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

The three-dimensional structure of a complex of an Fab from a murine IgG2b( lambda) antibody (NC10.14) with a high potency sweet tasting hapten, N-(p-c yanophenyl)-N'-(diphenylmethyl)-N'-(carboxymethyl)guanidine (NC174), has be en determined to 2.6 Angstrom resolution by X-ray crystallography. This com plex crystallized in the triclinic space group P1, with two molecules in th e asymmetric unit. In contrast to a companion monoclonal antibody (NC6.8) w ith a K-type light chain and similar high affinity for the NC174 ligand, th e NC10.14 antibody possessed a large and deep antigen combining site bounde d 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' beta-turn. NC174 wa s 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 orient ation 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 bindi ng of NC174 to the NC6.8 and NC10.14 antibodies illustrate the enormous str uctural and mechanistic diversity manifest by immune responses. (C) 2000 Ac ademic Press.