Saito Keisuke, Nakagawa Minesato, Ishikita Hiroshi
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.
Commun Chem. 2020 Jul 16;3(1):89. doi: 10.1038/s42004-020-00336-7.
Release of the protons from the substrate water molecules is prerequisite for O evolution in photosystem II (PSII). Proton-releasing water molecules with low pK values at the catalytic moiety can be the substrate water molecules. In some studies, one of the ligand water molecules, W2, is regarded as OH. However, the PSII crystal structure shows neither proton acceptor nor proton-transfer pathway for W2, which is not consistent with the assumption of W2 = OH. Here we report the pK values of the four ligand water molecules, W1 and W2 at Mn4 and W3 and W4 at Ca, of the MnCaO cluster. pK(W1) ≈ pK(W2) << pK(W3) ≈ pK(W4) in the MnCaO cluster in water. However, pK(W1) ≈ pK(D1-Asp61) << pK(W2) in the PSII protein environment. These results suggest that in PSII, deprotonation of W2 is energetically disfavored as far as W1 exists.
从底物水分子中释放质子是光系统II（PSII）中氧气生成的先决条件。在催化部分具有低pK值的释放质子的水分子可能是底物水分子。在一些研究中，配体水分子之一W2被视为OH。然而，PSII晶体结构既没有显示出W2的质子受体，也没有显示出质子转移途径，这与W2 = OH的假设不一致。在这里，我们报告了MnCaO簇中四个配体水分子的pK值，即Mn4处的W1和W2以及Ca处的W3和W4。在水中的MnCaO簇中，pK（W1）≈pK（W2）<< pK（W3）≈pK（W4）。然而，在PSII蛋白质环境中，pK（W1）≈pK（D1-Asp61）<< pK（W2）。这些结果表明，在PSII中，只要W1存在，W2的去质子化在能量上是不利的。
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