do not reduce cyt(III)-c, k < 10² dm³ mol^–1 s^–1. The structure and charge of the radicals and the conformation of cytochrome influence the rate of electron transfer. Reaction of about half the OH radicals with cyt(III)-c leads to reduction by attack at the protein coat. Two separate electron transfer processes occur subsequently, the rate constants, k₁= 2 × 10⁵ s^–1 and k₂= 3 × 10⁴ s^–1, being concentration independent. The extent of cyt(III)-c reduction by these free radicals does not correlate with their rate constants. G(reduction) ranged from 0.0 to 6.5. Reduction by ·CO^–₂ is stoichiometric and produces a cyt(III)-c spectrum having a smaller absorption at 270 nm region than is found classically. Reduction by ·His–OH, ·Met–OH and α-peptide radicals [graphic omitted] is effective, while ·Phe-OH is less effective. These result indicate that electron reduction involves several pathways and is concomitant with free radical reactions not involving the metal centre.