Temperature-induced functional and structural transformations of the photosystem II oxygen-evolving complex in spinach subchloroplast preparations.

Heat inactivation of the process of O2 evolution, temperature-induced Mn2+ release and structural transitions revealed by differential scanning calorimetry (DSC) were studied in photosystem II (PSII) enriched subchloroplast fragments, granal thylakoids and the isolated oxygen-evolving pigment-lipoprotein complexes (OEC). It was found that the temperature of semi-inactivation of O2 evolution, which coincided with Mn2+ release, declined from 45 degrees C to 40 degrees C and 34 degrees C in this series of preparations, in accordance with the decreased structural stability of OEC. This was paralleled by a decrease in the content of light harvesting complex (LHC) and by an increase in the accessibility of OEC to hydrophilic electron acceptors. Thermoinactivation processes were accompanied by a two-fold decrease in PSII particle size on the EFs surface of membrane fragments. A "bi-core" oxygen-evolving complex model is proposed to account for these findings.