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

Prince, R.C.; Bashford, C.L.; Takamiya, K.I.; van den Berg, W.H.; Dutton, P.L.

Journal of Biological Chemistry 253(12): 4137-4142


ISSN/ISBN: 0021-9258
PMID: 207691
Accession: 068524090

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The direct reductant of ferricytochrome c2 in the ubiquinone cytochrome b-c2 oxidoreductase of R. sphaeroides is a component (Z), with an oxidation-reduction midpoint potential of 155 mV at pH 7, which requires 2 electrons and 2 protons for its equilibrium reduction. The reduction of the ferricytochrome is 1st order in both ferricytochrome and reductant, indicating that the fraction has 2nd order characteristics. The behavior of the half-time of a 2nd 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 2nd order rate constant of the reaction. The ratio of reductant to electron transfer systems (which each contain a pair of cytochromes c2) is 0.8 .+-. 0.1, but each individual ferricytochrome can react with at least 4 or 5 moieties of reductant, so there can be extensive interactions between electron transfer systems. The apparent 2nd order rate constant is 3.0 .+-. 1.8 .times. 109 M-1 s-1, but since the reactants are membrane bound, their effective concentration is much higher than if they were in free solution, and the true 2nd order rate constant is probably closer to 104 M-1 s-1. Cytochromes c play an important role in many electron transfer systems, and 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.