Evidence from FTIR Difference Spectroscopy That D1-Asp61 Influences the Water Reactions of the Oxygen-Evolving Mn4CaO5 Cluster of Photosystem II
Evidence from FTIR Difference Spectroscopy That D1-Asp61 Influences the Water Reactions of the Oxygen-Evolving Mn4CaO5 Cluster of Photosystem II
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DOI：
10.1021/bi500309f
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发表时间：
2014-05-13
期刊：
影响因子：
2.9
通讯作者：
Debus, Richard J.
中科院分区：
文献类型：
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作者：
Debus, Richard J.
理解光合水氧化的机制需要表征作为底物或以其他方式与氧释放Mn 4CaO 5簇相互作用的水分子的反应。FTIR差谱是研究氢键水分子结构变化的有力工具。例如，具有相对较弱氢键的水分子的O-H伸缩模式可以在3600 cm（-1）附近监测，D-O-D弯曲模式可以在1210 cm（-1）附近监测，并且高度可极化的氢键网络可以在3000和2000 cm（-1）之间作为宽特征监测。前两个区域几乎没有来自蛋白质的重叠振动模式。在光系统II中，水氧化需要一个精确编排的质子和电子转移步骤序列，其中需要质子释放来防止Mn 4CaO 5簇的氧化还原电位上升到阻止其随后氧化的水平。质子释放通过一个或多个质子出口途径从Mn 4CaO 5簇到类囊体腔。越来越多的证据表明，D1-D 61是一个主要质子出口途径的初始残基。该残基直接与Mn 4CaO 5簇的第一和第二配位球中的水分子相互作用。在这项研究中，我们探讨了D1-D 61对水反应伴随着氧气的生产的影响，通过表征的D1-D 61 A突变的蓝藻，集胞藻属PCC 6803的FTIR特性。基于突变引起的1747 cm（-1）附近羰基伸缩区域的变化，我们得出结论，D1-D 61参与了先前通过FTIR研究确定的相同的广泛氢键网络。基于突变引起的弱氢键O-H伸缩区的变化，我们得出结论，D1-D 61与位于Cl-（1）离子附近的水分子相互作用，并且由于S-1到S-2和S-2到S-3的转变而去质子化或参与更强的氢键。在消除3100和2600 cm（-1）之间的宽特征的基础上，我们得出结论，其极化率或质子化状态在S1到S2转变期间增加的高度可极化的氢键网络涉及D1-D 61。在消除D-O-D弯曲区特征的基础上，我们得出结论，D1-D 61与H-O-H弯曲模式响应于S1到S2跃迁而改变的H2O分子之一形成氢键。D1-D 61 A突变体中该H2O分子的消除为该突变体中水氧化效率降低提供了一个理由。最后，我们讨论的原因，最近的结论，一个基板包含集群的五个水分子接受质子从Mn 4CaO 5集群在S，S2过渡和去质子化在随后的S状态转换过程中应重新评估。
Understanding the mechanism of photosynthetic water oxidation requires characterizing the reactions of the water molecules that serve as substrate or that otherwise interact with the oxygen-evolving Mn4CaO5 cluster. FTIR difference spectroscopy is a powerful tool for studying the structural changes of hydrogen bonded water molecules. For example, the O-H stretching mode of water molecules having relatively weak hydrogen bonds can be monitored near 3600 cm(-1), the D-O-D bending mode can be monitored near 1210 cm(-1), and highly polarizable networks of hydrogen bonds can be monitored as broad features between 3000 and 2000 cm(-1). The two former regions are practically devoid of overlapping vibrational modes from the protein. In Photosystem II, water oxidation requires a precisely choreographed sequence of proton and electron transfer steps in which proton release is required to prevent the redox potential of the Mn4CaO5 cluster from rising to levels that would prevent its subsequent oxidation. Proton release takes place via one or more proton egress pathways leading from the Mn4CaO5 cluster to the thylakoid lumen. There is growing evidence that D1-D61 is the initial residue of one dominant proton egress pathway. This residue interacts directly with water molecules in the first and second coordination spheres of the Mn4CaO5 cluster. In this study, we explore the influence of D1-D61 on the water reactions accompanying oxygen production by characterizing the FTIR properties of the D1-D61A mutant of the cyanobacterium, Synechocystis sp. PCC 6803. On the basis of mutation-induced changes to the carbonyl stretching region near 1747 cm(-1), we conclude that D1-D61 participates in the same extensive networks of hydrogen bonds that have been identified previously by FTIR studies. On the basis of mutation-induced changes to the weakly hydrogen-bonded O-H stretching region, we conclude that D1-D61 interacts with water molecules that are located near the Cl-(1) ion and that deprotonate or participate in stronger hydrogen bonds as a result of the S-1 to S-2 and S-2 to S-3 transitions. On the basis of the elimination of a broad feature between 3100 and 2600 cm(-1), we conclude that the highly polarizable network of hydrogen bonds whose polarizability or protonation state increases during the SI to S2 transition involves D1-D61. On the basis of the elimination of features in the D-O-D bending region, we conclude that D1-D61 forms a hydrogen bond to one of the H2O molecules whose H-O-H bending mode changes in response to the S, to S2 transition. The elimination of this H2O molecule in the D1-D61A mutant provides one rationale for the decreased efficiency of water oxidation in this mutant. Finally, we discuss reasons why the recent conclusion that a substrate-containing cluster of five water molecules accepts a proton from the Mn4CaO5 cluster during the S, to S2 transition and deprotonates during subsequent S state transitions should be reassessed.