Three molecules of ubiquinone bind specifically to mitochondrial cytochrome bc(1) complex

Bifurcated electron flow to high potential "Rieske" iron-sulfur cluster and low potential heme b(L) is crucial for respiratory energy conservation by the cytochrome bc(1) complex. The chemistry of ubiquinol oxidation has to e nsure the thermodynamically unfavorable electron transfer to heme b(L). To resolve a central controversy about the number of ubiquinol molecules invol ved in this reaction, we used high resolution magic-angle-spinning nuclear magnetic resonance experiments to show that two out of three n-decyl-ubiqui nones bind at the ubiquinol oxidation center of the complex. This substanti ates a proposed mechanism in which a charge transfer between a ubiquinol/ub iquinone pair explains the bifurcation of electron flow.