Influence of mutations of the N-cap residue, Gly4, on stability and structure of hen lysozyme

Hen lysozyme, with three α-helices (A, B, and C), is a c-type lysozyme. In these lysozymes, Ser24 and Asp88 located at the N-cap position in the B- and C-helix, respectively, are mostly conserved, but residue 4 at the N-cap position in A-helix is variable. To investigate the effect of mutation at position 4 on the stability of hen lysozyme, we prepared five mutant lysozymes and examined their stabilities and structures. Gly4Pro lysozyme (G4P), in which Gly4 was replaced by Pro, was less stable by 8.8 kJ/mol than the wild-type lysozyme, possibly because the side chain at position 7 is shifted away from the A-helix. The other mutant lysozymes were of almost equal stability to the wild-type lysozyme, although the hydrogen bonds of the amide groups at positions N1-N3 in the A-helix were absent or altered. These results indicated that various mutations at the N-cap position in the A-helix would be allowed as long as the negative charge of Glu7 at the N-terminus stabilized the A-helix.