Saito Keisuke, Nishio Shunya, Asada Mizue, Mino Hiroyuki, 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.
PNAS Nexus. 2023 Jul 28;2(8):pgad244. doi: 10.1093/pnasnexus/pgad244. eCollection 2023 Aug.
In photosystem II (PSII), one-electron oxidation of the most stable oxidation state of the MnCaO cluster (S) leads to formation of two distinct states, the open-cubane S conformation [Mn1(III)Mn2(IV)Mn3(IV)Mn4(IV)] with low spin and the closed-cubane S conformation [Mn1(IV)Mn2(IV)Mn3(IV)Mn4(III)] with high spin. In electron paramagnetic resonance (EPR) spectroscopy, the open-cubane S conformation exhibits a = 2 multiline signal. However, its protonation state remains unclear. Here, we investigated the protonation state of the open-cubane S conformation by calculating exchange couplings in the presence of the PSII protein environment and simulating the pulsed electron-electron double resonance (PELDOR). When a ligand water molecule, which forms an H-bond with D1-Asp61 (W1), is deprotonated at dangling Mn4(IV), the first-exited energy (34 cm) in manifold spin excited states aligns with the observed value in temperature-dependent pulsed EPR analyses, and the PELDOR signal is best reproduced. Consequently, the = 2 multiline signal observed in EPR corresponds to the open-cubane S conformation with the deprotonated W1 (OH).
在光系统II（PSII）中，锰钙氧簇（S）最稳定氧化态的单电子氧化会导致形成两种不同的状态，即具有低自旋的开放立方烷S构象[Mn1(III)Mn2(IV)Mn3(IV)Mn4(IV)]和具有高自旋的封闭立方烷S构象[Mn1(IV)Mn2(IV)Mn3(IV)Mn4(III)]。在电子顺磁共振（EPR）光谱中，开放立方烷S构象呈现出g = 2的多线信号。然而，其质子化状态仍不清楚。在此，我们通过在PSII蛋白质环境存在下计算交换耦合并模拟脉冲电子-电子双共振（PELDOR）来研究开放立方烷S构象的质子化状态。当与D1-天冬氨酸61（W1）形成氢键的配体水分子在悬空的Mn4(IV)处去质子化时，多重自旋激发态中的第一激发能（34 cm）与温度相关脉冲EPR分析中的观测值一致，并且PELDOR信号得到了最佳重现。因此，EPR中观测到的g = 2多线信号对应于具有去质子化W1（OH）的开放立方烷S构象。
