Anti-cooperative Oxidation of Ubiquinol by the Yeast Cytochrome bc 1 Complex

Abstract

We have investigated the interaction between monomers of the dimeric yeast cytochrome bc1 complex by analyzing the pre-steady and steady state activities of the isolated enzyme in the presence of antimycin under conditions that allow the first turnover of ubiquinol oxidation to be observable in cytochrome c1 reduction. At pH 8.8, where the redox potential of the iron-sulfur protein is ∼200 mV and in a bc1 complex with a mutated iron-sulfur protein of equally low redox potential, the amount of cytochrome c1 reduced by several equivalents of decyl-ubiquinol in the presence of antimycin corresponded to only half of that present in the bc1 complex. Similar experiments in the presence of several equivalents of cytochrome c also showed only half of the bc1 complex participating in quinol oxidation. The extent of cytochrome b reduced corresponded to two bH hemes undergoing reduction through one center P per dimer, indicating electron transfer between the two cytochrome b subunits. Antimycin stimulated the ubiquinol-cytochrome c reductase activity of the bc1 complex at low inhibitor/enzyme ratios. This stimulation could only be fitted to a model in which half of the bc1 dimer is inactive when both center N sites are free, becoming active upon binding of one center N inhibitor molecule per dimer, and there is electron transfer between the cytochrome b subunits of the dimer. These results are consistent with an alternating half-of-the-sites mechanism of ubiquinol oxidation in the bc 1 complex dimer.