Steady-state kinetic and calorimetric studies on the binding of Aspergillus niger glucoamylase with gluconolactone, 1-deoxynojirimycin, and beta-cyclodextrin.

Binding equilibria of Aspergillus niger glucoamylase with its several ligands were observed to analyze the binding modes of the ligands. Steady-state kinetic studies using p-nitrophenyl alpha-glucoside as a substrate showed that 1-deoxynojirimycin, which is a mixed type inhibitor for Rhizopus glucoamylase, was a competitive type inhibitor bound at the active site of the enzyme, but gluconolactone, which is also a mixed type inhibitor for Rhizopus glucoamylase, was a non-competitive type inhibitor forming a nonproductive ternary complex with the enzyme and the substrate. beta-Cyclodextrin, which binds to the starch-binding domain of the enzyme, did not inhibit the enzyme activity, showing that there was no interaction between the catalytic domain and the starch-binding domain for the binding of the substrate and beta-cyclodextrin. Isothermal titration calorimetry showed that one 1-deoxynojirimycin molecule and two beta-cyclodextrin molecules bind to the catalytic domain and the starch-binding domain of the enzyme, respectively, and there is no significant interaction between the binding of these ligands.