title: A common NH53K mutation in the combining site of antibodies raised against chlamydial LPS glycoconjugates significantly increases avidity.
abstract: The crystal structures of the antigen-binding fragment of the murine monoclonal antibody (mAb) S25-39 in the presence of several antigens representing chlamydial lipopolysaccharide (LPS) epitopes based on the bacterial sugar 3-deoxy-α-D-manno-oct-2-ulosonic acid (Kdo) have been determined at resolutions from 2.4 to 1.8 Å. The antigen-binding site of this antibody differs from the well-characterized antibody S25-2 by a single mutation away from the germline of asparagine H53 to lysine, yet this one mutation results in a significant increase in avidity across a range of antigens. A comparison of the two antibody structures reveals that the mutated Lys H53 forms additional hydrogen bonds and/or charged-residue interactions with the second Kdo residue of every antigen having two or more carbohydrate residues. Significantly, the NH53K mutation results from a single nucleotide substitution in the germline sequence common among a panel of antibodies raised against glycoconjugates containing carbohydrate epitopes of chlamydial LPS. Like S25-2, S25-39 displays significant induced fit of complementarity determining region (CDR) H3 upon antigen binding, with the unliganded structure possessing a conformation distinct from those reported earlier for S25-2. The four different observed conformations for CDR H3 suggest that this CDR has evolved to exploit the recognition potential of a flexible loop while minimizing the associated entropic penalties of binding by adopting a limited number of ordered conformations in the unliganded state. These observations reveal strategies evolved to balance adaptability and specificity in the germline antibody response to carbohydrate antigens.
authors.affiliation: Department of Biochemistry and Microbiology, University of Victoria , Victoria, BC V8P 3P6, Canada.
authors.forename: Ryan J
authors.initials: RJ
authors.lastname: Blackler
authors.forename: Sven
authors.initials: S
authors.lastname: Müller-Loennies
authors.forename: Cory L
authors.initials: CL
authors.lastname: Brooks
authors.forename: Dylan W
authors.initials: DW
authors.lastname: Evans
authors.forename: Lore
authors.initials: L
authors.lastname: Brade
authors.forename: Paul
authors.initials: P
authors.lastname: Kosma
authors.forename: Helmut
authors.initials: H
authors.lastname: Brade
authors.forename: Stephen V
authors.initials: SV
authors.lastname: Evans
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000911:Antibodies, Monoclonal / Q000737:chemistry; D000942:Antigens, Bacterial / Q000276:immunology; D001666:Binding Sites, Antibody / Q000276:immunology*; D002689:Chlamydia / Q000276:immunology*; D022801:Complementarity Determining Regions / Q000276:immunology; D000939:Epitopes / Q000235:genetics / Q000276:immunology; D007140:Immunoglobulin Fab Fragments / Q000737:chemistry; D007143:Immunoglobulin Heavy Chains / Q000235:genetics; D008070:Lipopolysaccharides / Q000276:immunology*; D051379:Mice
chemical_list: D000911:Antibodies, Monoclonal; D000942:Antigens, Bacterial; D022801:Complementarity Determining Regions; D000939:Epitopes; D007140:Immunoglobulin Fab Fragments; D007143:Immunoglobulin Heavy Chains; D008070:Lipopolysaccharides