Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803;
Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803.
Proc Natl Acad Sci U S A. 2019 Sep 17;116(38):18917-18922. doi: 10.1073/pnas.1910231116. Epub 2019 Sep 4.
Photosystem II (PSII) performs the solar-driven oxidation of water used to fuel oxygenic photosynthesis. The active site of water oxidation is the oxygen-evolving complex (OEC), a MnCaO cluster. PSII requires degradation of key subunits and reassembly of the OEC as frequently as every 20 to 40 min. The metals for the OEC are assembled within the PSII protein environment via a series of binding events and photochemically induced oxidation events, but the full mechanism is unknown. A role of proton release in this mechanism is suggested here by the observation that the yield of in vitro OEC photoassembly is higher in deuterated water, DO, compared with HO when chloride is limiting. In kinetic studies, OEC photoassembly shows a significant lag phase in HO at limiting chloride concentrations with an apparent H/D solvent isotope effect of 0.14 ± 0.05. The growth phase of OEC photoassembly shows an H/D solvent isotope effect of 1.5 ± 0.2. We analyzed the protonation states of the OEC protein environment using classical Multiconformer Continuum Electrostatics. Combining experiments and simulations leads to a model in which protons are lost from amino acid that will serve as OEC ligands as metals are bound. Chloride and DO increase the proton affinities of key amino acid residues. These residues tune the binding affinity of Mn and facilitate the deprotonation of water to form a proposed μ-hydroxo bridged MnMn intermediate.
光系统 II（PSII）进行太阳能驱动的水氧化，用于为产氧光合作用提供燃料。水氧化的活性位点是放氧复合物（OEC），一种 MnCaO 簇。PSII 需要频繁降解关键亚基并重新组装 OEC，频率高达每 20 到 40 分钟一次。OEC 的金属通过一系列结合事件和光化学诱导的氧化事件在 PSII 蛋白环境中组装，但完整的机制尚不清楚。这里通过观察到在氯有限的情况下，与 HO 相比，DO 中体外 OEC 光组装的产率更高，表明质子释放在此机制中起作用。在动力学研究中，在氯有限的情况下，OEC 光组装在 HO 中显示出明显的滞后阶段，表观 H/D 溶剂同位素效应为 0.14±0.05。OEC 光组装的生长阶段显示 H/D 溶剂同位素效应为 1.5±0.2。我们使用经典多构象连续静电学分析了 OEC 蛋白环境的质子化状态。将实验和模拟结合起来，得到了一个模型，其中金属结合时，氨基酸会失去质子，这些氨基酸将作为 OEC 配体。氯和 DO 增加了关键氨基酸残基的质子亲和力。这些残基调节 Mn 的结合亲和力，并促进水的去质子化，形成一个提议的 μ-羟桥接 MnMn 中间物。
