Recognition of a cell-surface oligosaccharide of pathogenic Salmonella by an antibody Fab fragment.

The 2.05 angstrom (A) resolution crystal structure of a dodecasaccharide-Fab complex revealed an unusual carbohydrate recognition site, defined by aromatic amino acids and a structured water molecule, rather than the carboxylic acid and amide side chains and a structured water molecule, rather than the carboxylic acid and amide side chains that are features of transport and other carbohydrate binding proteins. A trisaccharide epitope of a branched bacterial lipopolysaccharide fills this hydrophobic pocket (8 A deep by 7 A wide) in an entropy-assisted association (association constant = 2.05 x 10(5) liters per mole, enthalpy = -20.5 +/- 1.7 kilojoules per mole, and temperature times entropy = +10.0 +/- 2.9 kilojoules per mole). The requirement for the complementarity of van der Waals surfaces and the requirements of saccharide-saccharide and protein-saccharide hydrogen-bonding networks determine the antigen conformation adopted in the bound state.