Center for Computational Sciences , University of Tsukuba , Tennodai 1-1-1 , Tsukuba 305-8577 , Japan.
Graduate School of Natural Science and Technology, Faculty of Science , Okayama University , Okayama 700-8530 , Japan.
J Phys Chem B. 2018 Jun 28;122(25):6491-6502. doi: 10.1021/acs.jpcb.8b03465. Epub 2018 Jun 15.
The O release of the oxygen-evolving complex of the photosystem II (PSII) is one of the essential processes responsible for the highly efficient O production. Despite its importance, the detailed molecular mechanism is still unsolved. In the present study, we show that the O release is directly coupled with water insertion into the Mn cluster based on the quantum mechanics/molecular mechanics (QM/MM) calculations. In this mechanism, the O molecule first dissociates from the Mn sites in order, that is, the O atom coordinating to the Mn3 (O5a) first dissociates, then the other O atom coordinating to the Mn1 (O5d) dissociates in the next step in the late S state (1 → 2). Next, the O migrates to a space surrounded by the Val185 and His332 side chains as one water molecule coordinating to the Ca ion (W3) comes into the O bonded site (2 → 3). Finally, a pre-S state (4) is formed after a proton transfer from the inserted water to the other proton acceptor site (W2) (3 → 4). The highest activation barrier during these reactions was found at the O release step (2 → 3) that only requires E = 12.7 kcal mol ( G = 10.4 kcal mol). A series of the reactions (2 → 3) look like a chain crash of billiard balls because the W3 is inserted into the catalytic center from the water-abundant side (Ca ion side), and then the O moiety is pushed out to the opposite side (Val185 side). The hydrophobic residue of Val185 covers the active O5 site and forms an O-specific permeation tunnel. The present sequential reactions clearly demonstrate the efficient removal of the toxic O from the catalytic center and implications of the essential roles of Val185, Ca ions and water molecules, which are all present in the active site of PSII as the indispensable constituents.
放氧复合体（OEC）的 O 释放是负责高效 O 生成的基本过程之一。尽管其重要性，但其详细的分子机制仍未解决。在本研究中，我们基于量子力学/分子力学（QM/MM）计算表明，O 释放直接与水插入 Mn 簇偶联。在该机制中，O 分子首先依次从 Mn 位点解离，即首先解离与 Mn3（O5a）配位的 O 原子，然后在下一个迟 S 态（1→2）中，与 Mn1（O5d）配位的另一个 O 原子解离。接下来，O 迁移到由 Val185 和 His332 侧链包围的空间中，因为一个与 Ca 离子配位的水分子（W3）进入 O 结合位点（2→3）。最后，在插入水中的质子转移到另一个质子接受位点（W2）（3→4）后，形成预 S 态（4）。在这些反应中，最高的活化能垒出现在 O 释放步骤（2→3），仅需要 E = 12.7 kcal mol（ G = 10.4 kcal mol）。一系列反应（2→3）看起来像台球链碰撞，因为 W3 从水丰富侧（Ca 离子侧）插入到催化中心，然后将 O 部分推向相反的一侧（Val185 侧）。疏水残基 Val185 覆盖活性 O5 位点并形成 O 特异性渗透隧道。目前的顺序反应清楚地表明，从催化中心有效去除有毒 O，并暗示了 Val185、Ca 离子和水分子的重要作用，它们都是 PSII 活性位点中不可缺少的组成部分。
