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 ({up triangle, open}F) based on the tryptophan residues of the enzyme, and the binding parameters (Kd and {up triangle, open}Fmax) were evaluated from the dependence of {up triangle, open}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 {up triangle, open}F, suggesting that some difference exists in the conformation of the isomaltose-binding subsites between the two glucoamylases. © 1990.