Ammonia binding to the oxygen-evolving complex of photosystem II identifies the solvent-exchangeable oxygen bridge (μ-oxo) of the manganese tetramer
Ammonia binding to the oxygen-evolving complex of photosystem II identifies the solvent-exchangeable oxygen bridge (μ-oxo) of the manganese tetramer
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DOI：
10.1073/pnas.1304334110
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发表时间：
2013-09-24
影响因子：
11.1
通讯作者：
Cox, Nicholas
中科院分区：
文献类型：
--
作者：
Navarro, Montserrat Perez;Ames, William M.;Cox, Nicholas
光系统II的四锰五氧钙(Mn4O5Ca)团簇的两个底物水位的归属对于阐明生物O-O键的形成机制和随后的生物启发裂水催化剂的设计是至关重要的。我们最近使用脉冲EPR光谱证明了五个氧桥之一(u-oxo)与散装水交换的速度异常快，因此很可能是其中一个底物的候选者。氨，一种水的类似物，以前被证明与Mn4O5Ca簇结合，潜在地取代水/底物配体[Britt RD，等人]。(1989)化学学报111(10)：3522-3532]。在这里，我们通过EPR和时间分辨膜进样质谱仪的结合表明，氨的结合扰乱了可交换的Muoxo桥，而不会显著改变两种底物的结合/交换动力学。结合破缺对称密度泛函理论，我们的结果表明：(1)通过对当前晶体结构的标记，可交换的Muoxo桥为O5{[Umena Y，et al.(2011)性质473(7345)：55-60]}；(Ii)氨取代水配体到外部的锰(Mn-A4-W1)；和(Iii)当W1反式为O5时，氨结合拉长了Mn-A4-O5键，导致Mu-oxo桥共振的扰动和水交换率的微小变化。这些实验结果支持Siegbahn提出的O5和可能的氧自由基之间的O-O键的形成，并排除了W1作为第二底物水。
The assignment of the two substrate water sites of the tetramanganese penta-oxygen calcium (Mn4O5Ca) cluster of photosystem II is essential for the elucidation of the mechanism of biological O-O bond formation and the subsequent design of bio-inspired water-splitting catalysts. We recently demonstrated using pulsed EPR spectroscopy that one of the five oxygen bridges (mu-oxo) exchanges unusually rapidly with bulk water and is thus a likely candidate for one of the substrates. Ammonia, a water analog, was previously shown to bind to the Mn4O5Ca cluster, potentially displacing a water/substrate ligand [Britt RD, et al. (1989) J Am Chem Soc 111(10):3522-3532]. Here we show by a combination of EPR and time-resolved membrane inlet mass spectrometry that the binding of ammonia perturbs the exchangeable mu-oxo bridge without drastically altering the binding/exchange kinetics of the two substrates. In combination with broken-symmetry density functional theory, our results show that (i) the exchangable mu-oxo bridge is O5 {using the labeling of the current crystal structure [Umena Y, et al. (2011) Nature 473(7345):55-60]}; (ii) ammonia displaces a water ligand to the outer manganese (Mn-A4-W1); and (iii) as W1 is trans to O5, ammonia binding elongates the Mn-A4-O5 bond, leading to the perturbation of the mu-oxo bridge resonance and to a small change in the water exchange rates. These experimental results support O-O bond formation between O5 and possibly an oxyl radical as proposed by Siegbahn and exclude W1 as the second substrate water.