Functional and Structural Cooperativity in Photosystem II

Abstract

Objectives and Methods: To investigate the dependence of oxygen evolution in photosystem II (PSII) on the lipid content of the thylakoid membrane, experiments were performed since 1990 to determine the effect of the lipid/PSII molar ratio on the function of the complexes prepared with PSII particles incorporated into phosphatidylglycerol (PG) and digalactosyldiacylglycerol (DGDG) vesicles, i.e., PSII-PG and PSII-DGDG. Results: The major finding is the observation of the oxygen evolution (OE) enhancement in the PSII-lipid vesicles with increasing lipid/PSII molar ratios up to OE maxima which are seen in both PSII-PG and PSII-DGDG; then, at higher lipid/PSII molar ratios the OE decreases to values as low as 20 % of the activity in control (delipidated) PSII particles. Conclusions: To explain the results, a mechanism of lipid-mediated cooperative function of PSII units (CF model) was postulated by M Fragata, K Strzalka and EK Nénonéné [J. Photochem. Photobiol. B: Biol. 11 (1991) 329-342]. According to the CF model, the OE enhancement detected in the PSII-lipid vesicles has its origin in the formation of PSII aggregates of a yet undetermined size at the optimum lipid/PSII molar ratio. Recently, it became clear that the CF model has its structural counterpart in the PSII units arranged as dimers which were identified in the last few years (1996-2001). The data indicate that the `functional and structural cooperativity in PSII¿ is a mechanism which is instrumental not only in stimulating OE in the thylakoids but as well in the regulation of PSII function.