Thermodynamics of inhibitor binding to the catalytic site of glucoamylase from Aspergillus niger determined by displacement titration calorimetry

The binding of different inhibitors to glucoamylase G2 from Aspergillus niger and its temperature and pH dependencies have been studied by titration calorimetry. The enzyme binds the inhibitors 1-deoxynojirimycin and the pseudo-tetrasaccharide acarbose with association constants of 3 x 10(4) and 9 X 10(11) M(-1), respectively, at 27 degrees C, The binding free energy for both ligands is remarkably temperature-invariant in the interval from 9 to 54 degrees C as the result of large compensating changes in enthalpy and entropy. Acarbose and 1-deoxynojirimycin bound with slightly different free energy-pH profiles, with optima at 5.5 and 5.5-7.0, respectively. Variations in Delta H degrees and T Delta S degrees as a function of pH were substantially larger than variations in Delta G degrees in a partly compensatory manner. Two titratable groups at or near subsite 1 of the catalytic site were found to change their pK(a) slightly upon binding. The hydrogenated forms of acarbose, D-gluco- and L-ido-dihydroacarbose, bind with greatly reduced association constants of 3 x 10(7) and 2 x 10(5) M(-1), respectively, and the pseudo-disaccharide methyl acarviosinide, lacking the two glucose units at the reducing end compared to acarbose, has a binding constant of 8 x 10(6) M(-1); these values all result from losses in both enthalpy and entropy compared to acarbose. Three thio analogues of the substrate maltose, methyl alpha- and beta-4-thiomaltoside and methyl alpha-4,5'-dithiomaltoside, bind with affinities from 3 X 10(3) to 6 x 10(4) M(-1). All ligands bind with remarkably favorable contributions from entropy, indicating good structural complementarity with efficient solvent displacement in complex formation.