Structure of a single-chain antibody variable domain (Fv) fragment complexed with a carbohydrate antigen at 1.7-A resolution.

We describe here the 1.7-A resolution structure of a single-chain antibody variable domain (scFv) molecule, based on the carbohydrate-binding antibody Se155-4, complexed with the trisaccharide ligand alpha-D-Gal(1-->2)[alpha-D-Abe(1-->3)]alpha-D-Manp1-->OMe, where Abe is abequose. The scFv expressed in Escherichia coli has the variable region light chain to heavy chain polarity with the domains connected by a 19-residue linker. Although the linker is partially disordered in the crystal, the packing of the molecules suggests a monomeric state of the scFv. The carbohydrate adopts a different conformation about the Man-Gal linkage than was observed previously in the Fab-trisaccharide complex. Instead of a direct hydrogen bond between O2Abe and O2Gal, these two atoms are bridged by a water molecule in the present complex.