The three-dimensional structure of a complex of a murine Fab (NC10.14) with a potent sweetener (NC174): an illustration of structural diversity in antigen recognition by immunoglobulins

• Published in: Journal of molecular biology 2000-09, Vol.302 (4), p.853-872
• Main Authors: Guddat, Luke W., Shan, Lin, Broomell, Christopher, Ramsland, Paul A., Fan, Zhao-chang, Anchin, Jerry M., Linthicum, D.Scott, Edmundson, Allen B.
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animals; anti-sweetener Fab; Antibody Specificity - immunology; antigen recognition; antigen-antibody complex; Antigen-Antibody Complex - chemistry; Antigen-Antibody Complex - immunology; Binding Sites, Antibody; crystal structure; Crystallography, X-Ray; Haptens - chemistry; Haptens - immunology; Humans; Hydrogen Bonding; Immunoglobulin Fragments - chemistry; Immunoglobulin Fragments - immunology; Immunoglobulin Fragments - metabolism; Immunoglobulin Heavy Chains - chemistry; Immunoglobulin Heavy Chains - immunology; Immunoglobulin Heavy Chains - metabolism; Immunoglobulin Light Chains - chemistry; Immunoglobulin Light Chains - immunology; Immunoglobulin Light Chains - metabolism; Ligands; Mice; Models, Molecular; Molecular Sequence Data; Protein Structure, Quaternary; Protein Structure, Secondary; Protein Structure, Tertiary; receptor mimicry; Sequence Alignment; Sweetening Agents - chemistry; Sweetening Agents - metabolism
• ISSN: 0022-2836; 1089-8638
• DOI: 10.1006/jmbi.2000.4083

The three-dimensional structure of a complex of an Fab from a murine IgG2b(λ) antibody (NC10.14) with a high potency sweet tasting hapten, N-( p-cyanophenyl)- N′-(diphenylmethyl)- N″-(carboxymethyl)guanidine (NC174), has been determined to 2.6 Å resolution by X-ray crystallography. This complex crystallized in the triclinic space group P1, with two molecules in the asymmetric unit. In contrast to a companion monoclonal antibody (NC6.8) with a κ-type light chain and similar high affinity for the NC174 ligand, the NC10.14 antibody possessed a large and deep antigen combining site bounded primarily by the third complementarity-determining regions (CDR3s) of the light and heavy chains. CDR3 of the heavy chain dominated the site and its crown protruded into the external solvent as a type 1′ β-turn. NC174 was nested against HCDR3 and was held in place by two tryptophan side-chains (L91 and L96) from LCDR3. The diphenyl rings were accommodated on an upper tier of the binding pocket that is largely hydrophobic. At the floor of the site, a positively charged arginine side-chain (H95) stabilized the orientation of the electronegative cyano group of the hapten. The negative charge on the acetate group was partially neutralized by a hydrogen bond with the phenolic hydroxyl group of tyrosine H58. Comparisons of the modes of binding of NC174 to the NC6.8 and NC10.14 antibodies illustrate the enormous structural and mechanistic diversity manifest by immune responses.