The oxidation of yeast Complex III. Evidence for a very rapid electron equilibration between cytochrome c1 and the iron-sulfur center.

The reoxidation of reduced cytochrome c1 by potassium ferricyanide follows pseudo-first order kinetics with k = 4 x 10(4) M-1 s-1. However, the reoxidation of this cytochrome in two-electron reduced Complex III does not follow any simple rate law although the overall rate of reaction is essentially unchanged. The observed kinetics can be well fitted with a model in which ferricyanide reacts exclusively with cytochrome c1 together with very rapid electron transfer from the reduced iron-sulfur center to cytochrome c1. Neither removal of coenzyme Q from the complex nor prior incubation with antimycin A had any effect on the observed kinetics of reoxidation.