Rapid electron transfer between monomers when the cytochrome bc1 complex dimer is reduced through center N

Abstract

We have obtained evidence for electron transfer between cytochrome 6 subunits of the yeast bc1 complex dimer by analyzing pre-steady state reduction of cytochrome 6 in the presence of center P inhibitors. The kinetics and extent of cytochrome 6 reduced by quinol in the presence of variable concentrations of antimycin decreased non-linearly and could only be fitted to a model in which electrons entering through one center N can equilibrate between the two cytochrome 6 subunits of the bc1 complex dimer. The b H heme absorbance in a bc1 complex inhibited at center P and preincubated with substoichiometric concentrations of antimycin showed a red shift upon the addition of substrate, which indicates that electrons from the uninhibited center N in one monomer are able to reach the bH heme at the antimycin-blocked site in the other. The extent of cytochrome 6 reduction by variable concentrations of menaquinol could only be fitted to a kinetic model that assumes electron equilibration between center N sites in the dimer. Kinetic simulations showed that non-rate-limiting electron equilibration between the two bH hemes in the dimer through the two bL hemes is possible upon reduction through one center N despite the thermodynamically unfavorable bH to bL electron transfer step. We propose that electron transfer between cytochrome 6 subunits minimizes the formation of semiquinone-ferrocytochrome bH complexes at center N and favors ubiquinol oxidation at center P by increasing the amount of oxidized cytochrome b. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.