Comparison of sugar donor binding site residues of wild-type α3Gal-T with the bound UDP-Gal (green) (A) (PDB 1fg5) with the modeled structures of the mutants AAA (B), LGG (C), TGG (D), AGG (E), and SGG (F). The UDP-GalNAc structure (yellow) (from PDB 1oqm) is superimposed in these structures. H280 residue causes steric hindrance with the N-acetyl moiety of GalNAc. A mutation of H280 to S280 or A280 and a mutation of residues A281 and A282 to G281 and G282, respectively, accommodate the N-acetyl group from UDP-GalNAc in the sugar donor binding site.