title: The effect of pH, ubiquinone depletion and myxothiazol on the reduction kinetics of the prosthetic groups of ubiquinol:cytochrome c oxidoreductase.
abstract: (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.
mesh_terms: D000818:Animals; D002417:Cattle; D003573:Cytochrome b Group / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D014450:Electron Transport Complex III; D029968:Escherichia coli Proteins; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008689:Methacrylates; D008929:Mitochondria, Heart / Q000378:metabolism; D009097:Multienzyme Complexes / Q000378:metabolism*; D009247:NADH, NADPH Oxidoreductases / Q000378:metabolism*; D010084:Oxidation-Reduction; D011808:Quinone Reductases / Q000378:metabolism*; D013367:Submitochondrial Particles / Q000378:metabolism; D013844:Thiazoles / Q000494:pharmacology; D014451:Ubiquinone / Q000378:metabolism*
chemical_list: D003573:Cytochrome b Group; D029968:Escherichia coli Proteins; D008689:Methacrylates; D009097:Multienzyme Complexes; D013844:Thiazoles; D014451:Ubiquinone; C030517:myxothiazol; C017850:cytochrome b562, E coli; D009247:NADH, NADPH Oxidoreductases; D011808:Quinone Reductases; D014450:Electron Transport Complex III