Effect on product specificity of cyclodextrin glycosyltransferase by site-directed mutagenesis.

Cyclodextrin glycosyltransferase (CGTase; EC 2.4.1.19) catalyzes the degradation of starch into cyclodextrins through an intramolecular transglycosylation reaction. Tyr-89, Asn-94, and Tyr-100 are located near the putative active center. To analyze their roles in product specificity, Tyr-89, Asn-94, and Tyr-100 of CGTase from alkalophilic Bacillus sp. I-5 were replaced with different amino acids. Among the mutants, the N94S mutant protein produced about two times more alpha-cyclodextrin than the wild-type at all incubation times. The Y89F and Y100F mutant proteins were changed to more beta-specific enzymes. From these results it is suggested that the changing of the residues located at the near active site can change the product specificity of CGTase.