Ishikita Hiroshi, Saito Keisuke
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. 2025 May 6;64(9):1895-1906. doi: 10.1021/acs.biochem.5c00112. Epub 2025 Apr 7.
Photosystem II (PSII) is a multisubunit protein-pigment complex that drives the oxidation of water, producing molecular oxygen essential for life. At the core of PSII, the oxygen-evolving complex (OEC) facilitates sequential four-electron oxidation steps following the Kok cycle. Despite significant progress in structural and spectroscopic studies, fundamental questions remain regarding the precise mechanisms of substrate water incorporation, deprotonation pathways, and oxygen-oxygen bond formation. A key challenge is determining the protonation states of water ligands and oxo bridges in the OEC, as incorrect assignments can eventually lead to misinterpretation of reaction energetics and mechanisms. This Review examines recent structural, spectroscopic, and theoretical studies, with a particular focus on proton transfer pathways and the role of key residues in regulating OEC deprotonation, emphasizing the importance of systematically establishing protonation states at lower S-states before modeling higher oxidation states. By integrating structural data with fundamental chemical principles, we outline essential considerations for constructing a physically meaningful and mechanistically coherent model of water oxidation in PSII.
光系统II（PSII）是一种多亚基蛋白质 - 色素复合体，它驱动水的氧化，产生生命所必需的分子氧。在PSII的核心，放氧复合体（OEC）遵循Kok循环促进连续的四电子氧化步骤。尽管在结构和光谱研究方面取得了重大进展，但关于底物水掺入的精确机制、去质子化途径和氧 - 氧键形成等基本问题仍然存在。一个关键挑战是确定OEC中水配体和氧桥的质子化状态，因为错误的分配最终可能导致对反应能量学和机制的错误解读。本综述考察了最近的结构、光谱和理论研究，特别关注质子转移途径以及关键残基在调节OEC去质子化中的作用，强调在模拟更高氧化态之前系统地确定较低S态下的质子化状态的重要性。通过将结构数据与基本化学原理相结合，我们概述了构建PSII中水氧化的物理上有意义且机制上连贯的模型的基本考虑因素。
