Oxygenic photosynthesis forms the basis of aerobic life on earth by converting light energy into biologically useful chemical energy and by splitting water to generate molecular oxygen. The water-splitting and oxygen-evolving reaction is catalyzed by photosystem II (PSII), a huge, multisubunit membrane-protein complex located in the thylakoid membranes of organisms ranging from cyanobacteria to higher plants. The structure of PSII has been analyzed at 1.9-Å resolution by X-ray crystallography, revealing a clear picture of the MnCaO cluster, the catalytic center for water oxidation. This article provides an overview of the overall structure of PSII followed by detailed descriptions of the specific structure of the MnCaO cluster and its surrounding protein environment. Based on the geometric organization of the MnCaO cluster revealed by the crystallographic analysis, in combination with the results of a vast number of experimental studies involving spectroscopic and other techniques as well as various theoretical studies, the article also discusses possible mechanisms for water splitting that are currently under consideration.


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