title: Probing the role of chloride in Photosystem II from Thermosynechococcus elongatus by exchanging chloride for iodide.
abstract: The active site for water oxidation in Photosystem II (PSII) goes through five sequential oxidation states (S(0) to S(4)) before O(2) is evolved. It consists of a Mn(4)CaO(5) cluster and Tyr(Z), a redox-active tyrosine residue. Chloride ions have been known for long time to be required for the function of the enzyme. However, X-ray data have shown that they are located about 7Å away from the Mn(4)CaO(5) cluster, a distance that seems too large to be compatible with a direct involvement of chloride in the water splitting chemistry. We have investigated the role of this anion by substituting I(-) for Cl(-) in the cyanobacterium Thermosynechococcus elongatus with either Ca(2+) or Sr(2+) biosynthetically assembled into the Mn(4) cluster. The electron transfer steps affected by the exchanges were investigated by time-resolved UV-visible absorption spectroscopy, time-resolved EPR at room temperature and low temperature cw-EPR spectroscopy. In both Ca-PSII and Sr-PSII, the Cl(-)/I(-) exchange considerably slowed down the two S(3)Tyr(Z)(•)→(S(3)Tyr(Z)(•))'→S(0) reactions in which the fast phase, S(3)Tyr(Z)(•)→(S(3)Tyr(Z)(•))', reflects the electrostatically triggered expulsion of one proton from the catalytic center caused by the positive charge near/on Tyr(Z)(•) and the slow phase corresponds to the S(0) and O(2) formations and to a second proton release. The t(1/2) for S(0) formation increased from 1.1 ms in Ca/Cl-PSII to ≈6 ms in Ca/I-PSII and from 4.8 ms in Sr/Cl-PSII to ≈45 ms in Sr/I-PSII. In all cases the Tyr(Z)(•) reduction was the limiting step. The kinetic effects are interpreted by a model in which the Ca(2+) binding site and the Cl(-) binding site, although spatially distant, interact. This interaction is likely mediated by the H-bond and/or water molecules network(s) connecting the Cl(-) and Ca(2+) binding sites by which proton release may be channelled.
authors.affiliation: iBiTec-S, CNRS UMR 8221, CEA Saclay, 91191 Gif-sur-Yvette, France. alain.boussac@cea.fr
authors.forename: Alain
authors.initials: A
authors.lastname: Boussac
authors.forename: Naoko
authors.initials: N
authors.lastname: Ishida
authors.forename: Miwa
authors.initials: M
authors.lastname: Sugiura
authors.forename: Fabrice
authors.initials: F
authors.lastname: Rappaport
mesh_terms: D000042:Absorption; D002118:Calcium / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism*; D000458:Cyanobacteria / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D007454:Iodides / Q000378:metabolism*; D007700:Kinetics; D007834:Lasers; D045332:Photosystem II Protein Complex / Q000378:metabolism*; D014443:Tyrosine / Q000378:metabolism
chemical_list: D002712:Chlorides; D007454:Iodides; D045332:Photosystem II Protein Complex; D014443:Tyrosine; D002118:Calcium