The urea induced equilibrium denaturation behavior of glutaminyl-tRNA synthetase from Escherichia coli (GlnRS) in 0.25 M potassium L-glutamate, a naturally occurring osmolyte in E. coli, has been studied. Both the native to molten globule and molten globule to unfolded state transitions are shifted significantly toward higher urea concentrations in the presence of L-glutamate, suggesting that L-glutamate has the ability to counteract the denaturing effect of urea. D-Glutamate has a similar effect on the equilibrium denaturation of glutaminyltRNA synthetase, indicating that the effect of L-glutamate may not be due to substrate-like binding to the native state. The activation energy of unfolding is not significantly affected in the presence of 0.25 M potassium L-glutamate, indicating that the native state is not preferentially stabilized by the osmolyte. Dramatic increase of coefficient of urea concentration dependence (m) values of both the transitions in the presence of glutamate suggests destabilization and increased solvent exposure of the denatured states. Four other osmolytes, sorbitol, trimethylamine oxide, inositol, and triethylene glycol, show either a modest effect or no effect on native to molten globule transition of glutaminyl-tRNA synthetase. However, glycine betaine significantly shifts the transition to higher urea concentrations. The effect of these osmolytes on other proteins is mixed. For example, glycine betaine counteracts urea denaturation of tubulin but promotes denaturation of S228N -repressor and carbonic anhydrase. Osmolyte counteraction of urea denaturation depends on osmolyte-protein pair.Folding of a polypeptide chain into a precise three-dimensional structure has been a subject of intense study over the past several decades (1, 2). Despite much progress, a complete understanding still eludes us. Much attention is now devoted to protein folding in vivo, where the cellular environment profoundly influences folding (3, 4). This has led to the discovery of the chaperones. Another aspect of in vivo environments that differ significantly from in vitro environments normally used for protein folding studies is the presence of osmolytes. Osmolytes are small molecules that accumulate inside the cell at relatively high concentrations and protect the intracellular proteins against environmental stress. Thus, they play a crucial role in protein stabilization.Although some efforts have been directed toward understanding the effects of osmolytes on protein stability, little is known about their effect on folding intermediates and partially unfolded states of proteins. Partially unfolded states are not only of crucial importance in understanding the folding processes but may play a crucial role in many diseases that involve extracellular protein aggregation and amyloid fibril formation, such as Alzheimer's disease and Scrapie (5, 6). Similar intracellular protein aggregates (Lewy bodies) are also known to play an important role in other neurodegenerative diseases, such as Parkinson's disease (7). Ver...