Thermostability of Photosystem I Trimers and Monomers from the Cyanobacterium Arthrospira platensis

Abstract

The correlation between changes in the circular dichroism (CD) spectra of antenna pigments and the photooxidation kinetics of the primary electron donor (P700) in trimeric and monomeric complexes of photosystem 1 (PS1) after incubation at elevated temperatures was studied to assess the thermostability of photosystem I (PSI) complexes from the cyanobacterium Arthrospira platensis. It was shown that heating of the monomers and trimers to 60 and to 70°C, respectively, caused a 10% decrease in the amplitude of the CD signals of antenna chlorophyll. Thermal disturbance of the spatial organization of the antenna chlorophyll correlated with a lower concentration of photoactive P700. A decrease in the initial rate of P700 photooxidation was observed already after 40°C for both monomers and trimers. This decrease occurred simultaneously with a decrease in the intensity of the carotenoid CD band of trimers but not monomers, which is probably due to the different carotenoid compositions of trimers and monomers. It is assumed that the slowdown of P700 photooxidation kinetics in PS1 complexes at temperatures up to 50–60°C is associated not with impairment of energy migration to the reaction center (P700) but rather with an initial delay of P700 photooxidation due to the changes in the state of the acceptor part of PS1, in particular, the secondary acceptor phylloquinone.