title: The S0 state of the water oxidizing complex in photosystem II: pH dependence of the EPR split signal induction and mechanistic implications.
abstract: Water oxidation in photosystem II is catalyzed by the CaMn(4) cluster. The electrons extracted from the CaMn(4) cluster are transferred to P(680)(+) via the redox-active tyrosine residue D1-Tyr161 (Y(Z)). The oxidation of Y(Z) is coupled to a deprotonation creating the neutral radical Y(Z)(*). Light-induced oxidation of Y(Z) is possible down to extreme temperatures. This can be observed as a split EPR signal from Y(Z)(*) in a magnetic interaction with the CaMn(4) cluster, offering a way to probe for Y(Z) oxidation in active PSII. Here we have used the split S(0) EPR signal to study the mechanism of Y(Z) oxidation at 5 K in the S(0) state. The state of the hydrogen bond between Y(Z) and its proposed hydrogen bond partner D1-His190 is investigated by varying the pH. The split S(0) EPR signal was induced by illumination at 5 K between pH 3.9 and pH 9.0. Maximum signal intensity was observed between pH 6 and pH 7. On both the acidic and alkaline sides the signal intensity decreased with the apparent pK(a)s (pK(app)) approximately 4.8 and approximately 7.9, respectively. The illumination protocol used to induce the split S(0) EPR signal also induces a mixed radical signal in the g approximately 2 region. One part of this signal decays with similar kinetics as the split S(0) EPR signal ( approximately 3 min, at 5 K) and is easily distinguished from a stable radical originating from Car/Chl. We suggest that this fast-decaying radical originates from Y(Z)(*). The pH dependence of the light-induced fast-decaying radical was measured in the same pH range as for the split S(0) EPR signal. The pK(app) for the light-induced fast-decaying radical was identical at acidic pH ( approximately 4.8). At alkaline pH the behavior was more complex. Between pH 6.6 and pH 7.7 the signal decreased with pK(app) approximately 7.2. However, above pH 7.7 the induction of the radical species was pH independent. We compare our results with the pH dependence of the split S(1) EPR signal induced at 5 K and the S(0) --> S(1) and S(1) --> S(2) transitions at room temperature. The result allows mechanistic conclusions concerning differences between the hydrogen bond pattern around Y(Z) in the S(0) and S(1) states.
authors.affiliation: Department of Photochemistry and Molecular Science, Uppsala University, Angstrom Laboratory, S-751 20 Uppsala, Sweden.
authors.forename: Johannes
authors.initials: J
authors.lastname: Sjöholm
authors.forename: Kajsa G V
authors.initials: KG
authors.lastname: Havelius
authors.forename: Fikret
authors.initials: F
authors.lastname: Mamedov
authors.forename: Stenbjörn
authors.initials: S
authors.lastname: Styring
mesh_terms: D002118:Calcium / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D005609:Free Radicals; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008027:Light; D008345:Manganese / Q000737:chemistry; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism*; D017433:Protein Structure, Secondary; D011522:Protons; D018724:Spinacia oleracea / Q000737:chemistry / Q000378:metabolism; D013696:Temperature; D014443:Tyrosine / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism; D014867:Water / Q000378:metabolism*
chemical_list: D005609:Free Radicals; D045332:Photosystem II Protein Complex; D011522:Protons; C114979:tyrosyl radical YZ; D014867:Water; D014443:Tyrosine; D008345:Manganese; D002118:Calcium