HOW RAPID ARE THE INTERNAL REACTIONS OF THE UBIQUINOL-CYTOCHROME-C2 OXIDOREDUCTASE
HOW RAPID ARE THE INTERNAL REACTIONS OF THE UBIQUINOL-CYTOCHROME-C2 OXIDOREDUCTASE
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DOI：
10.1007/bf00114768
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发表时间：
1989-10-01
影响因子：
3.7
通讯作者：
WANG, ZG
中科院分区：
文献类型：
--
作者：
CROFTS, AR;WANG, ZG
本文研究了球形磷杆菌UQH_2：Cyt_2氧化还原酶部分翻转反应的温度依赖性。细胞色素组分的氧化还原性质在280-330K温度范围内表现出微弱的温度依赖关系，其系数约为1 mV/度；我们的结果表明，其他组分也表现出类似的依赖关系，因此在该温度范围内组分之间的标准自由能梯度没有明显变化。高势链(Rieske铁硫中心，细胞色素c1和c2，反应中心主施主)的反应速率对温度的依赖性很弱，表明该链中电子转移的活化能为每摩尔8 kJ。泛喹酚在络合物的QZ位上的氧化表现出强烈的温度依赖性，其活化能约为32kJ摩尔-1。细胞色素b-566到细胞色素b-561的电子转移在任何温度下都不是速率决定的，也不是对能垒的贡献。苯二酚氧化的活化能为32kJ摩尔~(-1)，对苯二酚库的所有状态(闪光前完全氧化、部分还原或完全还原)都是相同的。我们认为，活化势垒存在于催化位上的泛喹酚被氧化为半喹酮的反应中。该反应最经济的方案是使半喹酮中间体处于活化势垒所指示的能级。我们讨论了这个简单模型的合理性，以及与泛喹酚氧化中心机理有关的速率常数、稳定常数、中间对的氧化还原电势和半醌的结合常数的值。
The temperature dependence of the partial reactions leading to turn-over of the UQH2:cyt c2 oxidoreductase of phodobacter sphaeroides have been studied. The redox properties of the cytochrome components show a weak temperature dependence on the range 280-330 K, with coefficients of about 1 mV per degree; our results suggest that the other components show similar dependencies, so that no significant change in the gradient of standard free-energy between components occurs over this temperature range. The rates of the reactions of the high potential chain (the Rieske iron sulfur center, cytochromes c1 and c2, reaction center primary donor) show a weak temperature dependence, indicating an activation energy < 8 kJ per mole for electron transfer in this chain. The oxidation of ubiquinol at the Qz-site of the complex showed a strong temperature dependence, with an activation energy of about 32 kJ mole-1. The electron transfer from cytochrome b-566 to cytochrome b-561 was not rate determining at any temperature, and did not contribute to the energy barrier. The activation energy of 32 kJ mole-1 for quinol oxidation was the same for all states of the quinone pool (fully oxidized, partially reduced, or fully reduced before the flash). We suggest that the activation barrier is in the reaction by which ubiquinol at the catalytic site is oxidized to semiquinone. The most economical scheme for this reaction would have the semiquinone intermediate at the energy level indicated by the activation barrier. We discuss the plausibility of this simple model, and the values for rate constants, stability constant, the redox potentials of the intermediate couples, and the binding constant for the semiquinone, which are pertinent to the mechanism of the ubiquinol oxidizing site.