Investigation on the effect of nonpolar amino acids as macromolecular crowders on the stability of globular proteins

Addition of macromolecular crowders to biomolecules can mimic the interior of living cells, however, it is not always very evident whether the effect is general or dependent on the biomolecule-crowder pair specificity. In this study, we investigate the effect of nonpolar hydrophobic amino acids (Glycine, L-alanine, L-valine, L-leucine, L-isoleucine) as crowding agents on the structural and thermal stability of three proteins: human serum albumin (HSA), lysozyme (HEWL) and Ribonuclease-A (RNase-A) of varying α-helical content using temperature dependent circular dichroism (CD) spectroscopic technique. It has been observed that all these five amino acids do not substantially alter the secondary structure of proteins rather they stabilize their respective native forms. Thermal unfolding of each protein is found to be irreversible in nature with the extent of secondary structural loss during refolding being proportional to the α-helicity of the protein. Estimated thermodynamic parameters (van't Hoff enthalpy and heat capacity) show that enthalpic stabilization is protein specific and the added amino acids alter these parameters in different extent without following any specific trend. Our study affirms that conformational stability of proteins and the associated thermodynamic parameters do not necessarily bear a linear correlation with the hydrophobicity of amino acids; rather it is protein specific where the secondary structural content plays an important role.