Leu600 mutations decrease product inhibition of the β-cyclodextrin glycosyltransferase from Bacillus circulans STB01.

The limits that cyclodextrin products impose on their industrial production from starch by cyclodextrin glycosyltransferases (CGTases) are a severe problem. In this paper, mutants at residue Leu600 of the β-CGTase from Bacillus circulans STB01 were constructed in an effort to decrease the product inhibition exhibited by β-cyclodextrin. A kinetic analysis of the inhibition of the wild-type and mutant β-CGTases by β-cyclodextrin revealed that the mutations do not alter the inhibition mode, which is mixed-type. However, the values of the inhibition constants (Ki and Ki ') of the mutants L600I, L600E and L600R are higher than those of the wild-type enzyme, weakening the product inhibition. The mutant L600Y only exhibited a decrease in noncompetitive inhibition, with the value of Ki ' increasing by 40%. Comparison of the Km ' values and the 3D model structures of the wild-type and mutant CGTases suggests that this decrease in product inhibition is related to a decrease in binding affinity between the product cyclodextrin and the enzyme.