title: Energetics of proton release on the first oxidation step in the water-oxidizing enzyme.
abstract: In photosystem II (PSII), the Mn4CaO5 cluster catalyses the water splitting reaction. The crystal structure of PSII shows the presence of a hydrogen-bonded water molecule directly linked to O4. Here we show the detailed properties of the H-bonds associated with the Mn4CaO5 cluster using a quantum mechanical/molecular mechanical approach. When O4 is taken as a μ-hydroxo bridge acting as a hydrogen-bond donor to water539 (W539), the S0 redox state best describes the unusually short O4-OW539 distance (2.5 Å) seen in the crystal structure. We find that in S1, O4 easily releases the proton into a chain of eight strongly hydrogen-bonded water molecules. The corresponding hydrogen-bond network is absent for O5 in S1. The present study suggests that the O4-water chain could facilitate the initial deprotonation event in PSII. This unexpected insight is likely to be of real relevance to mechanistic models for water oxidation.
authors.affiliation: Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan.|Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8654, Japan.|Japan Science and Technology Agency (JST), PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Saito
authors.affiliation: Department of Life Sciences, Sir Ernst Chain Building, Imperial College London, London SW7 2AZ, UK.
authors.forename: A William
authors.initials: AW
authors.lastname: Rutherford
authors.affiliation: Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan.|Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8654, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Ishikita
mesh_terms: D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D008345:Manganese / Q000378:metabolism*; D008958:Models, Molecular; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000378:metabolism*; D011522:Protons; D046940:Synechococcus; D014867:Water / Q000378:metabolism*
chemical_list: D045332:Photosystem II Protein Complex; D011522:Protons; D014867:Water; D008345:Manganese