Water oxidation chemistry of photosystem II

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Abstract
The O 2 –evolving complex of photosystem II catalyses the light–driven four–electron oxidation of water to dioxygen in photosynthesis. In this article, the steps leading to photosynthetic O 2 evolution are discussed. Emphasis is given to the proton–coupled electron–transfer steps involved in oxidation of the manganese cluster by oxidized tyrosine Z (Y Z ), the function of Ca 2+ and the mechanism by which water is activated for formation of an O–O bond. Based on a consideration of the biophysical studies of photosystem II and inorganic manganese model chemistry, a mechanism for photosynthetic O 2 evolution is presented in which the O–O bond–forming step occurs via nucleophilic attack on an electron–deficient Mn V =O species by a calcium–bound water molecule. The proposed mechanism includes specific roles for the tetranuclear manganese cluster, calcium, chloride, Y Z and His190 of the D1 polypeptide. Recent studies of the ion selectivity of the calcium site in the O 2 –evolving complex and of a functional inorganic manganese model system that test key aspects of this mechanism are also discussed.