The S3 state of the oxygen-evolving complex in photosystem II is converted to the S2YZ• state at alkaline pH
The S3 state of the oxygen-evolving complex in photosystem II is converted to the S2YZ• state at alkaline pH
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DOI：
10.1021/bi010040v
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发表时间：
2001-09-11
期刊：
影响因子：
2.9
通讯作者：
Styring, S
中科院分区：
文献类型：
--
作者：
Geijer, P;Morvaridi, F;Styring, S
在这里，我们报告的EPR信号，诱导的pH值跳到碱性pH值的S-3状态的光系统II中的放氧复合物。S-3状态首先在pH 6下通过两次闪蒸形成。此后，在冷冻样品之前，在黑暗中改变pH。EPR信号的宽度为90-100 G，以g=2为中心。该信号是可逆诱导的，pK=8.5 +/-0.3，并且非常稳定，衰减半衰期为5-6 min。如果在黑暗中将pH从pH 8.6变为6.0，则信号消失，尽管S-3状态仍然存在。我们认为该信号是由Mn团簇与Y-Z之间的相互作用产生的，从而产生了自旋耦合的S2 YZcircle信号。我们的数据表明，在S-3状态下，Y-Z（圆圈）/Y-Z氧化还原对的电势对环境pH敏感。碱性pH降低了Y-Z（圆圈）/Y-Z电对的电势，使得Y-Z可以将电子返回到S-3状态，从而获得S2 YZ圆圈EPR信号。酪氨酸氧化还涉及质子从Y-Z释放，并且结果支持这样的机制，其中该质子被释放到本体介质中，推测是通过靠近的碱基。因此，通过碱性pH将平衡从S3 YZ改变为S2 YZ循环。在正常pH（pH 5.5-7）下，该平衡强烈地设定为S3 YZ状态。结果进行了讨论，与目前的水氧化模型。讨论了在不同pH值下Y-z（圆圈）/Y-z和S-3/S-2的相对氧化还原电位的结果。我们还比较了pH诱导的S2 YZcircle信号与来自Ca 2+耗尽的光系统II的S2 YZcircle信号。
Here we report an EPR signal that is induced by a pH jump to alkaline pH in the S-3 state of the oxygen-evolving complex in photosystem II. The S-3 state is first formed with two flashes at pH 6. Thereafter, the pH is changed in the dark prior to freezing of the sample. The EPR signal is 90-100 G wide and centered around g=2. The signal is reversibly induced with a pK=8.5 +/-0.3 and is very stable with a decay half-time of 5-6 min. If the pH is changed in the dark from pH 8.6 to 6.0, the signal disappears although the S-3 state remains. We propose that the signal arises from the interaction between the Mn cluster and Y-Z, resulting in the spin-coupled S2YZcircle signal. Our data suggest that the potential of the Y-Z(circle)/Y-Z redox couple is sensitive to the ambient pH in the S-3 state. The alkaline pH decreases the potential of the Y-Z(circle)/Y-Z couple so that Y-Z can give back an electron to the S-3 state, thereby obtaining the S2YZcircle EPR signal. The tyrosine oxidation also involves proton release from Y-Z, and the results support a mechanism where this proton is released to the bulk medium presumably via a close-lying base. Thus, the equilibrium is changed from S3YZ to S2YZcircle by the alkaline pH. At normal pH (pH 5.5-7), this equilibrium is set strongly to the S3YZ state. The results are discussed in relation to the present models of water oxidation. Consequences for the relative redox potentials of Y-z(circle)/Y-z and S-3/S-2 at different pH values are discussed. We also compare the pH-induced S2YZcircle signal with the S2YZcircle signal from Ca2+- depleted photosystem II.