Three-dimensional structure of the barley β-D-glucan glucohydrolase in complex with a transition state mimic

Glucophenylimidazole (PheGlcIm), a tetrahydroimidazopyridine-type inhibitor and H-4(3) conformer mimic of a glucoside, binds very tightly to a barley beta-D-glucan glucohydrolase, with a K-i constant of 2x10(-9) M and a DeltaG of 51 kJ mol(-1). PheGlcIm binds to the barley beta-D-glucan glucohydrolase similar to2x10(5) times tighter than laminarin, which is the best non-synthetic ground-state substrate found so far for this enzyme, 10(6) times tighter than 4-nitrophenyl beta-D-glucopyranoside, and 2x10(7) tighter than glucose. The three-dimensional structure of the beta-D-glucan glucohydrolase with bound PheGlcIm indicates that the complex resembles a hypothetical transition state during the hydrolytic cycle, that the enzyme derives substrate binding energy from the "aglycone" portion of the ligand, and that it also reveals an anti-protonation trajectory for hydrolysis. Continuous electron densities at the 1.6 sigma level form between the three active site residues Asp(95), His(207), and Asp(285), and the C6OH, C7OH, C8OH, and C9OH groups of PheGlcIm. These electron densities correspond to the most favorable interactions in the three-dimensional structure of the beta-D-glucan glucohydrolase-PheGlcIm complex and indicate atomic distances equal to or less than 2.55 Angstrom. The crystallographic data were corroborated with ab initio molecular orbital calculations. The data indicate that the E-4 conformation of the glucose part of PheGlcIm is critical for tight binding and provide the first evidence for probable substrate distortion during catalysis by this enzyme.