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

Release of calcium from PSII by monovalent metal ions. Charles Yocum^*,1, Zachary Nagel¹, ¹ Departments of MCD Biology and of Chemistry, Ann Arbor, MI, USA

ABSTRACT- Monovalent cations (Na⁺, K⁺, Rb⁺, Cs⁺) at high concentrations will extract the extrinsic 23 and 17 kDa polypeptides and release Ca²⁺ from PSII. Steady state assays show that these metals compete with Ca²⁺ for its binding site with the order of effectiveness Rb⁺, K⁺ > Cs⁺ >>>> Na⁺. This competition is unusual in view of the large ionic radii of the most effective competitors (> 1.3 Angstrom vs. 0.99 Angstrom for Ca²⁺); in one investigation on dark adapted samples, none of these metals appeared to compete for the Ca²⁺ site. The ability of monovalent ions to displace Ca²⁺ from dark-adapted PSII preparations has been examined. The order of effectiveness in releasing Ca²+ was found to be K⁺ > Cs⁺ >>>> Na⁺, which approximates the order in which these ions compete effectively for the Ca²⁺ site in the steady state. Ionic strength effects that would affect release of the extrinsic polypeptides do not modify the relative efficiency of Ca²⁺ release by these ions, while increasing the incubation temperature accelerates the reaction. Flash-induced Ca²⁺ release, induced by exposure of the sample to K⁺ after the flash, occurs optimally in S₃, similar to what has been reported for Na⁺. A possible explanation for the ability large monovalent ions to displace Ca²⁺ is that their hydration energies are substantially lower than that of Ca²⁺, which may facilitate exchange of the hydrated ion into a binding site normally occupied by Ca²⁺ when the monovalent ion is present at sufficiently high concentrations.

KEY WORDS: calcium, photosystem II, potassium, sodium