Effects of proline mutations on the unfolding and refolding of human lysozyme: the slow refolding kinetic phase does not result from proline cis-trans isomerization

The unfolding and refolding kinetics of six proline mutants of the human lysozyme (h-lysozyme) were carried out and compared to that of the wild-type protein. Our results show that the slow refolding phase observed in the h-lysozyme refolding kinetics cannot be ascribed to proline isomerization reactions. The h-lysozyme contains two proline residues at positions 71 and 103, both in the trans conformation in the native state. The refolding kinetics of the P71G/P103G mutant, in which both prolines have been replaced by a glycine, were found to be similar to those of the wild-type protein. The same slow phase amplitude of about 10% was found for both proteins, and the slow phase rate constants were also identical within experimental error. Other mutants such as P103G or P71G, in which only one of the two prolines has been replaced by a glycine, and A47P with its three prolines, gave identical slow refolding phases. The X-ray structure analysis and scanning microcalorimetric study of each protein (Herning et al., unpublished experiments) have confirmed that none of the considered mutations affects significantly protein structure and that no major changes in protein stability were brought about by these mutations. Therefore, comparison of the properties of the mutant and wild-type proteins is legitimate. Interestingly, the refolding kinetics of the V110P mutant, in which a proline residue has been introduced at position 110 (N-terminus of an alpha-helix), were clearly triphasic. For this mutant an additional very slow phase with properties similar to those expected from the proline hypothesis was detected. Equilibrium denaturation studies were conducted for each protein, and the refolding pathway of h-lysozyme is partly presented. We also discuss the effect of proline mutations on the energetics of the folding pathway of the h-lysozyme in water.