STABILIZATION OF OXYGEN EVOLUTION AND PRIMARY ELECTRON-TRANSPORT REACTIONS IN PHOTOSYSTEM-II AGAINST HEAT-STRESS WITH GLYCINEBETAINE AND SUCROSE

The protective action of co-solutes, such as sucrose and glycinebetain e, against the thermal inactivation of photosystem II function was stu died in untreated and Mn-depleted photosystem II preparations, It was shown that, in addition to the reactions that depend on the oxygen-evo lving activity of the photosystem, those that implicate more intimatel y the reaction center itself are protected by high concentrations of o smolytes, However, the temperature required to inhibit oxygen evolutio n totally in the presence of osmolytes is lower than that required to eliminate reactions, such as P680 (primary electron donor in photosyst em II) photo-oxidation and pheophytin photoreduction, which only invol ve charge separation and primary electron transport processes. The ene rgy storage measured from the thermal dissipation yield during photoac oustic experiments and the yield of variable fluorescence are also pro tected to a significant degree (up to 30%) at temperatures at which ox ygen evolution is totally inhibited. It is suggested that a cyclic ele ctron transport reaction around photosystem II may be preserved under these conditions and may be responsible for the energy storage measure d at relatively high temperatures. This interpretation is also support ed by thermoluminescence data involving the recombination between redu ced electron accepters and oxidized electron donors at -30 and -55 deg rees C, The data also imply that a high concentration of osmolyte allo ws the stabilization of the photosystem core complex together with the oxygen-evolving complex. The stabilization effect is understood in te rms of the minimization of protein-water interactions as proposed by t he theory of Arakawa and Timasheff (Biophys. J., 47 (1985) 411-414).