Replacement of an amino acid residue of cyclodextrin glucanotransferase of Bacillus ohbensis doubles the production of gamma-cyclodextrin.

Cyclodextrin glucanotransferase (CGTase; EC 2.4.1.19) produces cyclodextrin (CD) from starch through an intramolecular transglucosylation reaction. To obtain a better understanding of the amylolytic and cyclization mechanisms of CGTase, and furthermore to improve the production of gamma-CD, mutant CGTases were constructed by site-directed mutagenesis of the CGTase gene of Bacillus ohbensis replacing Tyr at position 188 by 19 other amino acids. All mutant enzymes retained both starch-degrading and CD synthesizing activities to various extents. Among them, a mutant enzyme having Trp instead of Tyr-188 produced 15% of gamma-CD from soluble starch, which is about twice as much as the amount produced by the wild-type enzyme.