The influence of salts on charge separation (P6804QA) and water oxidation of photosystem II complexes from thermophilic cyanobacteria. Active and inactive conformational states of photosystem II

The activities of photosynthetic water oxidation and of the primary stable charge separation have been investigated as a function of the concentration of various salts by measuring flash-induced oxygen yields and absorption changes at 320 nm due to Q A reduction. (1) At low salt concentrations, water oxidation is inhibited, whereas the charge separation is not affected. (2) The former can be reactivated by salts containing neithercalcium nor chloride (e.g., Na 2 SO 4 ), indicating that these ions are not essential cofactors of water oxidation. (3) At higher concentrations, certain salts (e.g. CaCl 2 ) reversibly inhibit water oxidation and in the range of molar concentration also the primary stable charge separation. (4) These activating and inhibiting effects are explained by equilibria between active and inactive conformational states of PS II. These equilibria depend on the concentrations and properties of the various salts. (5) Arguments are given that, also in higher plants, salt-dependent conformational states are responsible for the salt effects.