Second order kinetics of the reduction of cytochrome c2 by the ubiquinone cytochrome b-c2 oxidoreductase of Rhodopseudomonas sphaeroides.

The direct reductant of ferricytochrome c2 in the ubiquinone cytochrome b-c2 oxidoreductase of Rhodopseudomonas sphaeroides is a component (Z), with an oxidation-reduction midpoint potential of 155 mV at pH i’, which requires 2 electrons and 2 protons for its equilibrium reduction. The reduction of the ferricytochrome is first order in both ferricytochrome and reductant, indicating that the reaction has second order characteristics. The behavior of the half-time of a second order reaction as the ratio of reactants is varied experimentally has allowed an estimation of the concentration of the reductant in the membrane and thus an estimation of the apparent second order rate constant of the reaction. The ratio of reductant to electron transfer systems (which each contain a pair of cytochromes ~2) is 0.8 + 0.1, but each individual ferricytochrome can react with at least four or five moieties of reductant, so there can be extensive interactions between electron transfer systems. The apparent second order rate constant is 3.0 + 1.8 X lo9 M-’ s-l, but since the reactants are membrane bound, their effective concentration is much higher than if they were in free solution, and the “true” second order rate constant is probably closer to 10” M-I s-‘. Cytochromes c play an important role in many electron transfer systems, and it is likely that these results will have important implications in other bacterial systems and in mitochondria; several authors have already suggested the existence of a mitochondrial reductant with properties rather similar to those measured here.