title: Redox potentials of chlorophylls in the photosystem II reaction center.
abstract: Water oxidation generating atmospheric oxygen occurs in photosystem II (PSII), a large protein-pigment complex located in the thylakoid membrane. The recent crystal structures at 3.2 and 3.5 A resolutions provide novel details on amino acid side chains, especially in the D1/D2 subunits. We calculated the redox potentials for one-electron oxidation of the chlorophyll a (Chla) molecules in PSII, considering the protein environment in atomic detail. The calculated redox potentials for the dimer Chla (P(D1/D2)) and accessory Chla (Chl(D1/D2)) were 1.11-1.30 V relative to the normal hydrogen electrode at pH 7, which is high enough for water oxidation. The D1/D2 proteins and their cofactors contribute approximately 390 mV to the enormous upshift of 470 mV compared to the redox potential of monomeric Chla in dimethylformamide. The other subunits are responsible for the remaining 80 mV. The high redox potentials of the two accessory Chla Chl(D1/D2) suggests that they also participate in the charge separation process.
authors.affiliation: Institute of Chemistry, Department of Biology, Chemistry, and Pharmacy, Free University of Berlin, Takustrasse 6, D-14195 Berlin, Germany.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Ishikita
authors.forename: Bernhard
authors.initials: B
authors.lastname: Loll
authors.forename: Jacek
authors.initials: J
authors.lastname: Biesiadka
authors.forename: Wolfram
authors.initials: W
authors.lastname: Saenger
authors.forename: Ernst-Walter
authors.initials: EW
authors.lastname: Knapp
mesh_terms: D002734:Chlorophyll / Q000737:chemistry* / Q000378:metabolism*; D000077194:Chlorophyll A; D018360:Crystallography, X-Ray; D000458:Cyanobacteria; D019281:Dimerization; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism*; D021122:Protein Subunits / Q000737:chemistry / Q000378:metabolism; D055672:Static Electricity
chemical_list: D045332:Photosystem II Protein Complex; D021122:Protein Subunits; C422369:photosystem II, psbA subunit; D002734:Chlorophyll; D000077194:Chlorophyll A