Hyperthermostable mutants of Bacillus licheniformis alpha-amylase: multiple amino acid replacements and molecular modelling.

We have identified previously two critical positions for the thermostability of the highly thermostable alpha-amylase from Bacillus licheniformis. We have now introduced all 19 possible amino acid residues to these two positions, His133 and Ala209. The most favourable substitutions were to Ile and Val, respectively, which both increased the half-life of the enzyme at 80 degrees C by a factor of approximately 3. At both positions a stabilizing effect of hydrophobic residues was observed, although only in the case of position 133 could a clear correlation be drawn between the hydrophobicity of the inserted amino acid and the gain in protein stability. The construction of double mutants showed a cumulative effect of the most favourable and/or deleterious substitutions. Computer modelling was used to generate a 3-D structure of the wild-type protein and to model substitutions at position 209, which lies in the conserved (alpha/beta)8 barrel domain of alpha-amylase; Ala209 would be located at the beginning of the third helix of the barrel, in the bottom of a small cavity facing the fourth helix. The model suggests that replacement by, for example, a valine could fill this cavity and therefore increase intra- and interhelical compactness and hydrophobic interactions.