Increasing the thermostability of a neutral protease by replacing positively charged amino acids in the N-terminal turn of α-helices

Abstract
The 247–260 and 289–299 α-helices of Bacillus subtilis neutral protease have a lysine in their N-terminal turn. These lysines were replaced by Ser or Asp in order to improve electrostatic interactions with the α-helix dipole. After replacing Lys by Ser at positions 249 or 290, the thermostability of the enzyme was increased by 0.3 and 1.0°C, respectively. The Asp249 and Asp290 mutants exhibited a stabilization of 0.6 and 1.2°C, respectively. The results show the feasibility of stabilizing enzymes by introducing favourable residues at the end of α-helices.

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