Аннотация:The stability and functioning of proteins in an aqueous solution can be modified by the presence of osmolytes. The data presented here provide detailed information on the effect of a natural osmolyte - disaccharide trehalose on the light-driven oxygen evolution/consumption and electron transfer in photosystem II (PS II). It was found that in O2-evolving PS II preparations, trehalose significantly stimulated the steady-state rate of oxygen evolution but did not affect the rate of primary quinone QA oxidation. The asymmetric incorporation of PS II in proteoliposomes (>90% of complexes facing donor side outside) enabled us to study the effect of osmolyte exclusively on the water-oxidizing side. Similarity of the rate of oxygen evolution in PS II core particles in solution in the presence of detergent and in proteoliposomes suggests that this disaccharide interacts exclusively with the water-oxidizing complex (WOC). Data on the effect of the osmolyte on the electrogenic proton transfer reactions during the S2→S3 and S4→S0 state transitions of the WOC also confirm this assumption. On the other hand, in Mn-depleted PS II samples (apo-WOC-PSII) incapable of oxygen evolution, the addition of trehalose increased the rate of oxygen photoconsumption associated with electron transport in the reaction center (RC). It is concluded that structural changes in the PS II complexes caused by the addition of osmolyte enhance the capability of apo-WOC-PSII to accept electrons from manganese (both exogenous and endogenous), which in turn leads to a considerable suppression of the donor side photoinhibition of PS II. Trehalose matrix effects on charge-recombination kinetics in type I and type II photosynthetic RC complexes are also briefly described.