Investigation of the mechanism of proton translocation by NADH:ubiquinone oxidoreductase (complex I) from bovine heart mitochondria: does the enzyme operate by a Q-cycle mechanism?
Résumé
NADH:ubiquinone oxidoreductase (complex I) is the first enzyme of the membrane bound electron transport chain in mitochondria. It conserves energy, from the reduction of ubiquinone by NADH, as a proton motive force across the inner membrane, but the mechanism of energy transduction is not known. The structure of the hydrophilic arm of thermophilic complex I supports the idea that proton translocation is driven at (or close to) the point of quinone reduction, rather than at the point of NADH oxidation, with a chain of iron-sulphur clusters transferring electrons between the two active sites. Here we describe experiments to determine whether complex I, isolated from bovine heart mitochondria, operates via a Q-cycle mechanism analogous to that observed in the cytochrome bc 1} complex. No evidence for the 'reductant-induced oxidation' of ubiquinol could be detected - therefore, no support for a Q-cycle mechanism was obtained. Unexpectedly, in the presence of NADH, complex I inhibited by either rotenone or piericidin A was found to catalyse the exchange of redox states between different quinone and quinol species, providing a possible route for future investigations into the mechanism of energy transduction.
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