Variants at position 603 of the CGTase from Bacillus circulans STB01 for reducing product inhibition

Abstract

Inhibition of cyclodextrin glycosyltransferases (CGTases) by their product cyclodextrins limits the efficiency of cyclodextrin production. In an effort to produce variants with good activity but reduced product inhibition, six mutants were constructed at position 603 of the CGTase from Bacillus circulans STB01, which exhibits mixed-type product inhibition. In a kinetic analysis, N603I showed reduced noncompetitive inhibition while N603K, N603H and N603R showed increased noncompetitive inhibition. Unexpectedly, N603E and N603D exhibited reductions in both competitive and noncompetitive product inhibition. Noncompetitive product inhibition is closely related to the interaction between the cyclodextrin and the enzyme in maltose binding site 2 (MBS2). Structural models led to the suggestion that there is increased interaction between maltose binding sites 1 and 2 in mutants N603E and N603D, which may have led to the unexpected results. N603D exhibited a 23.9% greater cyclodextrin yield per gram of enzyme than the wild-type, suggesting it has potential for industrial use. Further reductions in product inhibition may be gained through studies of maltose binding site interactions.