Structure-function relationships in the Mn4CaO5 water-splitting cluster

Abstract

Mn4CaO5 cluster is the catalytic center for photosynthetic water-splitting harbored in photosystem II (PSII), a huge, multi-subunit membrane- protein complex located in the thylakoid membranes from cyanobacteria to higher plants. The structure of PSII has been analyzed at 1.9 Å resolution by X-ray crystallography, revealing a clear picture of the Mn4CaO5 cluster. In this chapter, principles of crystallization and crystal structure analysis are briefly introduced, followed by descriptions of the structure of the Mn4CaO5 cluster and its implications in the mechanism of water-splitting. Based on the geometric organization of the Mn4CaO5 cluster, the location of four terminal water ligands, the possible oxidation states of the four Mn ions reported so far, as well as the structural changes revealed by replacing the Ca ion with Sr, a mechanism for water-splitting was proposed and discussed.