Evidence for a concerted mechanism of ubiquinol oxidation by the cytochrome bc(1) complex

To better understand the mechanism of divergent electron transfer from ubiq uinol to the iron-sulfur protein and cytochrome b(L) within the cytochrome bc(1) complex, we have examined the effects of antimycin on the presteady s tate reduction kinetics of the bc(1) complex in the presence or absence of endogenous ubiquinone, When ubiquinone is present, antimycin slows the rate of cytochrome c(1) reduction by similar to 10-fold but had no effect upon the rate of cytochrome c(1) reduction in bc(1) complex lacking endogenous u biquinone, In the absence of endogenous ubiquinone cytochrome c(1), reducti on was slower than when ubiquinone was present and was similar to that in t he presence of ubiquinone plus antimycin, These results indicate that the l ow potential redox components, cytochrome b(H) and b(L) exert negative cont rol on the rate of reduction of cytochrome c(1) and the Rieske iron-sulfur protein at center P, If electrons cannot equilibrate from cytochrome b(H) a nd b(L) to ubiquinone, partial reduction of the low potential components sl ows reduction of the high potential components. We also examined the effect s of decreasing the midpoint potential of the iron-sulfur protein on the ra tes of cytochrome b reduction. As the midpoint potential decreased, there w as a parallel decrease in the rate of b reduction, demonstrating that the r ate of b reduction is dependent upon the rate of ubiquinol oxidation by the iron-sulfur protein. Together these results indicate that ubiquinol oxidat ion is a concerted reaction in which both the low potential and high potent ial redox components control ubiquinol oxidation at center P, consistent wi th the protonmotive Q cycle mechanism.