EVIDENCE FOR THE EXTENDED HELICAL NATURE OF POLYSACCHARIDE EPITOPES -THE 2.8 ANGSTROM RESOLUTION STRUCTURE AND THERMODYNAMICS OF LIGAND-BINDING OF AN ANTIGEN-BINDING FRAGMENT SPECIFIC FOR ALPHA-(2-]8)-POLYSIALIC ACID

The antigen binding fragment from an IgG(2a) kappa murine monoclonal a ntibody with specificity for alpha-(2-->8)-linked sialic acid polymers has been prepared and crystallized in the absence of hapten. Crystals were grown by vapor diffusion equilibrium with 16-18% polyethylene gl ycol 4000 solutions. The structure was solved by molecular replacement methods and refined to a conventional R factor of 0.164 for data to 2 .8 Angstrom. The binding site is observed to display a shape and distr ibution of charges that is complementary to that of the predicted conf ormation of the oligosaccharide epitope. A thermodynamic description o f ligand binding has been compiled for oligosaccharides ranging in len gth from 9 to 41 residues, and the data for the largest ligand has bee n used in a novel way to estimate the size of the antigen binding site . A model of antigen binding is presented that satisfies this thermody namic data, as well as a previously reported requirement of conformati onal specificity of the oligosaccharide. X-ray crystallographic and th ermodynamic evidence are consistent with a binding site that accommoda tes at least eight sialic acid residues.