pH-induced intramolecular electron transfer between the iron-sulfur protein and cytochrome c(1) in bovine cytochrome bc(1) complex

Structural analysis of the bc(1) complex suggests that the extramembrane do main of iron-sulfur protein (ISP) undergoes substantial movement during the catalytic: cycle. Binding of Qo site inhibitors to this complex affects th e mobility of ISP, Taking advantage of the difference in the pH dependence of the redox midpoint potentials of cytochrome c(1) and ISP, we have measur ed electron transfer between the [2Fe-2S] cluster and heme c(1) in native a nd inhibitor-treated partially reduced cytochrome bc(1) complexes. The rate of the pH-induced cytochrome c(1) reduction can be estimated by convention al stopped-flow techniques (t(1/2), 1-2 ms), whereas the rate of cytochrome c(1) oxidation is too high for stopped-flow measurement, These results sug gest that oxidized ISP has a higher mobility than reduced ISP and that the movement of reduced ISP may require an energy input from. another component . In the 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole (UHDBT)-inhibited com plex, the rate of cytochrome c(1) reduction is greatly decreased to a t(1/2 ) of approximately 2.8 s. An even lower rate is observed with the stigmatel lin-treated complex, These results support the idea that UHDBT and stigmate llin arrest the [2Fe-2S] cluster at a fixed position, 31 Angstrom from heme c, making electron transfer very slow.