The energy landscape for ubihydroquinone oxidation at the Qo site of the bc1 complex in Rhodobacter sphaeroides
The energy landscape for ubihydroquinone oxidation at the Qo site of the bc1 complex in Rhodobacter sphaeroides
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DOI：
10.1074/jbc.274.48.33931
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发表时间：
1999-11-26
影响因子：
4.8
通讯作者：
Crofts, AR
中科院分区：
文献类型：
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作者：
Hong, SJ;Ugulava, N;Crofts, AR
BC(1)络合物氧化对苯二酚的部分反应活化能在5.5-8.9范围内与pH无关。酶-底物复合体的形成需要两种底物，来自脂相的泛氢对苯二酚结合和铁硫蛋白的外在结构域。泛氢苯二酚氧化反应的活化能与底物浓度无关，说明酶-底物复合体形成后活化步骤处于反应过程中。在所有pH值下，限制反应速率和活化能最高的部分反应是结合泛氢苯二酚的氧化。泛氢苯二酚氧化速率的pH依赖性反映了氧化铁-硫蛋白上的PK，以及酶-底物复合体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性质，并排除了高活化势垒处于第二电子转移或由QH(2)的去质子化引起的模型。在第一次电子转移后产物的分离和在Q(O)位形成的半喹酮的移动将允许电子快速转移到血红素b(L)。这也将使半喹酮不被铁硫蛋白氧化，这解释了分叉的效率。
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.