THERMAL INACTIVATION OF THE OXYGEN-EVOLVING COMPLEX OF THE FUNCTIONALCORE OF PHOTOSYSTEM-II IN CHLOROPLASTS

The rates of thermal inactivation of the oxygen evolution in three sub chloroplast preparations has been compared The preparations are the is olated oxygen-evolving pigment-protein-lipid complex (OEC) which is th e functional core of Photosystem H (PS-II), PS-II subchloroplast parti cles, and the initial granal thylakoids of spinach chloroplasts. Subst antial increase in thermal lability of these preparations was found in the series. PS-II subchloroplast particles < granal thylakoids < OEC. The temperatures for half-inactivation of oxygen evolution are 45, 40 , and 34-degrees-C, respectively. The thermally induced inhibition is irreversible and is accompanied by sudden release of endogenous Mn2+ w hich occurs in the range of the half-inactivation temperatures. The th ermal lability series correlates with the exposure of hydrophilic grou ps of the functional core to the aqueous phase in the corresponding pr eparations. This exposure was estimated from the ratio between the rat es of oxygen evolution in the presence of hydrophilic and hydrophobic electron acceptors. An increase in hydrophilicity of the surface of th e PS-II core occurs on isolation of OEC from membranes using detergent . This effect is clearly related to the interaction of the OEC with th e light-harvesting complex. Three thermally induced structural transit ions in the functional core of PS-II have been observed by differentia l scanning microcalorimetry. The low-temperature or A-transition corre lates strictly with the corresponding half-inactivation temperature of oxygen evolution in the studied preparations. The A-transition occurs below the melting temperatures of proteins in the OEC. From physicoch emical analysis of this data, it is proposed that the water-oxidizing system is located in a region of hydrophobic contacts between two PS-I I reaction centers. The native state of the OEC has also been suggeste d to be a dimer of two core complexes. We assume that the water-oxidiz ing system cannot be isolated in the form of a single enzyme complex. The hydrophobic locus forming the contact region of the two reaction c enters may play a key role in the structure of water-oxidizing system and in stabilization of the highly reactive oxidized intermediates tha t are produced during its function. The low thermal stability of the w ater-oxidizing system may be due to thermally induced dissociation of the dimer complex.