Abstract
We have investigated the interaction between monomers of the dimeric yeast cytochrome bc(1) 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 c(1) reduction. At pH 8.8, where the redox potential of the iron-sulfur protein is approximately 200 mV and in a bc(1) complex with a mutated iron-sulfur protein of equally low redox potential, the amount of cytochrome c(1) reduced by several equivalents of decyl-ubiquinol in the presence of antimycin corresponded to only half of that present in the bc(1) complex. Similar experiments in the presence of several equivalents of cytochrome c also showed only half of the bc(1) complex participating in quinol oxidation. The extent of cytochrome b reduced corresponded to two b(H) 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 bc(1) complex at low inhibitor/enzyme ratios. This stimulation could only be fitted to a model in which half of the bc(1) 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.
Highlights
Some center P inhibitors have been shown to completely block bc1 complex activity upon binding to only half of the dimeric complex [6], suggesting anti-cooperative interaction
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
The results show that the number of center N sites blocked in the dimer determines whether ubiquinol oxidation can occur in one or both of the monomers, and that electron communication exists between cytochrome b subunits of the two monomers
Summary
Some center P inhibitors have been shown to completely block bc1 complex activity upon binding to only half of the dimeric complex [6], suggesting anti-cooperative interaction. 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.
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