Relationship between the stability of hen egg-white lysozymes mutated at sites designed to interact with α-helix dipoles and their secretion amounts in yeast

The positively charged lysine at the C-terminals of three long α-helices (5-15, 25-35, and 88-99) was replaced with alanine (K13A, K33A, K97A) or aspartic acid (K13D, K33D, K97D) in hen lysozyme by genetic engineering. The denaturation transition point (Tm) and Gibbs energy change ΔG of the mutant lysozymes decreased remarkably, suggesting that the positive charge at the C-terminals of helices is involved in the stabilization of the helix dipole. On the other hand, the non-charged asparagine at the N-terminal of the long α-helices (25-35 and 88-99) was replaced with negatively charged aspartic acid (N27D and N93D). The Tm and ΔG of N27D increased, suggesting that the dipole moment of the N-terminal of the helices is diminished by replacement with negatively charged amino acid strengthening the stability of the helices. The stabilities of those hen egg white lysozymes mutated at the N- or Cterminal sites of the three long α-helices were related with their secretion amounts in yeast (Pichia pastoris). The secretion amounts of these mutant lysozymes in yeast were closely correlated with their stability.