Explorations in the “Inner Sanctum of the Photosynthetic Process,” the Water Oxidizing System

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Abstract
This article summarizes the present state of knowledge about the organization of the water oxidizing polypeptide complex of photosynthesis and the mechanism of its operation. Spectacular advances have been made in recent years, but many questions remain to be answered. Among them are those concerned with the organization and binding of the functional Mn, its specific role in the oxidant storage that precedes the formation of one oxygen from two water molecules, and its cooperation with other redox-active constituents. Much is still to be learned also about the function of the individual polypeptides and about the role of the cofactors Ca2+ and Cl−. Differences between the organization of the water oxidizing complexes in green plants and cyanobacteria suggest, furthermore, that in vivo their water oxidizing mechanisms must operate under different, still unrecognized constraints.