About 3 billion years ago an enzyme emerged which would dramatically change the chemical composition of our planet and set in motion an unprecedented explosion in biological activity. This enzyme used solar energy to power the thermodynamically and chemically demanding reaction of water splitting. In so doing it provided biology with an unlimited supply of reducing equivalents needed to convert carbon dioxide into the organic molecules of life while at the same time produced oxygen to transform our planetary atmosphere from an anaerobic to an aerobic state. The enzyme which facilitates this reaction and therefore underpins virtually all life on our planet is known as Photosystem II (PSII). It is a pigment-binding, multisubunit protein complex embedded in the lipid environment of the thylakoid membranes of plants, algae and cyanobacteria. Today we have detailed understanding of the structure and functioning of this key and unique enzyme. The journey to this level of knowledge can be traced back to the discovery of oxygen itself in the 18th-century. Since then there has been a sequence of mile stone discoveries which makes a fascinating story, stretching over 200 years. But it is the last few years that have provided the level of detail necessary to reveal the chemistry of water oxidation and O–O bond formation. In particular, the crystal structure of the isolated PSII enzyme has been reported with ever increasing improvement in resolution. Thus the organisational and structural details of its many subunits and cofactors are now well understood. The water splitting site was revealed as a cluster of four Mn ions and a Ca ion surrounded by amino-acid side chains, of which seven provide direct ligands to the metals. The metal cluster is organised as a cubane structure composed of three Mn ions and a Ca2+ linked by oxo-bonds with the fourth Mn ion attached to the cubane. This structure has now been synthesised in a non-protein environment suggesting that it is a totally inorganic precursor for the evolution of the photosynthetic oxygen-evolving complex. In summary, the overall structure of the catalytic site has given a framework on which to build a mechanistic scheme for photosynthetic dioxygen generation and at the same time provide a blue-print and incentive to develop catalysts for artificial photo-electrochemical systems to split water and generate renewable solar fuels.

中文翻译：

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“光系统 II：光合作用的水分解酶和我们大气中氧气的来源”

大约 30 亿年前，出现了一种酶，这种酶将极大地改变我们星球的化学成分，并引发前所未有的生物活动爆炸。这种酶利用太阳能为水分解的热力学和化学要求的反应提供动力。通过这样做，它为生物学提供了无限量的还原当量，将二氧化碳转化为生命的有机分子，同时产生氧气，将我们的行星大气从厌氧状态转变为需氧状态。促进这种反应并因此支撑我们星球上几乎所有生命的酶被称为光系统 II (PSII)。它是一种色素结合的多亚基蛋白质复合物，嵌入植物、藻类和蓝细菌的类囊体膜的脂质环境中。今天，我们详细了解了这种关键而独特的酶的结构和功能。达到这一知识水平的旅程可以追溯到 18 世纪氧气本身的发现。从那时起，一系列里程碑式的发现成为一个引人入胜的故事，绵延 200 多年。但最近几年提供了揭示水氧化和 O-O 键形成化学所需的详细程度。特别是，已报道分离的 PSII 酶的晶体结构随着分辨率的不断提高而不断提高。因此，它的许多亚基和辅因子的组织和结构细节现在已经很好地理解了。水分解位点显示为由氨基酸侧链包围的四个 Mn 离子和一个 Ca 离子簇，其中七种为金属提供直接配体。金属簇组织为立方结构，由三个 Mn 离子和一个 Ca2+通过氧键与附着在立方烷上的第四个 Mn 离子连接。这种结构现在已经在非蛋白质环境中合成，表明它是光合放氧复合物进化的完全无机前体。总之，催化位点的整体结构为构建光合双氧生成机制提供了一个框架，同时为开发用于人工光电化学系统分解水和分解水的催化剂提供了蓝图和激励。生产可再生太阳能燃料。