Electrogenic events in the ubiquinone-cytochrome bc2 oxidoreductase of rhodopseudomonas sphaeroides

The reductant of ferricytochrome c₂ in Rhodopseudomonas sphaeroides is a component, Z, which has an equilibrium oxidation-reduction reaction involving two electrons and two protons with a midpoint potential of 155 mV at pH 7. Under energy coupled conditions, the reduction of ferricytochrome c₂ by ZH₂ is obligatorily coupled to an apparently electrogenic reaction which is monitored by a red shift of the endogeneous carotenoids. Both ferricytochrome c₂ reduction and the associated carotenoid bandshift are similarly affected by the concentrations of ZH₂ and ferricytochrome c₂, pH, temperature the inhibitors diphenylamine and antimycin, and the presence of ubiquinone. The second-order rate constant for ferricytochrome c₂ reduction at pH 7.0 and at 24°C was 2 · 109 M^?1 · s^?1, but this varied with pH, being 5.1 · 10⁸ M^?1 · s^?1 at pH 5.2 and 4.3 · 10⁹ M^?1 · s^?1 at pH 9.3. At pH 7 the reaction had an activation energy of 10.3 kcal/mol.