Relationship between local structure and stability in hen egg white lysozyme mutant with alanine substituted for glycine

We prepared five mutant lysozymes in which glycines whose dihedralangles are located in the region of the left-handed helix, Gly49,Gly67, Gly71, Gly102 and Gly117, were mutated to an alanineresidue. From analyses of their thermal stabilities using differentialscanning calorimetry, most of them were more destabilized thanthe native lysozyme, except for the G102A mutant, which hasa stability similar to that of the native lysozyme at pH 2.7.As for the destabilized mutant lysozymes, their X-ray crystallographicanalyses showed that their global structures did not changebut that the local structures changed slightly. By examiningthe dihedral angles at the mutation sites based on X-ray crystallographicresults, it was found that the dihedral angles at these mutationsites tended to adopt favorable values in a Ramachandran plotand that the extent and direction of their shifts from the originalvalue had similar tendencies. Therefore, the change in dihedralangles may be the cause of the slight local structural changesaround the mutation site. On the other hand, regarding the mutationof G102A, the global structure was almost identical with thatof the native structure but the local structure was drasticallychanged. Therefore, it was suggested that the drastic localconformational change might be effective in releasing the unfavorableinteraction of the native state at the mutation site.