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?
Complex I (NADH:ubiquinone oxidoreductase) 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 protonmotive 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-sulfur 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 bc1 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.
Sherwood
Steven
S
Medical Research Council Dunn Human Nutrition Unit, Wellcome Trust/MRC Building, Hills Road, Cambridge CB2 2XY, UK.
Hirst
Judy
J
D000818:Animals; D002417:Cattle; D042967:Electron Transport Complex I / Q000737:chemistry / Q000378:metabolism*; D004734:Energy Metabolism; D008929:Mitochondria, Heart / Q000201:enzymology*; D008954:Models, Biological; D011522:Protons; D011809:Quinones; D014451:Ubiquinone / Q000031:analogs & derivatives / Q000378:metabolism
D011522:Protons; D011809:Quinones; D014451:Ubiquinone; D042967:Electron Transport Complex I; C003741:ubiquinol