Maltose binding site 2 mutations affect product inhibition of Bacillus circulans STB01 cyclodextrin...

The efficiency of enzymatic cyclodextrin production using cyclodextrin glycosyltransferases (CGTases) is limited by product inhibition. In this study, maltose binding site 2 (MBS2) of the β-CGTase from Bacillus circulans STB01 was modified to decrease product inhibition. First, two point mutants were prepared at position 599 (A599V and A599N). Then, two double mutants incorporating alanine at position 633 (A599N/Y633A and A599V/Y633A) were prepared. Finally, the entire MBS2 region was replaced by that of the α-CGTase from Paenibacillus macerans JFB05-01 to form multipoint mutant MBS2 β → α. All five mutants exhibited mixed-type product inhibition, although both the competitive and uncompetitive components of this inhibition were decreased. The total cyclization activities of A599N, A599V and A599V/Y633A were 15.6%, 76.8% and 70.9% lower than that of the wild-type, respectively, while that of A599N/Y633A was 22.4% higher. Among the mutants, only MBS2 β → α showed catalytic efficiency (k_cat/K_m) comparable with that of the wild-type. Moreover, A599N, A599N/Y633A and MBS2 β → α produced cyclodextrin yields 13.1%, 15.8% and 19.7% greater than that of the wild-type, respectively. These results suggest that A599N, A599N/Y633A and MBS2 β → α may be more suitable than the wild-type for cyclodextrin production.