Digalactosyl diacylglycerol-mediated Stability toward Thermal Denaturation of Oxygen Evolution in Photosystem II Core Complex

Abstract

The influence of digalactosyldiacylglycerol (DGDG), one of the photosynthetic membrane lipids, on heat inactivation of the process of oxygen evolution has been studied in vitro in photosystem II (PSII) core complex. It was found that the temperature of semi-inactivation of oxygen evolution in the complex increased from 40 to about 43 degrees C in the presence of DGDG. Furthermore, when incubated for 5 min at 45° C, the oxygen evolution in PSII core complex was completely lost, whilst DGDG-complexed PSII core complex still retained a 16% of activity (100% for 25° C). In addition, a one-hour incubation at 38° C inactivated absolutely the oxygen evolution for the PSII core complex. By contrast, there remained about 20% of activity (zero time for 100%) for the complex in the presence of DGDG on the same condition. These results might indicate the role of DGDG in the protection of PSII core complex against the deleterious effects of temperature. It was suggested that the DGDG-mediated stability toward thermal denaturation of oxygen evolution in PSII core complex is due to the redistribution of secondary bonds in PSII core complex proteins through interactions with the polar and/or fatty acid parts of DGDG, which results in DGDG-induced restraint on the inhibitory effect of heat.