Department of Physiology and Biophysics, University of Illinois at Urbana-Champaign, 524 Burrill Hall, 407 S. Goodwin Street, 61801, Urbana, Il, U.S.A..
Photosynth Res. 1989 Jan;22(1):69-87. doi: 10.1007/BF00114768.
The temperature dependence of the partial reactions leading to turn-over of the UQH2:cyt c 2 oxidoreductase of Rhodobacter sphaeroides have been studied. The redox properties of the cytochrome components show a weak temperature dependence over the range 280-330 K, with coefficients of about 1 m V 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 c 1 and c 2, 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.
已研究导致球形红杆菌 UQH2：cyt c 2 氧化还原酶周转的部分反应的温度依赖性。细胞色素组分的氧化还原性质在 280-330 K 范围内具有较弱的温度依赖性，系数约为每度 1 mV；我们的结果表明其他组分显示出类似的依赖性，因此在该温度范围内，组件之间的标准自由能梯度没有明显变化。高电位链（Rieske 铁硫中心、细胞色素 c 1 和 c 2、反应中心初级供体）的反应速率显示出较弱的温度依赖性，表明电子转移在该链中活化能<8 kJ/mol。复合物 Qz 位点上的 ubiquinol 氧化显示出强烈的温度依赖性，活化能约为 32 kJ/mol。在任何温度下，细胞色素 b-566 到细胞色素 b-561 的电子转移都不是速率决定因素，也不会对能量障碍做出贡献。32 kJ/mol 的 ubiquinol 氧化活化能对于醌库的所有状态（完全氧化、部分还原或在闪光之前完全还原）都是相同的。我们认为，激活障碍存在于催化位点上的 ubiquinol 氧化为半醌的反应中。对于该反应，最经济的方案是将半醌中间体置于由活化能垒指示的能级。我们讨论了这个简单模型的合理性，以及与 ubiquinol 氧化位点的机制相关的速率常数、稳定常数、中间偶对的氧化还原电位以及半醌的结合常数的值。
