PARENT SESSION
Posters P7A Mechanisms of water oxidation. Abstracts (347-381)

Calcium release from PSII without loss of extrinsic polypeptides: The roles of temperature and potassium. Robert McCarrick^*,1, Zachary Nagel¹, Charles Yocum¹, ¹ Chemistry Department, University of Michigan, Ann Arbor, MI, US

ABSTRACT- Ca²⁺ depletion of PSII utilizes 2 M NaCl, which extracts Ca²⁺ and the 23 and 17 kDa extrinsic polypeptides, or incubation of PSII at pH 3, which retains the majority of the extrinsic polypeptides. We have attempted to find alternate, milder conditions that will produce efficient Ca²⁺ release. Intact PSII samples incubated at 25° C with 150 mM K⁺ and EDTA exhibit a rapid loss of activity when assayed in the absence of Ca²⁺. High levels of EDTA-insensitive activity are reconstituted by incubation with Ca²⁺. This occurs at a much slower rate than depletion, possibly because K⁺ is trapped in the Ca²⁺ binding site and must be displaced as part of the reconstitution reaction. Results from SDS-PAGE show that there is no significant loss of the extrinsic polypeptides as a result of this treatment. Ca²⁺ release is sensitive to both temperature and light. When the same incubation conditions are used at 4° C, no loss of activity is observed, while at 25° C illumination with room light nearly doubles the rate of Ca²⁺ depletion. Although all of these properties of Ca²⁺ release are consistent within an individual PSII preparation, there are variations in depletion half-times, which range from 15 to 60 min depending on the PSII preparation. This variation may be reflected in the temperature dependence of the extraction process, which suggests that Ca²⁺ loss requires more conformational flexibility than is possessed by PSII at 4° C. The role of K⁺ may be two-fold, to weaken binding of the extrinsic polypeptides at room temperature as well as to competitively displace Ca²⁺ from its binding site. If this is the case, factors such as residual detergent content may affect the binding affinity of the extrinsic polypeptides and contribute to the variation we detect in rates of Ca²⁺ depletion.

KEY WORDS: potassium, depletion, calcium, photosystem II