Thermodynamic Stability of Hen Egg White Lysozyme and Its Mutants by Site-directed Mutagenesis

The folding transition thermodynamics parameters and activity of hen egg white lysozyme and its mutants, R21E, A31V, I55L, S91A, D101A, SIS, TIT, SVS, TVS by site-directed mutagenesis in urea solutions of various concentration were measured. The values of free energy of stabilization (delta G(T)(H(2)O)) are 16.87-28.67 kJ/mol at 25 degrees, and those at midpoint of urea concentration for folding transition [C](1/2) are 4.89-6.24 M for the mutants, respectively. The possible folding transition mechanism in urea solution, the change of transition thermodynamic stability induced by site-directed mutagenesis, and the contribution of hydrophobic interaction and the volume change of amino acid residues to protein stability were discussed.