Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8654, Japan.
Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan.
Biochemistry. 2020 Sep 8;59(35):3216-3224. doi: 10.1021/acs.biochem.0c00177. Epub 2020 Jul 12.
In photosystem II, water oxidation occurs at the oxygen-evolving complex (OEC). The presence of a hydronium ion (HO) was proposed at the Cl binding site and Ca-depleted OEC. Using a quantum mechanical/molecular mechanical approach, we report the stability of HO in the PSII protein environment. Neither release of the proton from ligand water molecule W2 at the OEC nor formation of HO at Cl is energetically favorable. In contrast, HO can exist at the Ca-depleted OEC. Even when HO exists in Ca-depleted PSII, the H-bond network of the redox-active tyrosine (TyrZ) remains unaltered, retaining the unusually short low-barrier H-bond with D1-His190, and the redox potential of TyrZ, (TyrZ), remains unaltered. These observations explain why the oxidation of the Ca-depleted MnO cluster by TyrZ (i.e., the S to S transition) is not inhibited at low pH. It seems likely that Ca plays a role in not only (i) maintaining the H-bond network and facilitating TyrZ oxidation [tuning (TyrZ)] but also (ii) providing the valence of +2, decreasing the p of the ligand molecule (W1), and facilitating the release of the proton from W1 in the S to S transition together with Cl.
在光系统 II 中，水的氧化发生在氧释放复合体（OEC）处。在 Cl 结合位点和钙耗尽的 OEC 中，提出了质子化氢离子（HO）的存在。我们使用量子力学/分子力学方法，报告了 HO 在 PSII 蛋白质环境中的稳定性。从 OEC 处的配体水分子 W2 中释放质子，或者在 Cl 处形成 HO，在能量上都没有优势。相比之下，HO 可以存在于钙耗尽的 OEC 中。即使在钙耗尽的 PSII 中存在 HO，氧化还原活性酪氨酸（TyrZ）的氢键网络也保持不变，保持与 D1-His190 的不寻常短低势氢键，并且 TyrZ 的氧化还原电位（TyrZ）保持不变。这些观察结果解释了为什么在低 pH 下，TyrZ 对钙耗尽的 MnO 簇的氧化（即 S 到 S 跃迁）不会被抑制。似乎钙不仅起到了（i）维持氢键网络和促进 TyrZ 氧化[调谐（TyrZ）]的作用，而且（ii）提供了+2 价，降低了配体分子（W1）的 p 值，并在 S 到 S 跃迁中与 Cl 一起促进了 W1 质子的释放。
