Hong S, Ugulava N, Guergova-Kuras M, Crofts A R
Center for Biophysics, University of Illinois, Urbana, Illinois 61801, USA.
J Biol Chem. 1999 Nov 26;274(48):33931-44. doi: 10.1074/jbc.274.48.33931.
Activation energies for partial reactions involved in oxidation of quinol by the bc(1) complex were independent of pH in the range 5. 5-8.9. Formation of enzyme-substrate complex required two substrates, ubihydroquinone binding from the lipid phase and the extrinsic domain of the iron-sulfur protein. The activation energy for ubihydroquinone oxidation was independent of the concentration of either substrate, showing that the activated step was in a reaction after formation of the enzyme-substrate complex. At all pH values, the partial reaction with the limiting rate and the highest activation energy was oxidation of bound ubihydroquinone. The pH dependence of the rate of ubihydroquinone oxidation reflected the pK on the oxidized iron-sulfur protein and requirement for the deprotonated form in formation of the enzyme-substrate complex. We discuss different mechanisms to explain the properties of the bifurcated reaction, and we preclude models in which the high activation barrier is in the second electron transfer or is caused by deprotonation of QH(2). Separation to products after the first electron transfer and movement of semiquinone formed in the Q(o) site would allow rapid electron transfer to heme b(L). This would also insulate the semiquinone from oxidation by the iron-sulfur protein, explaining the efficiency of bifurcation.
bc(1)复合物催化醌醇氧化过程中部分反应的活化能在pH值5.5 - 8.9范围内与pH无关。酶 - 底物复合物的形成需要两种底物，即来自脂质相的泛醌醇结合以及铁硫蛋白的外在结构域。泛醌醇氧化的活化能与任何一种底物的浓度无关，这表明活化步骤发生在酶 - 底物复合物形成之后的反应中。在所有pH值下，具有限速作用且活化能最高的部分反应是结合态泛醌醇的氧化。泛醌醇氧化速率对pH的依赖性反映了氧化态铁硫蛋白上的pK值以及酶 - 底物复合物形成过程中对去质子化形式的需求。我们讨论了不同的机制来解释分叉反应的特性，并且排除了高活化能屏障存在于第二次电子转移中或由QH(2)去质子化引起的模型。第一次电子转移后产物的分离以及在Q(o)位点形成的半醌的移动将允许快速电子转移至血红素b(L)。这也将使半醌免受铁硫蛋白的氧化，解释了分叉反应的效率。
