Naturally occurring organic osmolytes: from cell physiology to disease prevention

Osmolytes are naturally occurring organic compounds, which represent different chemical classes including amino acids, methylamines, and polyols. By accumulating high concentrations of osmolytes, organisms adapt to perturbations that can cause structural changes in their cellular proteins. Osmolytes shift equilibrium toward natively-folded conformations by raising the free energy of the unfolded state. As osmolytes predominantly affect the protein backbone, the balance between osmolyte-backbone interactions and amino acid side chain-solvent interactions determines protein folding. Abnormal cell volume regulation significantly contributes to the pathophysiology of several disorders, and cells respond to these changes by importing, exporting, or synthesizing osmolytes to maintain volume homeostasis. In recent years, it has become quite evident that cells regulate many biological processes such as protein folding, protein disaggregation, and protein-protein interactions via accumulation of specific osmolytes. Many genetic diseases are attributed to the problems associated with protein misfolding/aggregation, and it has been shown that certain osmolytes can protect these proteins from misfolding. Thus, osmolytes can be utilized as therapeutic targets for such diseases. In this review article, we discuss the role of naturally occurring osmolytes in protein stability, underlying mechanisms, and their potential use as therapeutic molecules.