Water oxidation in PSII-H atom abstraction revisited

A model for the water oxidation reaction in Photosystem II (PSII) is presented, based on an H atom abstraction mechanism. The model rationalises the S-state dependence of observed substrate water exchange kinetics [Biochim. Biophys. Acta 1503 (2001) 197] and assumes that H transfer occurs to an oxidised micro-oxo bridge oxygen on the S(3)-->S(4)-->S(0) transition. The model requires that only one Mn-pair and a Ca ion be directly involved in the substrate binding and catalytic function. The multiline signal observed in the S(0) state is shown to plausibly arise from such a system. A detailed molecular model of the three-metal site, assuming ligation by those residues identified by mutagenesis as Ca/Mn ligands is presented. This bears a resemblance to the dinuclear Mn site in Mn catalase and is generally consistent with the electron density map of cyanobacterial PSII recently presented [Proc. Natl. Acad. Sci. U. S. A. 100 (2003) 98].