Oxygen Evolution

A unique capability of the Photosystem II (PS II) reaction center is the ability to extract electrons from water, producing molecular oxygen as a byproduct. Four photon-induced charge separations at the site of the chlorophyll moiety P680 couple sequentially to oxidation events at an Oxygen Evolving Complex (OEC), resulting in the formation of molecular oxygen. A tetranuclear manganese cluster is at the heart of the OEC. Recent biochemical and spectroscopic results have given new insights into the structure of this Mn cluster, its ligation to the PS II polypeptides, and the role of essential cofactors Ca2+ and Cl−. Models for the oxygen evolution mechanism are discussed, including new models that assign a direct role in water splitting to the redox active tyrosine Y Z • .