Journal
JOURNAL OF PHYSICAL CHEMISTRY B
Volume 119, Issue 43, Pages 13742-13754Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.jpcb.5b03559
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Funding
- NIH [GM 55302]
- Office of Science, Office of Basic Energy Sciences (OBES), Division of Chemical Sciences, Geosciences, and Biosciences of the Department of Energy (DOE) [DE-AC02-05CH11231]
- NIH National Center for Research Resources, Biomedical Technology Program
- DOE Office of Biological and Environmental Research
- DOE Office of Basic Energy Science
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Ca2+-depleted and Ca2+-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 Ca2+ 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 S-1 (S-1') and S-2 (S-2') states, the S-2'Y-z(center dot) state, at which point the catalytic cycle of water oxidation is inhibited, and the Ca2+-reconstituted S-1 state. Polarized Mn K-edge XANES and EXAFS spectra exhibit pronounced dichroism. Polarized EXAFS data of all states of Ca2+ depleted PS II investigated show only minor changes in distances and orientations of the Mn Mn vectors compared to the Ca2+-containing OEC, 3 which may be attributed to some loss of rigidity of the core structure. Thus, removal of the Ca2+ ion does not lead to fundamental distortion or rearrangement of the tetranudear Mn cluster, which indicates that the Ca2+ ion in the OEC is not critical for structural maintenance of the cluster, at least in the S-1 and S-2 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 Ca2+ removal are discussed, attributing to the Ca2+ ion a fundamental role in organizing the surrounding (substrate) water framework and in proton-coupled electron transfer to Y-z(center dot) (D1-Tyr161).
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