STRUCTURE OF A SINGLE-CHAIN ANTIBODY VARIABLE DOMAIN (FV) FRAGMENT COMPLEXED WITH A CARBOHYDRATE ANTIGEN AT 1.7-ANGSTROM RESOLUTION

We describe here the 1.7-Angstrom resolution structure of a single-cha in antibody variable domain (scFv) molecule, based on the carbohydrate -binding antibody Se155-4, complexed with the trisaccharide ligand 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-resid ue 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 lin kage than was observed previously in the Fab-trisaccharide complex. In stead of a direct hydrogen bond between O2(Abe) and O2(Gal), these two atoms are bridged by a water molecule in the present complex.