Osmoprotectants or compatible solutes are small organic molecules with neutral charge and low toxicity at high concentrations that act as osmolytes and help organisms to survive in extreme osmotic stress. Osmoprotectants can be placed in three chemical classes: betaines and associated molecules, sugars and polyols, and amino acids. These molecules accumulate in cells and balance the osmotic difference between the cell's surroundings and the cytosol. In plants, their accumulation can increase survival during stresses such as drought. In extreme cases, such as in bdelloid rotifers, tardigrades, brine shrimp, and nematodes, these molecules can allow cells to survive being completely dried out and let them enter a state of suspended animation called cryptobiosis.
Intracellular osmoprotectant concentrations are regulated in response to environmental conditions such as osmolarity and temperature via regulation of specific transcription factors and transporters. They have been shown to play a protective role by maintaining enzyme activity through freeze-thaw cycles and at higher temperatures. It is currently believed that they function by stabilizing protein structures by promoting preferential exclusion from the water layers on the surface of hydrated proteins. This favors the native conformation and displaces inorganic salts that would otherwise cause misfolding.
