Insights into Proton-Transfer Pathways during Water Oxidation in Photosystem II
Insights into Proton-Transfer Pathways during Water Oxidation in Photosystem II
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DOI：
10.1021/acs.jpcb.9b06244
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发表时间：
2019-10-03
影响因子：
3.3
通讯作者：
BrudviG, Gary W.
中科院分区：
文献类型：
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作者：
Ghosh, Ipsita;Khan, Sahr;BrudviG, Gary W.
光系统II（PSII）对水的氧化作用包括在释氧复合体（OEC）中释放O-2、电子和质子。氨基酸残基和OEC周围的沃茨的氢键网络促进了这些过程。探测OEC的质子转移途径至关重要，因为质子释放有助于维持有效水氧化所需的电荷平衡。在这项研究中，我们在蓝细菌集胞藻PCC 6803中产生了OEC周围二级壳氨基酸残基的点突变：D2-K317，D1-S169，CP 43-R357，D1-D 61和D1-N181。我们采用直接的实验方法来研究在不同的pH值范围从3-8的O-2的进化速率。在野生型和突变的PSII中，pH依赖性遵循钟形曲线，从中我们可以推导出有效的酸性pK（a）。有效酸性pK（a）提供了对参与水氧化速率决定步骤期间质子转移过程的氨基酸残基的质子化状态的了解。在D1-S169 A PSII和D2-K317 A PSII中存在额外的有效pK（a），表明在水氧化的速率决定步骤中可能存在多个质子转移途径。我们还研究了在不同pL（L = H或D）的H2O和D2 O中O-2的释放速率，以确定参与质子转移过程的氨基酸残基。我们发现，取代带正电荷的赖氨酸与中性丙氨酸在D2-K317 A PSII和天冬氨酸与丙氨酸在D1-D 6 IA PSII显着增强的动力学溶剂同位素效应（KSIE），表明质子转移成为限速在这些突变的PSII的最佳pH值。然而，D1-N181 A、D1-S169 A和CP 43-R357 K PSII的KSIE保持不变。因此，扰动由D1-D 61和D2-K317残基限定的通道强烈地阻碍了质子转移机制，进而阻碍了PSII的水氧化反应。因此，我们的研究提供了一个直接的实验探针，以确定D1-D 61和D2-K317残基参与质子转移过程。这些结果，从而，为我们提供了更深入的了解质子转移过程中的水氧化机制。
Water oxidation by photosystem II (PSII) involves the release of O-2, electrons, and protons at the oxygen-evolving complex (OEC). These processes are facilitated by a hydrogen-bonded network of amino acid residues and waters surrounding the OEC. It is crucial to probe the proton-transfer pathways from the OEC as proton release helps to maintain the charge balance required for efficient water oxidation. In this study, we generate point mutations in the cyanobacterium Synechocystis sp. PCC 6803 at secondary-shell amino acid residues surrounding the OEC: D2-K317, D1-S169, CP43-R357, D1-D61, and D1-N181. We employ direct experimental methods to study the O-2 evolution rate under varying pH ranging from 3-8. The pH dependence follows a bell-shaped curve in both wild-type and mutated PSII from which we can derive the effective acidic pK(a). The effective acidic pK(a) provides insights into the protonation states of the amino acid residues participating in the proton-transfer process during the rate-determining step of water oxidation. The presence of an additional effective pK(a) in D1-S169A PSII and D2-K317A PSII indicates the possibility of multiple proton-transfer pathways during the ratedetermining step of water oxidation. We also studied the O-2 evolution rate in H2O and D2O with varying pL (L = H or D) to identify the amino acid residues participating in the proton-transfer process. We find that replacing the positively charged lysine with a neutral alanine in D2-K317A PSII and aspartate with alanine in D1-D6IA PSII significantly enhances the kinetic solvent isotope effect (KSIE), indicating that proton transfer becomes rate-limiting at the optimal pH in these mutated PSII. However, the KSIE remains unchanged for D1-N181A, D1-S169A, and CP43-R357K PSII. Thus, perturbing the channel defined by the D1-D61 and D2-K317 residues strongly hampers the proton-transfer mechanism, and in turn, the water oxidation reaction of PSII. Hence, our study provides a direct experimental probe to identify that the D1-D61 and D2-K317 residues participate in the proton-transfer process. These results, thereby, provide us a deeper understanding of the proton-transfer processes in the water oxidation mechanism.