In cyanobacteria, Glu-244 and Tyr-246 of the Photosystem II (PS II) D1 protein are hydrogen bonded to two water molecules that are part of a hydrogen-bond network between the bicarbonate ligand to a non-heme iron and the cytosol. Ala substitutions were introduced in Synechocystis sp. PCC 6803 to investigate the roles of these residues and the hydrogen-bond network on electron transfer between the primary plastoquinone acceptor, QA, and the secondary plastoquinone acceptor, QB, of the quinone-Fe-acceptor complex. All mutants assembled PS II; however, an increase in the PS II to PS I ratio was apparent, particularly in the E244A:Y246A double mutant. The mutants also showed impaired oxygen evolution and retarded chlorophyll a fluorescence decays following single turnover actinic flashes, which appeared to be primarily due to reduced QB binding in the E244A strain and an enhanced back reaction with the S2 state of the oxygen-evolving complex in the Y246A mutant. Impaired PS II in the Y246A and E244A:Y246A mutants resulted in inactivation of the psbA gene encoding D1. The Y246A and E244A:Y246A mutants also showed high light sensitivity whereas the E244A mutant showed enhanced resilience towards photodamage. Unlike the control strain, all of the mutants were insensitive to the addition of formate or bicarbonate in assays following chlorophyll decay kinetics that reflect electron transfer between QA and QB, suggesting the bicarbonate binding environment was perturbed. Our data also indicate that waters W582 and W622 (PDB: 4UB6) have essential roles in maintaining the architecture of the acceptor side of PS II.

中文翻译：

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Photosystem II的碳酸氢盐结合环境的D1：Glu244和D1：Tyr246具有中等的高光敏感性和通过醌-铁-受体配合物的电子转移。

在蓝细菌中，光系统II（PS II）D1蛋白的Glu-244和Tyr-246被氢键合到两个水分子上，这两个水分子是碳酸氢盐配体与非血红素铁和胞质溶胶之间氢键网络的一部分。Ala替代被引入到Synechocystis sp.。PCC 6803旨在研究这些残基和氢键网络对醌-铁-受体复合物的一级质体醌受体QA和次级质体醌受体QB之间电子转移的作用。所有突变体组装了PS II；但是，PS II与PS I的比率明显增加，尤其是在E244A：Y246A双重突变体中。这些突变体还显示出氧气释放受损和叶绿素a延迟（单次光化光化闪光后荧光衰减），这似乎主要归因于E244A菌株中QB结合减少以及Y246A突变体中与放氧复合物的S2状态增强的逆反应。Y246A和E244A：Y246A突变体中的PS II受损导致编码D1的psbA基因失活。Y246A和E244A：Y246A突变体也显示出高的光敏性，而E244A突变体显示出增强的对光损伤的适应性。与对照菌株不同，所有突变体在反映了QA和QB之间电子转移的叶绿素衰变动力学之后的测定中对甲酸或碳酸氢盐的添加不敏感，表明碳酸氢盐的结合环境受到干扰。我们的数据还表明，水域W582和W622（PDB：4UB6）在维护PS II受体侧的结构方面具有重要作用。