Complexes of glucoamylase with maltoside heteroanalogues: Bound ligand conformations by use of transferred NOE experiments and molecular modeling

Transferred nuclear Overhauser effect (trNOE) experiments have been perform ed to investigate the conformations of the competitive inhibitors, methyl 5 '-thio-4-N-alpha-maltoside 3a and methyl 5'-thio-4-S-alpha-maltoside 4 when bound to the catalytic subunit of the enzyme glucoamylase. These NMR data suggest that, although each of the free ligands populates two conformationa l families, both heteroanalogues are bound by the enzyme in conformations i n the area of the global energy minimum. These conformations have been used as initial points for docking into the active site of the enzyme taken fro m a X-ray crystal structure of the related glucoamylase-D-gluco-dihydroacar bose 2 complex. Minimization of the resulting complexes has yielded structu res fur the bound complexes. Corroboration of the structures is provided by fast T-1 rho-relaxation effects for certain ligand protons as a result of close contacts with protons in the enzyme active site. The results auger we ll for the combined use of transferred NOE spectroscopy and molecular model ing based on X-ray crystal structures of complexes of suitable congeners fo r the rapid analysis of ligand-receptor interactions.