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Reference: Covian R and Trumpower BL (2009) The Rate-limiting Step in the Cytochrome bc1 Complex (Ubiquinol-Cytochrome c Oxidoreductase) Is Not Changed by Inhibition of Cytochrome b-dependent Deprotonation: IMPLICATIONS FOR THE MECHANISM OF UBIQUINOL OXIDATION AT CENTER P OF THE bc1 COMPLEX. J Biol Chem 284(21):14359-67

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Abstract


Quinol oxidation at center P of the cytochrome bc1 complex involves bifurcated electron transfer to the Rieske iron-sulfur protein and cytochrome b. It is unknown whether both electrons are transferred from the same domain close to the Rieske protein, or if an unstable semiquinone anion intermediate diffuses rapidly to the vicinity of the bL heme. We have determined the pre-steady state rate and activation energy (Ea) for quinol oxidation in purified yeast bc1 complexes harboring either a Y185F mutation in the Rieske protein, which decreases the redox potential of the FeS cluster, or a E272Q cytochrome b mutation, which eliminates the proton acceptor in cytochrome b. The rate of the bifurcated reaction in the E272Q mutant (<10% of the wild-type) was even lower than that of the Y185F enzyme (~20% of wild-type). However, the E272Q enzyme showed the same Ea (61 kJ mol-1) with respect to wild-type (62 kJ mol-1), in contrast with the Y185F mutation, which increased Ea to 73 kJ mol-1. The rate and Ea of the slow reaction of quinol with oxygen that is observed after cytochrome b is reduced was unaffected by the E272Q substitution, while the Y185F mutation only modified its rate. The Y185F/E272Q double mutation resulted in a synergistic decrease in the rate of quinol oxidation (0.7% of wild-type). These results are inconsistent with a sequential "movable-semiquinone" mechanism, but are consistent with a model in which both electrons are transferred simultaneously from the same domain in center P.

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Journal Article
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Covian R, Trumpower BL
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