Figure 4.

Active site interactions and ligand binding

A. Active site interactions with the GalNAc ligand. Hydrogen bonds are shown in red, van der Waals interactions in blue, with the initial nucleophilic attack shown as a red arrow. B. Active site residues with the GalNAc ligand. Residues are colored as in Figure 2C, and the ligand is shown with σ_A-weighted 2F_o-F_c electron density contoured at 2σ. C. The galactose ligand with σ_A-weighted 2F_o-F_c electron density contoured at 1.5σ. D. The glycerol ligand with σ_A-weighted 2F_o-F_c electron density contoured at 1.2σ. E and F. Two view of the electron density for the 2,2-difluoro-galactose ligand covalently attached to the catalytic nucleophile D156. The map is a σ_A-weighted 2F_o-F_c electron density contoured at 2.0σ.