Role of the Rieske iron-sulfur protein midpoint potential in the protonmotive Q-cycle mechanism of the cytochrome bc(1) complex

The midpoint potential of the [2Fe-2S] cluster of the Rieske iron-sulfur pr otein (E-m7 = +280 mV) is the primary determinant of the rate of electron t ransfer from ubiquinol to cytochrome c catalyzed by the cytochrome bc(1) co mplex. As the midpoint potential of the Rieske cluster is lowered by alteri ng the electronic environment surrounding the cluster, the ubiquinol-cytoch rome c reductase activity of the be, complex decreases; between 220 and 280 mV the rate changes 2.5-fold. The midpoint potential of the Rieske cluster also affects the presteady-state kinetics of cytochrome b and cl;reduction . When the midpoint potential of the Rieske cluster is more positive than t hat of the heme of cytochrome c(1) reduction of cytochrome b is biphasic. T he fast phase of b reduction is linked to the optically invisible reduction of the Rieske center, while the rate of the second, slow phase matches tha t of c, reduction. The rates of b and c(1) reduction become slower as the p otential of the Rieske cluster decreases and change from biphasic to monoph asic as the Rieske potential approaches that of the ubiquinone/ubiquinol co uple. Reduction of b and c(1) remain kinetically linked as the midpoint pot ential of the Rieske cluster is varied by 180 mV and under conditions where the presteady state reduction is biphasic or monophasic. The persistent li nkage of the rates of b and c(1) reduction is accounted for by the bifurcat ed oxidation of ubiquinol that is unique to the Q-cycle mechanism.