Semin Boris K, Davletshina Lira N, Rubin Andrei B
Department of Biophysics, Faculty of Biology, Lomonosov Moscow State University, 119234, Moscow, Russia,
Photosynth Res. 2015 Aug;125(1-2):95-103. doi: 10.1007/s11120-015-0155-4. Epub 2015 May 15.
Effects of pH, Ca(2+), and Cl(-) ions on the extraction of Mn cations from oxygen-evolving complex (OEC) in Ca-depleted photosystem II (PSII(-Ca)) by exogenous reductants hydroquinone (H2Q) and H2O2 were studied. Two of 4 Mn cations are released by H2Q and H2O2 at pHs 5.7, 6.5, and 7.5, and their extraction does not depend on the presence of Ca(2+) and Cl(-) ions. One of Mn cations ("resistant" Mn cation) cannot be extracted by H2Q and H2O2 at any pH. Extraction of 4th Mn ion ("flexible" Mn cation) is sensitive to pH, Ca(2+), and Cl(-). This Mn cation is released by reductants at pH 6.5 but not at pHs 5.7 and 7.5. A pH dependence curve of the oxygen-evolving activity in PSII(-Ca) membranes (in the presence of exogenous Ca(2+)) has a bell-shaped form with the maximum at pH 6.5. Thus, the increase in the resistance of flexible Mn cation in OEC to the action of reductants at acidic and alkaline pHs coincides with the decrease in oxygen evolution activity at these pHs. Exogenous Ca(2+) protects the extraction of flexible Mn cation at pH 6.5. High concentration of Cl(-) anions (100 mM) shifts the pH optimum of oxygen evolution to alkaline region (around pH 7.5), while the pH of flexible Mn extraction is also shifted to alkaline pH. This result suggests that flexible Mn cation plays a key role in the water-splitting reaction. The obtained results also demonstrate that only one Mn cation in Mn4 cluster is under strong control of calcium. The change in the flexible Mn cation resistance to exogenous reductants in the presence of Ca(2+) suggests that Ca(2+) can control the redox potential of this cation.
研究了pH值、Ca(2+)和Cl(-)离子对通过外源还原剂对苯二酚(H2Q)和H2O2从缺钙光系统II (PSII(-Ca))的放氧复合体(OEC)中提取Mn阳离子的影响。在pH值为5.7、6.5和7.5时，4个Mn阳离子中的2个可被H2Q和H2O2释放，其提取不依赖于Ca(2+)和Cl(-)离子的存在。其中一个Mn阳离子("抗性"Mn阳离子)在任何pH值下都不能被H2Q和H2O2提取。第4个Mn离子("灵活"Mn阳离子)的提取对pH值、Ca(2+)和Cl(-)敏感。该Mn阳离子在pH 6.5时被还原剂释放，但在pH 5.7和7.5时不释放。PSII(-Ca)膜(在外源Ca(2+)存在下)放氧活性的pH依赖性曲线呈钟形，在pH 6.5时达到最大值。因此，OEC中灵活Mn阳离子在酸性和碱性pH值下对还原剂作用的抗性增加与这些pH值下放氧活性的降低相吻合。外源Ca(2+)在pH 6.5时保护灵活Mn阳离子的提取。高浓度的Cl(-)阴离子(100 mM)将放氧的最适pH值移至碱性区域(约pH 7.5)，而灵活Mn提取的pH值也移至碱性pH值。该结果表明灵活Mn阳离子在水裂解反应中起关键作用。所得结果还表明，Mn4簇中只有一个Mn阳离子受钙的严格控制。在Ca(2+)存在下灵活Mn阳离子对外源还原剂抗性的变化表明Ca(2+)可以控制该阳离子的氧化还原电位。
