Kubota T, Yoshikawa S, Matsubara H
Department of Biology, Faculty of Science, Osaka University.
J Biochem. 1992 Jan;111(1):91-8. doi: 10.1093/oxfordjournals.jbchem.a123725.
The electron transfer from ubiquinol-2 to ferricytochrome c mediated by ubiquinol:cytochrome c oxidoreductase [E.C. 1.10.2.2] purified from beef heart mitochondria, which contained one equivalent of ubiquinone-10 (Q10), was investigated under initial steady-state conditions. The Q10-depleted enzyme was as active as the Q10-containing one. Double reciprocal plots for the initial steady-state rate versus one of the two substrates at various fixed levels of the other substrate gave parallel straight lines in the absence of any product. Intersecting straight lines were obtained in the presence of a constant level of one of the products, ferrocytochrome c. The other product, ubiquinone-2, did not show any significant effect on the enzymic reaction. Ferrocytochrome c non-competitively inhibited the enzymic reaction against either ubiquinol-2 or ferricytochrome c. These results indicate a Hexa-Uni ping-pong mechanism with one ubiquinol-2 and two ferricytochrome c molecules as the substrates, which involves the irreversible release of ubiquinone-2 as the first product and the irreversible isomerization between the release of the first ferrocytochrome c and the binding of the second ferricytochrome c. Considering the cyclic electron transfer reaction mechanism, this scheme suggests that the binding of quinone or quinol to the enzyme and electron transfer between the iron-sulfur center and cytochrome c1 are rigorously controlled by the electron distribution within the enzyme.
细胞色素c氧化还原酶[E.C. 1.10.2.2]介导的从泛醇-2到高铁细胞色素c的电子转移进行了研究，该酶含有一当量的泛醌-10（Q10），研究在初始稳态条件下进行。去除Q10的酶与含有Q10的酶活性相同。在不存在任何产物的情况下，以初始稳态速率对两种底物之一在另一种底物的各种固定水平下绘制的双倒数图给出了平行直线。在存在恒定水平的产物之一亚铁细胞色素c的情况下获得了相交直线。另一种产物泛醌-2对酶促反应没有任何显著影响。亚铁细胞色素c对针对泛醇-2或高铁细胞色素c的酶促反应具有非竞争性抑制作用。这些结果表明了一种六-单乒乓机制，其中一个泛醇-2分子和两个高铁细胞色素c分子作为底物，这涉及作为第一个产物的泛醌-2的不可逆释放以及第一个亚铁细胞色素c的释放与第二个高铁细胞色素c的结合之间的不可逆异构化。考虑到循环电子转移反应机制，该方案表明醌或泛醇与酶的结合以及铁硫中心与细胞色素c1之间的电子转移受到酶内电子分布的严格控制。
