title: Probing the functional role of Ca2+ in the oxygen-evolving complex of photosystem II by metal ion inhibition.
abstract: Photosynthetic oxygen evolution in photosystem II (PSII) takes place in the oxygen-evolving complex (OEC) that is comprised of a tetranuclear manganese cluster (Mn4), a redox-active tyrosine residue (YZ), and Ca2+ and Cl- cofactors. The OEC is successively oxidized by the absorption of 4 quanta of light that results in the oxidation of water and the release of O2. Ca2+ is an essential cofactor in the water-oxidation reaction, as its depletion causes the loss of the oxygen-evolution activity in PSII. In recent X-ray crystal structures, Ca2+ has been revealed to be associated with the Mn4 cluster of PSII. Although several mechanisms have been proposed for the water-oxidation reaction of PSII, the role of Ca2+ in oxygen evolution remains unclear. In this study, we probe the role of Ca2+ in oxygen evolution by monitoring the S1 to S2 state transition in PSII membranes and PSII core complexes upon inhibition of oxygen evolution by Dy3+, Cu2+, and Cd2+ ions. By using a cation-exchange procedure in which Ca2+ is not removed prior to addition of the studied cations, we achieve a high degree of reversible inhibition of PSII membranes and PSII core complexes by Dy3+, Cu2+, and Cd2+ ions. EPR spectroscopy is used to quantitate the number of bound Dy3+ and Cu2+ ions per PSII center and to determine the proximity of Dy3+ to other paramagnetic centers in PSII. We observe, for the first time, the S2 state multiline electron paramagnetic resonance (EPR) signal in Dy3+- and Cd2+-inhibited PSII and conclude that the Ca2+ cofactor is not specifically required for the S1 to S2 state transition of PSII. This observation provides direct support for the proposal that Ca2+ plays a structural role in the early S-state transitions, which can be fulfilled by other cations of similar ionic radius, and that the functional role of Ca2+ to activate water in the O-O bond-forming reaction that occurs in the final step of the S state cycle can only be fulfilled by Ca2+ and Sr2+, which have similar Lewis acidities.
authors.affiliation: Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107, USA.
authors.forename: Cheng-I
authors.initials: CI
authors.lastname: Lee
authors.forename: K V
authors.initials: KV
authors.lastname: Lakshmi
authors.forename: Gary W
authors.initials: GW
authors.lastname: Brudvig
mesh_terms: D001665:Binding Sites / Q000187:drug effects; D002104:Cadmium / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D003300:Copper / Q000494:pharmacology*; D004419:Dysprosium / Q000494:pharmacology; D004578:Electron Spin Resonance Spectroscopy; D010100:Oxygen / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000037:antagonists & inhibitors / Q000378:metabolism*; D018724:Spinacia oleracea / Q000737:chemistry
chemical_list: D045332:Photosystem II Protein Complex; D002104:Cadmium; D004419:Dysprosium; D003300:Copper; D010100:Oxygen; D002118:Calcium