The basic properties of the electronic structure of the oxygen-evolving complex of photosystem II are not perturbed by Ca2+ removal
2012 (English)In: Journal of Biological Chemistry, ISSN 0021-9258, E-ISSN 1083-351X, Vol. 287, no 29, 24721-24733 p.Article in journal (Refereed) Published
Ca(2+) is an integral component of the Mn(4)O(5)Ca cluster of the oxygen-evolving complex in photosystem II (PS II). Its removal leads to the loss of the water oxidizing functionality. The S(2)' state of the Ca(2+)-depleted cluster from spinach is examined by X- and Q-band EPR and (55)Mn electron nuclear double resonance (ENDOR) spectroscopy. Spectral simulations demonstrate that upon Ca(2+) removal, its electronic structure remains essentially unaltered, i.e. that of a Mn tetramer. No redistribution of the Mn valence states and only minor perturbation of the exchange interactions between the Mn ions were found. Interestingly, the S(2)' state in spinach PS II is very similar to the native S(2) state of Thermosynechococcus elongatus in terms of spin state energies and insensitivity to methanol addition. These results assign the Ca(2+) a functional as opposed to a structural role in water splitting catalysis, such as i) being essential for efficient proton-coupled electron transfer between Y(Z) and the Mn cluster and/or ii) providing an initial binding site for substrate water. Additionally, a novel (55)Mn(2+) signal, detected by Q-band pulse EPR and ENDOR, was observed in Ca(2+)-depleted PS II. Mn(2+) titration, monitored by (55)Mn ENDOR, revealed a specific Mn(2+) binding site with a submicromolar K(D). Ca(2+) titration of Mn(2+)-loaded, Ca(2+)-depleted PS II demonstrated that the site is reversibly made accessible to Mn(2+) by Ca(2+) depletion and reconstitution. Mn(2+) is proposed to bind at one of the extrinsic subunits. This process is likely relevant for the formation of the Mn(4)O(5)Ca cluster during photoassembly and/or D1 repair.
Place, publisher, year, edition, pages
American Society for Biochemistry and Molecular Biology, 2012. Vol. 287, no 29, 24721-24733 p.
Calcium, Electron paramagnetic resonance (EPR), ENDOR spectroscopy, Manganese, Metalloproteins, Photosystem II, Oxygen-evolving complex (OEC), Photoassembly / photoactivation, Water-oxidizing complex (WOC), Zero-field splitting
IdentifiersURN: urn:nbn:se:umu:diva-55068DOI: 10.1074/jbc.M112.365288ISI: 000306651100065PubMedID: 22549771OAI: oai:DiVA.org:umu-55068DiVA: diva2:525452