CONTRIBUTION OF THE HYDROPHOBIC EFFECT TO THE STABILITY OF HUMAN LYSOZYME - CALORIMETRIC STUDIES AND X-RAY STRUCTURAL-ANALYSES OF THE 9 VALINE TO ALANINE MUTANTS

To clarify the contribution of the hydrophobic effect to the conformat ional stability of human lysozyme, a series of Val to Ala mutants were constructed. The thermodynamic parameters for the denaturation of the se nine mutant proteins were determined using differential scanning ca lorimetry (DSC), and the crystal structures were solved at high resolu tion. The denaturation Gibbs energy (Delta Delta G) and enthalpy (Delt a Delta H) values of the mutant proteins ranged from +2.2 to -6.3 kJ/m ol and from +7 to -17 kJ/mol, respectively. The structural analyses sh owed that the mutation site and/or the residues around it in some prot eins shifted toward the created cavity, and the substitutions affected not only the mutations site but also other parts far from the site, a lthough the structural changes were not as great. Correlation between the changes in the thermodynamic parameters and the structural feature s of mutant proteins was examined, including the five Ile to Val mutan t human lysozymes [Takano et al. (1995) J. Mel. Biol. 254, 62-76]. The re was no simple general correlation between Delta Delta G and the cha nges in hydrophobic surface area exposed upon denaturation (Delta Delt a ASA(HP)) We found only a new correlation between the Delta Delta G a nd Delta Delta ASA(HP) of all of the hydrophobic residues if the effec t of the secondary structure propensity was taken into account.