title: A calcium-specific site influences the structure and activity of the manganese cluster responsible for photosynthetic water oxidation.
abstract: EPR studies have revealed that removal of calcium using citric acid from the site in spinach photosystem II which is coupled to the photosynthetic O2-evolving process produces a structural change in the manganese cluster responsible for water oxidation. If done in the dark, this yields a modified S1' oxidation state which can be photooxidized above 250 K to form a structurally altered S2' state, as seen by formation of a "modified" multiline EPR signal. Compared to the "normal" S2 state, this new S2'-state EPR signal has more lines (at least 25) and 25% narrower 55Mn hyperfine splittings, indicative of disruption of the ligands to manganese. The calcium-depleted S2' oxidation state is greatly stabilized compared to the native S2 oxidation state, as seen by a large increase in the lifetime of the S2' EPR signal. Calcium reconstitution results in the reduction of the oxidized tyrosine residue 161YD+ (Em approximately 0.7-0.8 V, NHE) within the reaction center D1 protein in both the S1' and S2' states, as monitored by its EPR signal intensity. We attribute this to reduction by Mn. Thus a possible structural role which calcium plays is to bring YD+ into redox equilibrium with the Mn cluster. Photooxidation of S2' above 250 K produces a higher S state (S3 or S4) having a new EPR signal at g = 2.004 +/- 0.003 and a symmetric line width of 163 +/- 3 G, suggestive of oxidation of an organic donor, possibly an amino acid, in magnetic contact with the Mn cluster. This EPR signal forms in a stoichiometry of 1-2 relative to YD+.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Chemistry, Princeton University, New Jersey 08544.
authors.forename: M
authors.initials: M
authors.lastname: Sivaraja
authors.forename: J
authors.initials: J
authors.lastname: Tso
authors.forename: G C
authors.initials: GC
authors.lastname: Dismukes
mesh_terms: D002118:Calcium / Q000378:metabolism*; D002734:Chlorophyll / Q000378:metabolism*; D003624:Darkness; D004578:Electron Spin Resonance Spectroscopy; D007700:Kinetics; D008027:Light; D045342:Light-Harvesting Protein Complexes; D008345:Manganese / Q000378:metabolism*; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D010940:Plant Proteins / Q000378:metabolism*; D013057:Spectrum Analysis; D014867:Water / Q000378:metabolism*
chemical_list: D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D014867:Water; D002734:Chlorophyll; D008345:Manganese; D002118:Calcium