Quinones such as ubiquinone are the lipid soluble electron and proton carriers in the membranes of mitochondria, chloroplasts and oxygenic bacteria. Quinones undergo controlled redox reactions bound to specific sites in integral membrane proteins such as the cytochrome bc(1) oxidoreductase. The quinone reactions in bacterial photosynthesis are amongst the best characterized, presenting a model to understand how proteins modulate cofactor chemistry. The free energy of ubiquinone redox reactions in aqueous solution and in the Q(A) and Q(B) sites of the bacterial photosynthetic reaction centers (RCs) are compared. In the primary Q(A) site ubiquinone is reduced only to the anionic semiquinone (Q(*-)) while in the secondary Q(B) site the product is the doubly reduced, doubly protonated quinol (QH(2)). The ways in which the protein modifies the relative energy of each reduced and protonated intermediate are described. For example, the protein stabilizes Q(*-) while destabilizing Q(=) relative to aqueous solution through electrostatic interactions. In addition, kinetic and thermodynamic mechanisms for stabilizing the intermediate semiquinones are compared. Evidence for the protein sequestering anionic compounds by slowing both on and off rates as well as by binding the anion more tightly is reviewed.

中文翻译：

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生物电子转移链中蛋白质对醌电化学的修饰：来自光合作用反应中心的示例。


泛醌等醌类是线粒体、叶绿体和产氧细菌膜中的脂溶性电子和质子载体。醌经历受控氧化还原反应，与整合膜蛋白（例如细胞色素 bc(1) 氧化还原酶）的特定位点结合。细菌光合作用中的醌反应是最具特征的反应之一，它提供了一个模型来理解蛋白质如何调节辅因子化学。比较了水溶液中以及细菌光合反应中心 (RC) 的 Q(A) 和 Q(B) 位点中泛醌氧化还原反应的自由能。在初级 Q(A) 位点，泛醌仅还原为阴离子半醌 (Q(*-))，而在次级 Q(B) 位点，产物是双重还原、双重质子化的对苯二酚 (QH(2))。描述了蛋白质改变每种还原和质子化中间体的相对能量的方式。例如，蛋白质通过静电相互作用稳定 Q(*-)，同时使 Q(=) 相对于水溶液不稳定。此外，还比较了稳定中间体半醌的动力学和热力学机制。回顾了蛋白质通过减慢开启和关闭速率以及更紧密地结合阴离子来隔离阴离子化合物的证据。