The calcium and chloride requirements of the O2 evolving complex

Manganese oxidation, catalyzed by chlorophyll photochemistry in photosystem II, is the key step in the pathway of redox reactions leading formation of O2 from H2O. A unique feature of Mn redox chemistry is its dependence on the presence of both Ca2+ and Cl−; the active site of H2O oxidation consists of four Mn atoms, and one atom each of Ca2+ and Cl−. The best current structural model of the inorganic ion cluster, based on polarized EXAFS experiments on crystals of the photosystem, shows Ca2+ ligated by carboxyl groups from amino acid residues (alanine, glutamate) that bridge to Mn atoms. Chloride is not resolved in the current structures. Calcium is an essential structural element of the cluster during its assembly, and is required for efficient Mn oxidation. There is also compelling evidence that Ca2+ is involved in catalysis of H2O oxidation. Chloride is also required for Mn oxidation by photosystem II, and it has been demonstrated that the anion is required for the Mn redox reactions immediately preceding oxidation of H2O to O2. Although there are no structural data on the site of Cl− binding in photosystem II, spectroscopic probing and ligand competition experiments position it in the vicinity of the Mn cluster.