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Calcium Ligation in Photosystem II under Inhibiting Conditions

School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332

Correspondence: Address reprint requests to Bridgette A. Barry, Tel.: 404-385-6085; Fax: 404-894-2295; E-mail: bridgette.barry{at}chemistry.gatech.edu.

In oxygenic photosynthesis, PSII carries out the oxidation of water and reduction of plastoquinone. The product of water oxidation is molecular oxygen. The water splitting complex is located on the lumenal side of the PSII reaction center and contains manganese, calcium, and chloride. Four sequential photooxidation reactions are required to generate oxygen from water; the five sequentially oxidized forms of the water splitting complex are known as the S_n states, where n refers to the number of oxidizing equivalents stored. Calcium plays a role in water oxidation; removal of calcium is associated with an inhibition of the S state cycle. Although calcium can be replaced by other cations in vitro, only strontium maintains activity, and the steady-state rate of oxygen evolution is decreased in strontium-reconstituted PSII. In this article, we study the role of calcium in PSII that is limited in water content. We report that strontium substitution or ¹⁸OH₂ exchange causes conformational changes in the calcium ligation shell. The conformational change is detected because of a perturbation to calcium ligation during the S₁ to S₂ and S₂ to S₃ transition under water-limited conditions.

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