Determination of the binding rate constants of stigmatellin and UHDBT to bovine cytochrome bc(1) complex by cytochrome c(1) oxidation.

Based on the high electron transfer rate between the [2Fe-2S] cluster and heme c(1) and the elevation of the redox midpoint potential of iron sulfur protein (ISP) upon binding of certain Qo inhibitors, the binding rate constants of stigmatellin and UHDBT to the cytochrome bc(1) complex were determined using a stopped-flow rapid scanning technique. Assuming that the intramolecular electron transfer from ISP to cytochrome c(1) is much faster than the binding of inhibitors, the rate of the inhibitor binding can be determined by the rate of cytochrome c(1) oxidation. The binding rate constants were calculated to be 1.0x10(5) and 2.3x10(5) M(-1) s(-1) at pH 7.5 for stigmatellin and UHDBT, respectively. The binding rate constant of UHDBT is pH dependent and that of stigmatellin is not.