Effect of amino acids on the thermal stability of hen lysozyme both in the absence and presence of denaturant

Abstract

The present work examines the effect of amino acids (arginine, alanine, glycine, proline, threonine, and serine) on the thermal stability of native lysozyme (pH 4.5) both in the absence and presence of GdnHCl. It has been found that the thermal stability of lysozyme decreases in the presence arginine while it increases in the presence of alanine, glycine, proline, threonine, and serine. Among the amino acids used, the extent of the increase in the thermal stability of native lysozyme is more for proline and least for alanine. The stabilizing effect of amino acids is mainly due to preferential hydration of protein or the preferential exclusion of the amino acids [26].The destabilizing effect of arginine can be attributed to its side chain similarity with GdnH+ ions [35]. Both GdnH+ and arginine bind to peptide groups and aromatic side chains of protein [36]. As usual, GdnHCl decreases the thermal stability of native protein and the inclusion of stabilizing amino acids results in the counteraction of the destabilizing effect of the GdnHCl. This may be due to the fact that the addition of these amino acids alters the balance between the preferential binding of GdnHCl and preferential exclusion of amino acids, resulting in the increased hydration of the protein and thus helps in the counteraction of the destabilizing action of the GdnHCl. The counteraction effect of stabilizing amino acids on the destabilizing action of the denaturant was found to be more for serine and least for alanine.