Contribution of a hydrogen bond to the thermal stability of the mutant human lysozyme C77/95S.

The structural stability due to a disulfide bridge between Cys77 and Cys95 of the wild-type human lysozyme is partly recovered by a putative hydrogen bond introduced in to the mutant human lysozyme C77/95S, where Cys77 and Cys95 have been replaced by serines (Yamada et al. (1994) Biol. Pharm. Bull., 17, 192 (1994). In order to understand quantitatively the role of the hydrogen bond in the thermal stability of the mutant human lysozyme, we constructed further mutant proteins, C77SC95A in which Cys77 and Cys95 were replaced by serine and alanine, respectively, and C77AC95S, in which Cys77 and Cys95 were replaced by alanine and serine, respectively. From the thermal unfolding studies of these mutant proteins, both C77SC95A and C77AC95S were shown to be destabilized up to -0.81 and -1.32 kcal/mol, respectively, as far as the free energy changes of unfolding were concerned by compared with C77/95A, where both Cys77 and Cys95 were replaced by two alanines. Considering that these decreases in conformational stability are attributable to hydrophobic effects, the hydrogen bond between Ser77 and Ser95, buried in the hydrophobic cavity in C77/95S, was estimated as 3.0 kcal/mol.