Counteracting Effect of Charged Amino Acids Against the Destabilization of Proteins by Arginine.

Studies on osmolyte-induced effects on proteins help in enhancing protein stability under stressed conditions for various applications. Using mixtures of osmolytes could indeed widen their applications. The combinatorial effects of osmolytes with methylamines are majorly found in the literature; however, such studies are limited on the amino acid class of osmolytes. The present study examines the effect of charged amino acids Arg, Asp, and Lys on the stability of RNase A and α-LA. The thermal stabilities of the proteins in the presence of osmolytes are monitored by absorption changes, and the structural changes are analyzed using fluorescence quenching and near-UV circular dichroism (CD). These results are compared with our previous report on the effect of Glu. Arg destabilizes both the proteins whereas Asp, Lys, and Glu stabilize the proteins. The extent of stability provided by Asp and Glu is almost same and higher than Lys in RNase A. However, the stability acquired in the presence of Asp and Lys is comparable for α-LA and Glu provides higher stability. Further, the quenching and CD results suggest that the addition of amino acids do not alter the structure of the proteins significantly. The counteracting abilities of the stabilizing amino acids (stAAs) against Arg are then investigated. The results show that Glu could counteract Arg at the lowest fraction in the mixture. Lys requires nearly equimolar concentration whereas Asp needs almost double the concentration to counteract Arg induced destabilization of the proteins. At higher concentrations, the counteracting ability of Asp and Lys is similar for both the proteins. The counteracting ratio might slightly vary among the proteins, and it is not necessary that the amino acid providing higher stability to the protein could more effectively counteract Arg. This could be due to the change in the extent of preferential hydration of the proteins by stAAs in the presence of Arg. The results suggest that the addition of stAAs could be an effective strategy to increase the protein stability in biotechnology and biopharma applications.