title: The dimeric structure of the cytochrome bc(1) complex prevents center P inhibition by reverse reactions at center N.
abstract: Energy transduction in the cytochrome bc(1) complex is achieved by catalyzing opposite oxido-reduction reactions at two different quinone binding sites. We have determined the pre-steady state kinetics of cytochrome b and c(1) reduction at varying quinol/quinone ratios in the isolated yeast bc(1) complex to investigate the mechanisms that minimize inhibition of quinol oxidation at center P by reduction of the b(H) heme through center N. The faster rate of initial cytochrome b reduction as well as its lower sensitivity to quinone concentrations with respect to cytochrome c(1) reduction indicated that the b(H) hemes equilibrated with the quinone pool through center N before significant catalysis at center P occurred. The extent of this initial cytochrome b reduction corresponded to a level of b(H) heme reduction of 33%-55% depending on the quinol/quinone ratio. The extent of initial cytochrome c(1) reduction remained constant as long as the fast electron equilibration through center N reduced no more than 50% of the b(H) hemes. Using kinetic modeling, the resilience of center P catalysis to inhibition caused by partial pre-reduction of the b(H) hemes was explained using kinetics in terms of the dimeric structure of the bc(1) complex which allows electrons to equilibrate between monomers.
authors.affiliation: Department of Biochemistry, Dartmouth Medical School, 7200 Vail Building, Hanover, NH 03755, USA.
authors.forename: Raul
authors.initials: R
authors.lastname: Covian
authors.forename: Bernard L
authors.initials: BL
authors.lastname: Trumpower
mesh_terms: D001665:Binding Sites; D045303:Cytochromes b / Q000737:chemistry / Q000378:metabolism; D003575:Cytochromes c1 / Q000737:chemistry / Q000378:metabolism; D019281:Dimerization; D014450:Electron Transport Complex III / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000378:metabolism*; D004583:Electrons; D004734:Energy Metabolism; D006873:Hydroquinones / Q000494:pharmacology; D007700:Kinetics; D012441:Saccharomyces cerevisiae / Q000201:enzymology; D029701:Saccharomyces cerevisiae Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism
chemical_list: D006873:Hydroquinones; D029701:Saccharomyces cerevisiae Proteins; D045303:Cytochromes b; D003575:Cytochromes c1; D014450:Electron Transport Complex III