TEMPERATURE-INDUCED FUNCTIONAL AND STRUCTURAL TRANSFORMATIONS OF THE PHOTOSYSTEM-II OXYGEN-EVOLVING COMPLEX IN SPINACH SUBCHLOROPLAST PREPARATIONS

Heat inactivation of the process of O-2 evolution, temperature-induced Mn2+ release and structural transitions revealed by differential scan ning calorimetry (DSC) were studied in photosystem II (PSII) enriched subchloroplast fragments, granal thylakoids and the isolated oxygen-ev olving pigment-lipoprotein complexes (OEC). It was found that the temp erature of semi-inactivation of O-2 evolution, which coincided with Mn 2+ release, declined from 45 degrees C to 40 degrees C and 34 degrees C in this series of preparations, in accordance with the decreased str uctural stability of OEC. This was paralleled by a decrease in the con tent of light harvesting complex (LHC) and by an increase in the acces sibility of OEC to hydrophilic electron accepters. Thermoinactivation processes were accompanied by a two-fold decrease in PSII particle siz e on the EF(s) surface of membrane fragments. A ''bi-core'' oxygen-evo lving complex model is proposed to account for these findings.