The function of calcium and chloride in reductant-mediated inhibition of photosystem II.

Abstract

The catalytic site of the oxygen evolving complex in photosystem II is composed of four manganese atoms, calcium and chloride. Despite intensive studies, the involvement of Ca$\sp{2+}$ and Cl$\sp-$ in photosynthetic oxygen evolution remains poorly understood. The research described in this dissertation attempts to elucidate the function of Ca$\sp{2+}$ and Cl$\sp-$ in the oxygen evolving complex by studying interactions between these inorganic ions and the manganese cluster. To examine such interactions, studies were carried out using compounds which destroy the activity of the oxygen evolving complex by reducing manganese atoms to Mn$\sp{2+},$ which is subsequently released from photosystem II. The ability of Ca$\sp{2+}$ and Cl$\sp-$ to affect such reductant-mediated inhibition was characterized. The results presented in this dissertation demonstrate that both Ca$\sp{2+}$ and Cl$\sp-$ attenuate the inhibition of photosystem II created by reductants, but the protective effects of these ions appear to arise from different mechanisms. For Ca$\sp{2+},$ the metal produces effects that are consistent with stabilization of the ligand environment of photosystem II manganese atoms that have been altered by reduction. For Cl$\sp-,$ the anion may interfere with reduction of the manganese cluster either by blocking the access of reductants to their site of action, or by binding directly to manganese to counter reductant attack.