title: Water oxidation in PSII-H atom abstraction revisited.
abstract: 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].
authors.affiliation: Department of Chemistry, Faculty of Science, Australian National University, Building 33, Canberra 0200, Australia. Ron.Pace@anu.edu.au
authors.forename: Ron J
authors.initials: RJ
authors.lastname: Pace
authors.forename: Karin A
authors.initials: KA
authors.lastname: Ahrling
mesh_terms: D002118:Calcium / Q000737:chemistry; D020134:Catalytic Domain; D004578:Electron Spin Resonance Spectroscopy; D004734:Energy Metabolism; D006859:Hydrogen / Q000737:chemistry; D007700:Kinetics; D008345:Manganese / Q000737:chemistry; D008958:Models, Molecular; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism*; D014867:Water / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D045332:Photosystem II Protein Complex; D014867:Water; D008345:Manganese; D006859:Hydrogen; D002118:Calcium