The kinetics of reduction of yeast complex III by a substrate analog

The kinetics of reduction of the cytochrome and quinone constituents of yeast complex III by the substrate homolog Q1H2 have been measured under a variety of conditions. The maximum rates of reduction of cytochromes b and c1 and of the endogenous Q6 by Q1H2 were sufficiently fast to support the Vmax for the reduction of cytochrome c by this substrate. The absorbance at 562 nm showed an initial increase which was subsequently followed by a decrease. This decrease was synchronous with the appearance of reduced cytochrome c1 and is interpreted as reflecting the absorbance contribution of c1 at 562 nm under conditions where the steady state level of the b cytochromes is constant. Prereduction of c1 and the Fe/S cluster did not affect the initial very rapid reduction of b, but the second phase was eliminated. Antimycin abolished the very rapid rate of reduction of cytochrome b in untreated complex III and completely inhibited the reduction of cytochrome b in complex III in which c1 and the Fe/S cluster had been prereduced. However, the reduction of the endogenous quinone was essentially unaffected by these treatments. Antimycin had no effect on the reduction of c1. Funiculosin also suppressed the very rapid reduction of b while both myxothiazol and 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole did not modify this phase of the reaction; no secondary decrease in absorbance was observed in the presence of any of these inhibitors. Most of the observed kinetic changes could be reproduced by simulation of the Q-cycle; simple linear and branched schemes were unable to reproduce the data.