Compatible solutes are non-toxic to protein structure and function, and alleviate inhibitory effects of high ion concentrations on enzyme activity. Some, such as trehalose, do not respond to osmotic stress by accumulating but are protective even at low concentra-tions.43-44 The majority of the compatible solutes, however, also seem to have osmoregu-latory functions and accumulate in response to osmotic stress. They may function as osmolytes as well as osmoprotectants. The net increase of solutes lowers the osmotic potential of the cell, which supports the maintenance of water balance under osmotic stress.
The main function of compatible solutes may be stabilization of proteins, protein complexes or membranes under environmental stress. In in vitro experiments, solutes at high concentrations have been found to reduce the inhibitory effects of ions on enzyme activity.17,45-47 Solute addition increased thermal stability of enzymes,33,48-49 and prevented dissociation of the oxygen-evolving complex of photosystem II.50 One argument often raised against these studies is that the effective concentration necessary for protection in vitro is very high, approximately 500 mM. Such high concentrations are rarely found in vivo. However, when we consider the high concentration of proteins in cells, the concentration necessary for protection can, we think, be much lower than that required for protection in in vitro assays. In addition, it may not be the solute concentration in solution that is important. Glycinebetaine (which may be present in high or low amounts), for example, protects thylakoid membranes and plasma membranes against freezing damage or heat destabiliza-tion,51-53 indicating that the local concentration on membranes or protein surfaces may be more important than the absolute concentration.
Two theoretical models have been proposed to explain protective or stabilizing effects of these solutes on protein structure and function. The first is termed the "preferential exclusion model"54 in which compatible solutes are largely excluded from the hydration shell of proteins that stabilizes protein structure or promotes or maintains protein/ protein interactions. Compatible solutes in this model would not disturb the native hydration water of proteins, but they would interact with the bulk water phase in the cytosol. The second model, the "preferential interaction model", in contrast, emphasizes interactions between solute and proteins.55 The protein's hydration shell is crucial for structural stability. During water deficit, these solutes may interact directly with hydrophobic domains of proteins and prevent their destabilization, or they may substitute for water molecules in the vicinity of such regions. While the two models seem to be mutually exclusive at first sight, the actual function of compatible solutes may in fact be explained by both models. The structures of different compatible solutes could accomo-date both hydrophobic, van de Waals interactions and charged interactions, but further experiments will be necessary to gain a better insight into the stabilizing effects of compatible solutes that have been documented in in vitro experiments.
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