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Transition-state mimicry is increasingly important both to understand enzyme mechanism and to direct the synthesis of putative therapeutic agents. X-ray crystallography is able to provide vital information on the interactions between an enzyme and the potential inhibitor. Here we report the structures, at approximately 2 Å resolution, of a family GH1 β-glycosidase from the hyperthermophilic archaeon Sulfolobus solfataricus, in complex with both covalently (derived from 2-fluoro-glycosides) and noncovalently (hydroximolactam) bound inhibitors. The enzyme has broad specificity, accommodating both gluco- and galacto-configured substrates, and the crystallographic data demonstrate that the only difference in the way these ligands bind lies in the interactions between Gln18, Glu432, and Trp433, and the hydroxyl group at the O3 and O4 positions. Inhibition by the differently configured ligands was also shown to be …