Water oxidation chemistry of photosystem II

Abstract

The O₂–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 O₂ 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 Ca²⁺ 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 O₂ 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 O₂–evolving complex and of a functional inorganic manganese model system that test key aspects of this mechanism are also discussed.