In photosystem II (PSII), photosynthetic water oxidation occurs at the O₂-evolving complex (OEC), a tetramanganese-calcium cluster that cycles through light-induced redox intermediates (S₀–S₄) to produce oxygen from two substrate water molecules. The OEC is surrounded by a hydrogen-bonded network of amino-acid residues that plays a crucial role in proton transfer and substrate water delivery. Previously, we found that D1-S169 was crucial for water oxidation and its mutation to alanine perturbed the hydrogen-bonding network. In this study, we demonstrate that the activation energy for the S₂ to S₁ transition of D1-S169A PSII is higher than wild-type PSII with a ~1.7–2.7× slower rate of charge recombination with Q_A⁻ relative to wild-type PSII. Arrhenius analysis of the decay kinetics shows an E_a of 5.87 ± 1.15 kcal mol⁻¹ for decay back to the S₁ state, compared to 0.80 ± 0.13 kcal mol⁻¹ for the wild-type S₂ state. In addition, we find that ammonia does not affect the S₂-state EPR signal, indicating that ammonia does not bind to the Mn cluster in D1-S169A PSII. Finally, a QM/MM analysis indicates that an additional water molecule binds to the Mn4 ion in place of an oxo ligand O5 in the S₂ state of D1-S169A PSII. The altered S₂ state of D1-S169A PSII provides insight into the S₂➔S₃ state transition.

在光系统II（PSII）中，光合水的氧化发生在O ₂演化复合物（OEC）上，该复合物是四锰钙簇，通过光诱导的氧化还原中间体（S ₀ –S ₄）循环，从两个底物水分子中产生氧气。OEC周围是氨基酸残基的氢键网络，该网络在质子转移和底物水输送中起关键作用。以前，我们发现D1-S169对水氧化至关重要，并且其突变为丙氨酸会干扰氢键网络。在这项研究中，我们证明了S ₂至S ₁的活化能D1-S169A PSII的过渡大于与具有Q一〜1.7-2.7×较慢速率的电荷重组野生型PSII更高_甲^-相对于野生型PSII。衰变动力学的Arrhenius分析显示，衰减回S ₁状态的E _a为5.87±1.15 kcal mol ^-1，而野生型S ₂状态的E _a为0.80±0.13 kcal mol ^-1。此外，我们发现氨气不会影响S ₂状态EPR信号，表明氨气不会与D1-S169A PSII中的Mn团簇结合。最后，QM / MM分析表明，另外一个水分子与Mn4离子结合，代替了S ₂中的氧代配体O5。D1-S169A PSII的状态。改变的小号₂ D1-S169A PSII的状态提供了洞察在S ₂ ➔S ₃状态转变。