Evidence for the extended helical nature of polysaccharide epitopes. The 2.8 A 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 IgG2a kappa murine monoclonal antibody 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 glycol 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 A. The binding site is observed to display a shape and distribution of charges that is complementary to that of the predicted conformation of the oligosaccharide epitope. A thermodynamic description of ligand binding has been compiled for oligosaccharides ranging in length from 9 to 41 residues, and the data for the largest ligand has been used in a novel way to estimate the size of the antigen binding site. A model of antigen binding is presented that satisfies this thermodynamic data, as well as a previously reported requirement of conformational specificity of the oligosaccharide. X-ray crystallographic and thermodynamic evidence are consistent with a binding site that accommodates at least eight sialic acid residues.
Evans
S V
SV
Institute for Biological Sciences, National Research Council of Canada, Ottawa.
Sigurskjold
B W
BW
Jennings
H J
HJ
Brisson
J R
JR
To
R
R
Tse
W C
WC
Altman
E
E
Frosch
M
M
Weisgerber
C
C
Kratzin
H D
HD
D000911:Antibodies, Monoclonal / Q000737:chemistry* / Q000378:metabolism; D001666:Binding Sites, Antibody; D003460:Crystallization; D018360:Crystallography, X-Ray; D000939:Epitopes / Q000737:chemistry* / Q000276:immunology; D006241:Haptens / Q000737:chemistry; D007140:Immunoglobulin Fab Fragments / Q000737:chemistry / Q000378:metabolism; D007074:Immunoglobulin G / Q000737:chemistry / Q000378:metabolism; D046911:Macromolecular Substances; D008958:Models, Molecular; D010446:Peptide Fragments / Q000737:chemistry*; D017433:Protein Structure, Secondary; D012794:Sialic Acids / Q000737:chemistry / Q000276:immunology*; D013816:Thermodynamics
D000911:Antibodies, Monoclonal; D000939:Epitopes; D006241:Haptens; D007140:Immunoglobulin Fab Fragments; D007074:Immunoglobulin G; D046911:Macromolecular Substances; D010446:Peptide Fragments; D012794:Sialic Acids; C021319:polysialic acid