Energetics of Ionized Water Molecules in the H-Bond Network near the Ca2+ and Cl- Binding Sites in Photosystem II.,Biochemistry

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Energetics of Ionized Water Molecules in the H-Bond Network near the Ca2+ and Cl- Binding Sites in Photosystem II.
Biochemistry ( IF 2.9 ) Pub Date : 2020-06-30 , DOI: 10.1021/acs.biochem.0c00177
Keisuke Saito _{1,

2} , Manoj Mandal ₂ , Hiroshi Ishikita _{1,

2}

Affiliation

In photosystem II, water oxidation occurs at the oxygen-evolving complex (OEC). The presence of a hydronium ion (H₃O⁺) was proposed at the Cl^– binding site and Ca²⁺-depleted OEC. Using a quantum mechanical/molecular mechanical approach, we report the stability of H₃O⁺ in the PSII protein environment. Neither release of the proton from ligand water molecule W2 at the OEC nor formation of H₃O⁺ at Cl^– is energetically favorable. In contrast, H₃O⁺ can exist at the Ca²⁺-depleted OEC. Even when H₃O⁺ exists in Ca²⁺-depleted PSII, the H-bond network of the redox-active tyrosine (TyrZ) remains unaltered, retaining the unusually short low-barrier H-bond with D1-His190, and the redox potential of TyrZ, E_m(TyrZ), remains unaltered. These observations explain why the oxidation of the Ca²⁺-depleted Mn₄O₅ cluster by TyrZ (i.e., the S₂ to S₃ transition) is not inhibited at low pH. It seems likely that Ca²⁺ plays a role in not only (i) maintaining the H-bond network and facilitating TyrZ oxidation [tuning E_m(TyrZ)] but also (ii) providing the valence of +2, decreasing the pK_a of the ligand molecule (W1), and facilitating the release of the proton from W1 in the S₂ to S₃ transition together with Cl^–.

中文翻译：

H-Bond网络中光电系统II中Ca2 +和Cl-结合位点附近的电离水分子的能量学。

在光系统II中，水氧化发生在放氧复合物（OEC）处。水合氢离子（H的存在₃ ö ⁺），提出了在C1- ^-结合位点和Ca ²⁺耗尽的OEC。使用量子力学/分子力学方法，我们报告了H ₃ O ⁺在PSII蛋白环境中的稳定性。在OEC上从配体水分子W2释放质子，或在Cl ^–上形成H ₃ O ⁺都不在能量上有利。相反，H ₃ O ⁺可以存在于贫Ca ^2+的OEC处。即使H ₃ O⁺存在于贫Ca ^2+的PSII中，氧化还原活性酪氨酸（TyrZ）的H键网络保持不变，保留了D1-His190异常短的低势垒H键，以及TyrZ，E的氧化还原电势_m（TyrZ），保持不变。这些观察结果解释了为什么在低pH下不抑制TyrZ对贫Ca ^2+的Mn ₄ O ₅簇的氧化（即S ₂到S _3的转变）。Ca ²⁺可能不仅在（i）维持H键网络和促进TyrZ氧化[调节E _m（TyrZ）]，而且（ii）提供+2的化合价，降低配体分子（W1）的p K _a，并促进质子从W1的S ₂过渡到S ₃跃迁以及Cl ^–。

更新日期：2020-09-08

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