Considerations on the mechanism of photosynthetic water oxidation - dual role of oxo-bridges between Mn ions in (i) redox-potential maintenance and (ii) proton abstraction from substrate water
Considerations on the mechanism of photosynthetic water oxidation - dual role of oxo-bridges between Mn ions in (i) redox-potential maintenance and (ii) proton abstraction from substrate water
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DOI：
10.1007/s11120-004-7080-2
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发表时间：
2005-06-01
影响因子：
3.7
通讯作者：
Haumann, M
中科院分区：
文献类型：
--
作者：
Dau, H;Haumann, M
本文讨论了光系统II (PS II) Mn络合物中光合水氧化的两个机制问题。(1)在四种Mn氧化转变中，预计任何纯Mn氧化都会导致氧化还原电位的增加，从而使后续氧化步骤无法进行（氧化还原电位问题）。未酸化氧桥的形成可以抵消潜在的增加。（II）没有任何高pk碱作为质子受体的O-O形成步骤在能量上是不利的（受体-碱问题）。在水氧化步骤中，Mn离子之间桥接位置的氧化物的pK可能随着Mn的减少而急剧增加，从而使它们成为有利的质子受体。在PS II中，在四个氧化转变过程中至少形成了两个未酸化的氧桥。因此(i)防止氧化还原电位增加和（ii）制备用于O-O形成步骤的质子受体。在O-O键形成步骤中，通过同时Mn还原和质子转移到桥接氧化物，相当于从底物水中的氢原子或氢化物转移到PS II的Mn配合物的Mn-oxo核心，促进了水的氧化。
Two mechanistic problems of photosynthetic water oxidation at the Mn complex of Photosystem II (PS II) are considered. (I) In the four Mn-oxidizing transitions, any pure Mn oxidation is predicted to cause an increase in redox potential that renders subsequent oxidation steps impossible (redox-potential problem). Formation of unprotonated oxo-bridges may counteract the potential increase. (II) The O-O formation step without any high-pK bases acting as proton acceptors is energetically unfavorable (acceptor-base problem). The pK of oxides in a bridging position between Mn ions may increase drastically upon reduction of Mn in the water-oxidation step (>10 units), thus rendering them favorable proton acceptors. It is proposed that in PS II, in the course of the four oxidizing transitions at least two unprotonated oxo-bridges are formed. Thereby (i) a redox potential increase is prevented and (ii) proton acceptors are prepared for the O-O formation step. Water oxidation in the O-O bond formation step is facilitated by simultaneous Mn reduction and proton transfer to bridging oxides amounting to hydrogen atom or hydride transfer from substrate water to the Mn-oxo core of the Mn complex of PS II.