Probing the functional role of Ca2+ in the oxygen-evolving complex of photosystem II by metal ion inhibition
Probing the functional role of Ca2+ in the oxygen-evolving complex of photosystem II by metal ion inhibition
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DOI：
10.1021/bi062033i
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发表时间：
2007-03-20
期刊：
影响因子：
2.9
通讯作者：
Brudvig, Gary W.
中科院分区：
文献类型：
--
作者：
Lee, Cheng-I;Lakshmi, K. V.;Brudvig, Gary W.
光系统II（PSII）中的光合放氧发生在由四核锰簇（Mn-4）、氧化还原活性酪氨酸残基（Y-Z）以及Ca 2+和Cl-辅因子组成的放氧复合物（OEC）中。OEC通过吸收4个量子的光而被连续氧化，这导致水的氧化和O-2的释放。Ca ~（2+）是水氧化反应中的重要辅助因子，它的缺乏会导致PSII中氧释放活性的丧失。在最近的X射线晶体结构中，Ca 2+已被揭示与PSII的Mn-4簇有关。虽然已经提出了几种机制的PSII的水氧化反应，在氧的演变中的作用的Ca 2+仍然不清楚。在这项研究中，我们探测的作用，Ca 2+在析氧监测的S-1到S-2状态转换PSII膜和PSII核心复合物后，抑制氧气的释放Dy 3+，Cu 2+，Cd 2+离子。通过使用阳离子交换程序，其中Ca 2+不被删除之前，除了所研究的阳离子，我们实现了高程度的可逆抑制PSII膜和PSII核心复合物的Dy 3+，Cu 2+，和Cd 2+离子。EPR光谱用于定量每个PSII中心结合的Dy 3+和Cu 2+离子的数量，并确定Dy 3+与PSII中其他顺磁中心的接近度。我们观察到，为第一次，S-2状态的多线电子顺磁共振（EPR）信号在Dy 3+和Cd 2+抑制PSII和得出结论，Ca 2+辅因子是不是特别需要的S-1到S-2状态转换的PSII。这一观察结果直接支持了这样的假设，即Ca 2+在早期S-状态转变中起结构作用，这可以由类似离子半径的其他阳离子实现，并且Ca 2+在S状态循环的最后一步中发生的O-O键形成反应中活化水的功能作用只能由Ca 2+和Sr 2+实现，它们具有类似的刘易斯酸性。
Photosynthetic oxygen evolution in photosystem II (PSII) takes place in the oxygen-evolving complex (OEC) that is comprised of a tetranuclear manganese cluster (Mn-4), a redox-active tyrosine residue (Y-Z), and Ca2+ and Cl- cofactors. The OEC is successively oxidized by the absorption of 4 quanta of light that results in the oxidation of water and the release of O-2. Ca2+ is an essential cofactor in the water-oxidation reaction, as its depletion causes the loss of the oxygen-evolution activity in PSII. In recent X-ray crystal structures, Ca2+ has been revealed to be associated with the Mn-4 cluster of PSII. Although several mechanisms have been proposed for the water-oxidation reaction of PSII, the role of Ca2+ in oxygen evolution remains unclear. In this study, we probe the role of Ca2+ in oxygen evolution by monitoring the S-1 to S-2 state transition in PSII membranes and PSII core complexes upon inhibition of oxygen evolution by Dy3+, Cu2+, and Cd2+ ions. By using a cation-exchange procedure in which Ca2+ is not removed prior to addition of the studied cations, we achieve a high degree of reversible inhibition of PSII membranes and PSII core complexes by Dy3+, Cu2+, and Cd2+ ions. EPR spectroscopy is used to quantitate the number of bound Dy3+ and Cu2+ ions per PSII center and to determine the proximity of Dy3+ to other paramagnetic centers in PSII. We observe, for the first time, the S-2 state multiline electron paramagnetic resonance (EPR) signal in Dy3+- and Cd2+-inhibited PSII and conclude that the Ca2+ cofactor is not specifically required for the S-1 to S-2 state transition of PSII. This observation provides direct support for the proposal that Ca2+ plays a structural role in the early S-state transitions, which can be fulfilled by other cations of similar ionic radius, and that the functional role of Ca2+ to activate water in the O-O bond-forming reaction that occurs in the final step of the S state cycle can only be fulfilled by Ca2+ and Sr2+, which have similar Lewis acidities.