Investigation of the Water Oxidising System of PS2 the Role of Calcium in Water Oxidation
Oxygen evolution from photosystem 2 of plants, algae and cyanobacteria requires the storage of four positive charges by a complex thought to contain four manganese ions. A model for oxygen evolution, based on oxygen release after single turnover laser flashes, was devised with five oxidation states So to S4 1 . Oxygen evolution occurs when S3 is oxidised to So. Calcium and chloride ions have been shown to be important for normal oxygen evolution, along with three extrinsic polypeptides of masses 18, 24 & 33 KDa. Several different washes have been developed to remove some or all of the extrinsic polypeptides from the reaction centre core. The absence of these polypeptides increases the concentration of calcium and chloride ions required for maximum oxygen evolution. Washing PS2 membranes with high salt concentrations removes the 18 and 24 KDa polypeptides2. The removal of calcium ions along with these polypeptides is a light dependent process3.The S3 to So transition is inhibited by the removal of calcium ions4,5. This suggests that calcium may be bound less firmly in certain ‘S’ states. EPR signals have only been identified for the S2 state.
KeywordsOxygen Evolution Calcium Binding Site Extrinsic Polypeptide Multiline Signal Reaction Centre Core
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