Conversion of a cyclodextrin glucanotransferase into an alpha-amylase: assessment of directed evolution strategies

Glycoside hydrolase family 13 (GH13) members have evolved to possess various distinct reaction specificities despite the overall structural similarity. In this study we investigated the evolutionary input required to effeciently interchange these specificities and also compared the effectiveness of laboratory evolution techniques applied, i.e., error-prone PCR and saturation mutagenesis. Conversion of our model enzyme, cyclodextrin glucanotransferase (CGTase), into an alpha-amylase like hydrolytic enzyme by saturation mutagenesis close to the catalytic core yielded a triple mutant (A231V/F260W/F184Q) with the highest hydrolytic rate ever recorded for a CGTase, similar to that of a highly active alpha-amylase, while cyclodextrin production was virtually abolished. Screening of a much larger, error-prone PCR generated library yielded far less effective mutants. Our results demonstrate that it requires only three mutations to change CGTase reaction specificity into that of another GH13 enzyme. This suggests that GH13 members may have diversified by introduction of a limited number of mutations to the common ancestor, and that interconversion of reaction specificites may prove easier than previously thought.