Fig. 4.

(a) Calystegine (8) in complex with a family 1 β-glucosidase (PDB code 2CBV⁹⁴); the residue below the inhibitor is the catalytic nucleophile and the residue to the right is the acid/base. Observed electron density (for the maximum likelihood weighted 2F_obs−F_calc map, contoured at 1σ) is shown for calystegine, showing it binds in a similar orientation to isofagomine. (b) Cellobio-derived form of isofagomine (9) in complex with a family 5 endoglucanase (PDB code 1OCQ¹⁰¹); the residue below the inhibitor is the catalytic nucleophile and the residue to the right is the acid/base. Observed electron density for the maximum likelihood weighted 2F_obs−F_calc map, contoured at 2.5σ, is shown in red and for the F_obs−F_calc map, contoured at 2.1σ, is shown in blue. The ‘difference’ density shows the presence of two hydrogen atoms on the nitrogen atom of isofagomine. (c) Phenylaminomethyl-substituted glucoimidazole in complex with a family 3 β-d-glucan glucohydrolase (PDB code 1X39¹³³); the residue below the inhibitor is the catalytic nucleophile and the residue to the right is the acid/base. The two tryptophan residues in the active site are proposed to make hydrophobic interactions with the phenyl ring of the inhibitor, but this interaction has not been observed in all enzyme complexes with substituted imidazole inhibitors. (d) Xylobio-derived isofagomine lactam in complex with a family 10 xylanase (PDB code 1OD8¹³⁷); the residue below the inhibitor is the catalytic nucleophile and the residue to the right is the acid/base. Observed electron density for the maximum likelihood weighted 2F_obs−F_calc map, contoured at 4σ, is shown in red and for the F_obs−F_calc map, contoured at 1.8σ, is shown in blue. The difference density shows the presence of a hydrogen atom on the nitrogen atom of the isofagomine lactam, indicating it exists as the amide tautomer and not the iminol as originally proposed.