Safikur Rahman, Faizan Ahmad
Centre for Interdisciplinary Research in Basic Sciences, Jamia Millia Islamia, New Delhi, 110025, India.
The potent denaturant, urea accumulates in large concentration in specific cells of various organisms. Urea disrupts protein structures, alters enzymes kinetic properties, interferes with protein-ligand interaction, and perturbs conformational and assembly state of urea sensitive proteins. The major strategy adopted by the organisms to cope up with these effects of urea is to accumulate a set of nitrogenous compounds, the methylamine solutes. In vitro experiments suggest that these solutes counteract the deleterious effects of urea on proteins. But the compensation is found to be organism and tissue specific varying from absolute to nil compensation along with the variation in the compensation ratio of urea: methylamine. We speculated that the extent of unfolding of the denatured polypeptide chain might be playing a key role in deciding the compensation ratio as osmolytes interact much more favorably with the peptide backbone than the amino acid side chains. So far almost all studies of the urea-methylamine counteraction were concentrated on the disulfide containing proteins. In fact, disulfides put a constraint to the accessibility of the peptide backbone to the solvent in the denatured state, and it therefore results in partial counteraction. We report here results of studies of urea-methylamine counteraction for proteins having no disulphide bonds. It has been observed that all the proteins show complete counteraction at 2:1.3 ratio in vitro contradicting the apparent ratio 2:1 obtained from tissue analysis. This observation led us to conclude that some unidentified osmotically active solutes or non-methylated osmolytes are present in the intracellular environment of the urea-rich cells playing some role in urea-methylamine compensation. Read more…