WHERE PLANTS MAKE OXYGEN - A STRUCTURAL MODEL FOR THE PHOTOSYNTHETIC OXYGEN-EVOLVING MANGANESE CLUSTER

In the photosynthetic evolution of oxygen, water oxidation occurs at a catalytic site that includes four manganese atoms together with the e ssential cofactors, the calcium and chlorine ions. A structural model and a determination of the manganese oxidation states based on x-ray a bsorption spectroscopy are presented. The salient features, in both hi gher plants and cyanobacteria, are a pair of di-mu-oxo bridged mangane se binuclear clusters linked by a mono-mu-oxo bridge, one proximal cal cium atom, and one halide. In dark-adapted samples, manganese occurs i n oxidation states (III) and (IV). Data from oriented membranes displa y distinct dichroism, precluding highly symmetrical structures for the manganese complex.