Recognition of a Cell-Surface Oligosaccharide of Pathogenic Salmonella by an Antibody Fab Fragment

The 2.05 angstrom (Å) 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 that are features of transport and other, carbohydrate binding proteins. A trisaccharide epitope of a branched bacterial lipopolysaccharide fills this hydrophobic pocket (8 Å deep by 7 Å wide) in an entropy-assisted association (association constant = 2.05 × 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