The effect of pH,ubiquinone depletion and myxothiazol on the reduction kinetics of the prosthetic groups of ubiquinol: Cytochrome c oxidoreductase

(1) The kinetics of the reduction by duroquinol of the prosthetic groups of QH₂: cytochrome c oxidoreductase and of the formation of ubisemiquinone have been studied using a combination of the freeze-quench technique, low-temperature diffuse-reflectance spectroscopy, EPR and stopped flow. (2) The formation of the antimycin-sensitive ubisemiquinone anion parallels the reduction of both high-potential and low-potential cytochrome b-562. (3) The rates of reduction of both the 2Fe-2S] clusters and cytochromes (c + c₁) are pH dependent. There is, however, a pH-dependent discrepancy between their rate of reduction, which can be correlated with the difference in pH dependencies of their midpoint potentials. (4) Lowering the pH or the Q content results in a slower reduction of part of the 2Fe-2S] clusters. It is suggested that one cluster is reduced by a quinol/semiquinone couple and the other by a semiquinone/quinone couple. (5) Myxothiazol inhibits the reduction of the 2Fe-2S] clusters, cytochrome c₁ and high-potential cytochrome b-562. (6) The results are consistent with a Q-cycle model describing the pathway of electrons through a dimeric QH₂: cytochrome c oxidoreductase.