A GENERAL RULE FOR THE RELATIONSHIP BETWEEN HYDROPHOBIC EFFECT AND CONFORMATIONAL STABILITY OF A PROTEIN - STABILITY AND STRUCTURE OF A SERIES OF HYDROPHOBIC MUTANTS OF HUMAN LYSOZYME

To get a general rule for the relationship between hydrophobic effect and conformational stability, five lie to Val and nine Val to Ala muta nts (3SS mutants) from 3SS (C77A/C95A) human lysozyme were constructed . As known from previous studies, the 3SS protein lacking a disulfide bond between Cys77 and Cys95 is destabilized by enthalpic factors, as revealed by a decrease of about 20 kJ/mol in the denaturation Gibbs en ergy change (Delta G) value, as compared to the wild-type protein, whi ch has four disulfide bonds. In this study, the stabilities and struct ures of the 3SS mutants were determined by differential scanning calor imetry and X-ray crystal analysis, respectively, and compared with tho se of the mutants (4SS mutants) from the wild-type (4SS) protein publi shed previously. The stabilities of all the 3SS mutants, except for V1 10A-3SS were decreased as compared with that of the 3SS protein, coinc iding with the results for the 4SS mutants. The change in the denatura tion Gibbs energy change (Delta Delta G) values of the 3SS mutants rel ative to the 3SS protein at the denaturation temperature (49.2 degrees C) of the 3SS protein at pH 2.7 were similar to those of the equivale nt 4SS mutants relative to the wild-type at 64.9 degrees C. The Delta Delta G values of the 3SS mutants correlated with the changes in hydro phobic surface area exposed upon denaturation (Delta Delta ASA(HP)) fo r all of the hydrophobic residues when the effects of the secondary st ructure propensity were considered. This correlation is identical with that previously found for the 4SS mutants. The linear relation betwee n Delta Delta G and Delta Delta ASA(HP) for all of the hydrophobic res idues with the same slope was found also for the mutants of T4 lysozym e already reported, indicating that this is a general relationship bet ween changes in conformational stability and changes in ASA values of hydrophobic residues due to mutations. (C) 1998 Academic Press.