Domain Structure and Ligand Binding Modes of Aspergillus niger Glucoamylase

Abstract

The domain structure of Aspergillus niger glucoamylase was studied by calorimetry and steady-state kinetics. Two forms of the enzyme were used; G1 is the whole protein with two main domains of the enzyme (the catalytic domain and the starch-binding domain), and G2 is an isozyme that lacks the starch-binding domain of Gi. Adiabatic differential scanning calorimetry (DSC) revealed that the catalytic domain and starch-binding domain were independent of each other concerning thermally induced unfolding, and that the former domain unfolded irreversibly and the latter reversibly . DSC and isothermal titration calorimetry also showed that there was no detectable interaction for the binding of ligands to those domains. Steady-state kinetic studies showed that the starch-binding domain did not change the kinetic parameters for the hydrolysis of p-nitrophenyl glucoside or the binding modes of inhibitors. These results lead us to the following conclusion; the essential role of the starch-binding domain seems simply to increase the local concentration of starch around the enzyme molecule .