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.

大约30亿年前，出现了一种酶，该酶会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性爆炸。这种酶利用太阳能为热力学和化学上需要的水分解反应提供动力。这样做为生物学提供了无限的还原当量供应，可将二氧化碳转化为生命的有机分子，同时产生氧气以将我们的行星大气从无氧状态转变为有氧状态。促进这种反应并因此支撑地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，藻类和蓝细菌类囊体膜的脂质环境中。今天，我们已经对该关键和独特酶的结构和功能有了详细的了解。达到这种知识水平的过程可以追溯到18世纪氧气本身的发现。从那时起，已有200多年的一系列里程碑式的发现成为一个令人着迷的故事。但是直到最近几年，才提供了揭示水氧化和O-O键形成化学过程所必需的详细程度。特别地，已经报道了分离的PSII酶的晶体结构随着分辨率的不断提高。因此，现在已经很好地理解了其许多亚基和辅因子的组织和结构细节。水分解位点显示为四簇Mn离子和一个Ca离子被氨基酸侧链包围，其中七个提供金属的直接配体。金属簇组织为由三个Mn离子和一个Ca2+通过氧键与附接到古巴的第四个Mn离子相连。现在已经在非蛋白质环境中合成了该结构，表明该结构是光合放氧复合物演化的完全无机前体。总而言之，催化位点的整体结构提供了一个框架，可在该框架上构建光合作用双氧生成的机理方案，同时为开发用于人造光电化学系统的催化剂提供了蓝图和激励措施，以分解水和水。产生可再生的太阳能。