Binding of isomaltose and maltose to the glucoamylase from Aspergillus niger, as studied by fluorescence spectrophotometry and steady-state kinetics.

The binding of maltose, isomaltose, and D-glucono-1,5-lactone to the glucoamylase [E.C.3.2.1.3] from Aspergillus niger was monitored by the fluorescence-intensity change (delta F) based on the tryptophan residues of the enzyme, and the binding parameters (Kd and delta Fmax) were evaluated from the dependence of delta F on the concentration of substrate and analogue. Maltose caused the fluorescence-intensity change, but isomaltose did not, although it is hydrolyzed by the enzyme. Both substrates bind to the glucoamylase of Rhizopus niveus and cause delta F, suggesting that some difference exists in the conformation of the isomaltose-binding subsites between the two glucoamylases.