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Photosynthetic water splitting by the Mn4Ca2+OX catalyst of photosystem II: its structure, robustness and mechanism
Quarterly Reviews of Biophysics ( IF 6.1 ) Pub Date : 2017-11-06 , DOI: 10.1017/s0033583517000105
James Barber 1
Affiliation  

The biological energy cycle of our planet is driven by photosynthesis whereby sunlight is absorbed by chlorophyll and other accessory pigments. The excitation energy is then efficiently transferred to a reaction centre where charge separation occurs in a few picoseconds. In the case of photosystem II (PSII), the energy of the charge transfer state is used to split water into oxygen and reducing equivalents. This is accomplished by the relatively low energy content of four photons of visible light. PSII is a large multi-subunit membrane protein complex embedded in the lipid environment of the thylakoid membranes of plants, algae and cyanobacteria. Four high energy electrons, together with four protons (4H+), are used to reduce plastoquinone (PQ), the terminal electron acceptor of PSII, to plastoquinol (PQH2). PQH2 passes its reducing equivalents to an electron transfer chain which feeds into photosystem I (PSI) where they gain additional reducing potential from a second light reaction which is necessary to drive CO2 reduction. The catalytic centre of PSII consists of a cluster of four Mn ions and a Ca2+ linked by oxo bonds. In addition, there are seven amino acid ligands. In this Article, I discuss the structure of this metal cluster, its stability and the probability that an acid-base (nucleophilic-electrophilic) mechanism catalyses the water splitting reaction on the surface of the metal-cluster. Evidence for this mechanism is presented from studies on water splitting catalysts consisting of organo-complexes of ruthenium and manganese and also by comparison with the enzymology of carbon monoxide dehydrogenase (CODH). Finally the relevance of our understanding of PSII is discussed in terms of artificial photosynthesis with emphasis on inorganic water splitting catalysts as oxygen generating photoelectrodes.

中文翻译:

光系统Ⅱ的Mn4Ca2+OX催化剂光合水分解:其结构、稳定性和机理

我们星球的生物能量循环由光合作用驱动,阳光被叶绿素和其他辅助色素吸收。然后激发能量被有效地转移到反应中心,在那里电荷分离在几皮秒内发生。在光系统 II (PSII) 的情况下,电荷转移状态的能量用于将水分解成氧气和还原当量。这是通过四个可见光光子的能量含量相对较低来实现的。PSII 是一种大型的多亚基膜蛋白复合物,嵌入植物、藻类和蓝细菌的类囊体膜的脂质环境中。四个高能电子,连同四个质子(4H+),用于将 PSII 的末端电子受体质体醌 (PQ) 还原为质体醌 (PQH)2)。质量保证2将其还原当量传递给电子转移链,该链进入光系统 I (PSI),在那里它们从驱动 CO 所必需的第二次光反应中获得额外的还原电位2减少。PSII 的催化中心由四个 Mn 离子簇和一个 Ca2+由氧键连接。此外,还有七种氨基酸配体。在本文中,我讨论了这种金属簇的结构、稳定性以及酸碱(亲核-亲电)机制在金属簇表面催化水分解反应的可能性。这种机制的证据来自对由钌和锰的有机络合物组成的水分解催化剂的研究,以及与一氧化碳脱氢酶 (CODH) 的酶学的比较。最后,我们在人工光合作用方面讨论了我们对 PSII 理解的相关性,重点是无机水分解催化剂作为产氧光电极。
更新日期:2017-11-06
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