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Type: Journal article
Title: Three-dimensional structure of the barley beta-D-glucan glucohydrolase in complex with a transition state mimic
Author: Hrmova, M.
De Gori, R.
Smith, B.
Vasella, A.
Varghese, J.
Fincher, G.
Citation: Journal of Biological Chemistry, 2004; 279(6):4970-4980
Publisher: Amer Soc Biochemistry Molecular Biology Inc
Issue Date: 2004
ISSN: 0021-9258
Statement of
Maria Hrmova, Ross De Gori, Brian J. Smith, Andrea Vasella, Joseph N. Varghese, and Geoffrey B. Fincher
Abstract: Glucophenylimidazole (PheGlcIm), a tetrahydroimidazopyridine-type inhibitor and ⁴H₃ conformer mimic of a glucoside, binds very tightly to a barley β-D-glucan glucohydrolase, with a Ki constant of 2 x 10⁻⁹ M and a ΔG of 51 kJ mol⁻¹. PheGlcIm binds to the barley β-D-glucan glucohydrolase ~2 x 10⁵ times tighter than laminarin, which is the best non-synthetic ground-state substrate found so far for this enzyme, 10⁶ times tighter than 4-nitrophenyl β-D-glucopyranoside, and 2 x 10⁷ tighter than glucose. The three-dimensional structure of the β-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 σlevel form between the three active site residues Asp⁹⁵, His²⁰⁷, and Asp²⁸⁵, 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 β-D-glucan glucohydrolase-PheGlcIm complex and indicate atomic distances equal to or less than 2.55 Å. The crystallographic data were corroborated with ab initio molecular orbital calculations. The data indicate that the ⁴E 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.
Keywords: Hordeum; Macromolecular Substances; Glucosidases; Enzyme Inhibitors; Crystallography, X-Ray; Molecular Mimicry; Catalytic Domain; Kinetics; Hydrogen-Ion Concentration; Models, Molecular; Static Electricity
RMID: 0020040049
DOI: 10.1074/jbc.M307188200
Appears in Collections:Agriculture, Food and Wine publications

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