title: Removal of Ca(2+) from the Oxygen-Evolving Complex in Photosystem II Has Minimal Effect on the Mn4O5 Core Structure: A Polarized Mn X-ray Absorption Spectroscopy Study.
abstract: Ca(2+)-depleted and Ca(2+)-reconstituted spinach photosystem II was studied using polarized X-ray absorption spectroscopy of oriented PS II preparations to investigate the structural and functional role of the Ca(2+) ion in the Mn4O5Ca cluster of the oxygen-evolving complex (OEC). Samples were prepared by low pH/citrate treatment as one-dimensionally ordered membrane layers and poised in the Ca(2+)-depleted S1 (S1') and S2 (S2') states, the S2'YZ(•) state, at which point the catalytic cycle of water oxidation is inhibited, and the Ca(2+)-reconstituted S1 state. Polarized Mn K-edge XANES and EXAFS spectra exhibit pronounced dichroism. Polarized EXAFS data of all states of Ca(2+)-depleted PS II investigated show only minor changes in distances and orientations of the Mn-Mn vectors compared to the Ca(2+)-containing OEC, which may be attributed to some loss of rigidity of the core structure. Thus, removal of the Ca(2+) ion does not lead to fundamental distortion or rearrangement of the tetranuclear Mn cluster, which indicates that the Ca(2+) ion in the OEC is not critical for structural maintenance of the cluster, at least in the S1 and S2 states, but fulfills a crucial catalytic function in the mechanism of the water oxidation reaction. On the basis of this structural information, reasons for the inhibitory effect of Ca(2+) removal are discussed, attributing to the Ca(2+) ion a fundamental role in organizing the surrounding (substrate) water framework and in proton-coupled electron transfer to YZ(•) (D1-Tyr161).
authors.affiliation: Physical Biosciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720-5230, United States.
authors.forename: Thomas
authors.initials: T
authors.lastname: Lohmiller
authors.affiliation: Physical Biosciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720-5230, United States.
authors.forename: Megan L
authors.initials: ML
authors.lastname: Shelby
authors.affiliation: Physical Biosciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720-5230, United States.
authors.forename: Xi
authors.initials: X
authors.lastname: Long
authors.affiliation: Physical Biosciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720-5230, United States.
authors.forename: Vittal K
authors.initials: VK
authors.lastname: Yachandra
authors.affiliation: Physical Biosciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720-5230, United States.
authors.forename: Junko
authors.initials: J
authors.lastname: Yano
mesh_terms: D002118:Calcium / Q000737:chemistry / Q000378:metabolism*; D056831:Coordination Complexes / Q000737:chemistry / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D008345:Manganese / Q000737:chemistry / Q000378:metabolism*; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000737:chemistry / Q000378:metabolism*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D056831:Coordination Complexes; D045332:Photosystem II Protein Complex; D008345:Manganese; D010100:Oxygen; D002118:Calcium