title: The kinetic mechanism of ubiquinol: cytochrome c reductase at steady state.
abstract: The steady-state kinetics of ubiquinol: cytochrome c reductase (cytochrome bc1 complex) is analyzed in this work. The graphical pattern of the titrations is clearly indicative of a ping-pong mechanism, but the two products ubiquinone and reduced cytochrome c behave competitively with their substrate and noncompetitively with the other substrate. Hence, the mechanism of the reductase is of a ping-pong two-site type. A minimal reaction scheme for the enzymatic mechanism is proposed and approximate values of its rate constants are deduced on the assumption that each substrate is in rapid equilibrium at its catalytic site. This has been substantiated by presteady-state measurements of the reduction and oxidation of cytochrome b by a short-chain homolog of ubiquinol. Values of the rate constants of the reaction scheme have been deduced from the steady-state titrations for a series of 2,3-dimethoxy-5-methyl quinols having different hydrophobic substituents in position 6 of the ring. The results provide a quantitative estimation of the specificity of the quinol catalytic site in the transmembrane portion of the bc1 complex. In particular, a reasonable correlation is found between the rate of the second-order reaction of quinols with the enzyme and their solubility in lipids.
authors.affiliation: Department of Biology, University of Bologna, Italy.
authors.forename: M D
authors.initials: MD
authors.lastname: Esposti
authors.forename: G
authors.initials: G
authors.lastname: Lenaz
mesh_terms: D000818:Animals; D001665:Binding Sites; D002417:Cattle; D003574:Cytochrome c Group; D014450:Electron Transport Complex III / Q000037:antagonists & inhibitors / Q000378:metabolism*; D006873:Hydroquinones; D066298:In Vitro Techniques; D007700:Kinetics; D008929:Mitochondria, Heart / Q000201:enzymology; D008956:Models, Chemical
chemical_list: D003574:Cytochrome c Group; D006873:Hydroquinones; D014450:Electron Transport Complex III