title: 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 b subunits of the yeast bc(1) complex dimer by analyzing pre-steady state reduction of cytochrome b in the presence of center P inhibitors. The kinetics and extent of cytochrome b 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 b subunits of the bc(1) complex dimer. The b(H) heme absorbance in a bc(1) 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 b(H) heme at the antimycin-blocked site in the other. The extent of cytochrome b 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 b(H) hemes in the dimer through the two b(L) hemes is possible upon reduction through one center N despite the thermodynamically unfavorable b(H) to b(L) electron transfer step. We propose that electron transfer between cytochrome b subunits minimizes the formation of semiquinone-ferrocytochrome b(H) complexes at center N and favors ubiquinol oxidation at center P by increasing the amount of oxidized cytochrome b.
authors.affiliation: Department of Biochemistry, Dartmouth Medical School, Hanover, New Hampshire 03755, USA.
authors.forename: Raul
authors.initials: R
authors.lastname: Covian
authors.forename: Bernard L
authors.initials: BL
authors.lastname: Trumpower
mesh_terms: D000968:Antimycin A / Q000031:analogs & derivatives* / Q000737:chemistry; D001665:Binding Sites; D001671:Biochemistry / Q000379:methods; D019281:Dimerization; D014450:Electron Transport Complex III / Q000737:chemistry*; D004583:Electrons; D006418:Heme / Q000737:chemistry; D006873:Hydroquinones / Q000737:chemistry; D007700:Kinetics; D008956:Models, Chemical; D011487:Protein Conformation; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D013053:Spectrophotometry; D013816:Thermodynamics; D013997:Time Factors; D014451:Ubiquinone / Q000737:chemistry
chemical_list: D006873:Hydroquinones; C032456:antimycin; D014451:Ubiquinone; D006418:Heme; D000968:Antimycin A; D014450:Electron Transport Complex III