Esposti M D, Lenaz G
Department of Biology, University of Bologna, Italy.
Arch Biochem Biophys. 1991 Sep;289(2):303-12. doi: 10.1016/0003-9861(91)90415-f.
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.
细胞色素c还原酶（细胞色素bc1复合物）的稳态动力学。滴定的图形模式清楚地表明是乒乓机制，但两种产物泛醌和还原型细胞色素c与它们的底物表现为竞争性，而与另一种底物表现为非竞争性。因此，还原酶的机制属于乒乓双位点类型。提出了酶促机制的最小反应方案，并在每种底物在其催化位点处于快速平衡的假设下推导出其速率常数的近似值。这已通过泛醇短链同系物对细胞色素b的还原和氧化的预稳态测量得到证实。反应方案的速率常数的值已从一系列在环的6位具有不同疏水取代基的2,3 - 二甲氧基 - 5 - 甲基喹醇的稳态滴定中推导出来。结果提供了对bc1复合物跨膜部分中喹醇催化位点特异性的定量估计。特别是，发现喹醇与酶的二级反应速率与其在脂质中的溶解度之间存在合理的相关性。
