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Kinetics of flash-induced electron transfer between bacterial reaction centres,mitochondrial ubiquinol: Cytochrome c oxidoreductase and cytochrome c
Authors:QS Zhu  HN Van Der Wal  R Van Grondelle  JA Berden
Institution:1. Laboratory of Biochemistry, B.C.P. Jansen Institute, University of Amsterdam, P.O. Box 20151, 1000 HD Amsterdam The Netherlands;2. Laboratory of Biophysics, State University of Leiden, Huygens Laboratory, Leiden The Netherlands
Abstract:Ascorbate-reduced horse heart cytochrome c reduces photo-oxidized bacterial reaction centres with a second-order rate constant of (5–8) · 108 M?1 · s?1 at an ionic strength of 50 mM. In the absence of cytochrome c, the cytochrome c1 in the ubiquinol:cytochrome c oxidoreductase is oxidized relatively slowly (k = 3.3 · 105 M?1 · s?1). Ferrocytochrome c binds specifically to ascorbate-reduced reductase, with a Kd of 0.6 μM, and only the free cytochrome c molecules are involved in the rapid reduction of photo-oxidized reaction centres. The electron transfer between ferricytochrome c and ferrocytochrome c1 of the reductase is rapid, with a second-order rate constant of 2.1 · 108 M?1 · s?1 at an ionic strength of 50 mM. The rate of electron transfer from the Rieske iron-sulphur cluster to cytochrome c1 is even more rapid. The cytochrome b of the ubiquinol:cytochrome c oxidoreductase can be reduced by electrons from the reaction centres through two pathways: one is sensitive to antimycin and the other to myxothiazol. The amount of cytochrome b reduced in the absence of antimycin is dependent on the redox potential of the system, but in no case tested did it exceed 25% of the amount of photo-oxidized reaction centres.
Keywords:Electron transfer  Reaction center  (Bovine heart  Rps  sphaeroides)  (oxido)reductase  ubiquinol  Q  ubiquinone-10  HMHQQ  UHDBT  Mops  4-morpholinepropanesulphonic acid
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