Molecular Organization and Biodynamics of the Photosystem II Reaction Center

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The efficient energy transformation in the primary process of photosynthesis is ensured by the highly ordered organization of photochemical reaction centers, in physical, chemical and biological sense. In photosystem II (PSII), the primary charge separation takes place in a pigmentprotein complex consisted of D1 and D2 proteins and some other components, which has recently been isolated and the chemical and physical properties have been analyzed (Nanba and Satoh, 1987). In the present study, the organization of PSII reaction center has been analyzed in two aspects. In the first part, stoichiometries of protein subunits and of pigments and redox cofactors of the isolated reaction center have been determined. The orientation of the primary donor (P-680) has been analyzed by EPR spectroscopy and it is deduced that the triplet state is localized on a chlorophyll, the tetrapyrrolic plane of which is tilted about 30ﾟ to the membrane. In the second part, the analysis has been focussed on some aspects of light-regulated turnover of D1 subunit of the PSII reaction center, which include the mechanism of light-regulated synthesis of D1 protein in isolated chloroplasts and the characterization of an enzyme involved in the C-terminal processing of D1 precursor protein. Determination of the levels of ATP in chloroplasts and the rate of synthesis of D1 protein under various conditions suggested that the level of ATP in soluble, stromal fraction controls the synthesis of D1 protein. The enzyme involved in the processing taking place after the synthesis has been extracted and purified.