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Abstract's Details

Characterization of the Binding and Function of the Ca2+ and Cl- Cofactors of the Water Oxidation Process in Photosystem II
Abstract IDBIO-10 
PresenterXi  Long
Presentation TypePoster
Full Author ListY. Pushkar (1,2), X. Long (1,2), J. Yano (1,2), K. Sauer (1,2), A. Boussac (3), V. Yachandra (1,2)
Affiliations(1) Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA-94740-5230 USA
(2) Department of Chemistry, University of California, Berkeley, CA 94720-5230 USA
(3) Service de Bioénergétique, DBJC, URA CNRS 2096, CEA Sacley, 91191 Gif sur Yvette, France
CategoryBio/Life Sciences
AbstractThe Mn4Ca cluster of the oxygen evolving complex (OEC) of photosystem II (PS II), cycles through five oxidation states (Si-states, i=0-4) coupling the one electron photochemistry of the reaction center with the 4 electron redox chemistry of water oxidation. Ca2+ and Cl- are obligatory cofactors for oxygen evolution. Using EXAFS at Ca and Sr (ion that can functionally substitute Ca in the OEC) K-edges we obtained information about cofactor location and function in the mechanism of water oxidation. For the Cl- cofactor, however, information about its location and function remains largely controversial. Literature contains the range of views from statements that Cl is not required for oxygen evolution activity to hypothesis about its direct binding to the Mn4Ca cluster. A variety of model compounds with Cl covalently bound to the Mn atom have characteristic pre-edge features in the Cl K-edge spectrum. They are due to a forbidden 1s to 3d transition, which becomes partially allowed due to mixing of ligand p-orbitals with metal d-orbitals. Preliminary Cl XAS data with spinach PS II samples free of loosely bound Cl demonstrated no specific pre-edge features. EXAFS at Mn K-edge of the PS II protein complex from the thermophilic cyanobacterium, Thermosynechococcus elongatus, where Cl has been biosynthetically replaced with Br did not show pronounced differences from the Cl containing controls in the S1, S2 and S3 states. Those two preliminary results suggest the absence of direct Cl-Mn interaction in the OEC. EXAFS at Br K-edge on PS II from spinach with Cl replaced with Br by a dialysis protocol demonstrated an additional peak corresponding to an interaction with Mn or Ca atom at about 4 Å. Possible roles of The Cl ion: i) regulation of the potential and reactivity of the Mn4Ca cluster; ii) prevention of the premature binding and oxidation of H2O, and others will be presented.
Footnotes 
Funding Acknowledgement