Max-Volmer-Laboratorium für Biophysikalische Chemie, Technische Universität Berlin, Strasse des 17, Juni 135, 10623 Berlin, Germany.
J Photochem Photobiol B. 2011 Jul-Aug;104(1-2):35-43. doi: 10.1016/j.jphotobiol.2011.01.023. Epub 2011 Feb 4.
The essential steps of photosynthetic water splitting take place in Photosystem II (PSII) and comprise three different reaction sequences: (i) light induced formation of the radical pair P680(+)Q(A)(-), (ii) P680(+) driven oxidative water splitting into O(2) and four protons, and (iii) two step plastoquinone reduction to plastoquinol by Q(A)(-). This mini-review briefly summarizes our state of knowledge on energetics, kinetics and mechanism of oxidative water splitting. Essential features of the two types of reactions involved are described: (a) P680(+) reduction by the redox active tyrosine Y(z) and (b) sequence of oxidation steps induced by Y(z)(ox) in the water-oxidizing complex (WOC). The rate of the former reaction is limited by the non-adiabatic electron transfer (NET) step and the multi-phase kinetics shown to originate from a sequence of relaxation processes. In marked contrast, the rate of the stepwise oxidation by Y(z)(ox) of the WOC up to the redox level S(3) is not limited by NET but by trigger reactions which probably comprise proton shifts and/or conformational changes. The overall rate of the final reaction sequence leading to formation and release of O(2) is assumed to be limited by the electron transfer step from the S(3) state of WOC to Y(z)(ox) due to involvement of an endergonic redox equilibrium. Currently discussed controversial ideas on possible pathways are briefly outlined. Several crucial points of the mechanism of oxidative water splitting, like O-O bond formation, role of local proton shift(s), details of hydrogen bonding, are still not clarified and remain a challenging topic of future research.
光合作用水分解的基本步骤发生在光系统 II（PSII）中，包括三个不同的反应序列：（i）光诱导形成自由基对 P680(+)Q(A)(-)，（ii）P680(+)驱动氧化水分解为 O(2)和四个质子，以及（iii）两步质体醌还原为质体醇由 Q(A)(-)。这篇综述简要总结了我们对氧化水分解的能量学、动力学和机制的了解。描述了所涉及的两种类型反应的基本特征：(a) P680(+)由氧化还原活性酪氨酸 Y(z)还原，以及(b) Y(z)(ox)在水氧化复合物（WOC）中诱导的氧化步骤顺序。前一个反应的速率受非绝热电子转移（NET）步骤的限制，并且显示出多相动力学源于一系列弛豫过程。相比之下，WOC 中 Y(z)(ox)逐步氧化至氧化还原水平 S(3)的速率不受 NET 限制，而是受触发反应限制，这些反应可能包括质子转移和/或构象变化。形成和释放 O(2)的最终反应序列的总速率被假定受 WOC 的 S(3)状态到 Y(z)(ox)的电子转移步骤限制，这是由于涉及到一个吸热氧化还原平衡。目前讨论的关于可能途径的有争议的观点被简要概述。氧化水分解机制的几个关键要点，如 O-O 键形成、局部质子转移的作用、氢键的细节，仍然没有得到澄清，仍然是未来研究的一个具有挑战性的课题。
