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 alpha-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 Delta G 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 alpha-helices (25-35 and 88-99) was replaced with negatively charged aspartic acid (N27D and N93D). The Tm and Delta G 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 C-terminal sites of the three long alpha-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.