Structure of an Fab fragment against a C-terminal peptide of hCG at 2.0 A resolution.

3A2 is an antibody raised against human chorionic gonadotropin and recognizes a linear epitope on the C-terminal peptide of the human chorionic gonadotropin beta-subunit. Its three-dimensional structure has been determined to 2-A resolution using molecular replacement and refined to a conventional R-factor of 18.2%. The protein exhibits the typical immunoglobulin fold, and the model contains 944 ordered water molecules and one sulfate ion. A comparison of the complementarity-determining regions of the Fab3A2 with those from the Protein Data Bank following the canonical structure method reveals a canonical main chain conformation. This antibody belongs to the canonical structure class (combination of canonical conformations of the complementarity determining loops) that shows a preference for haptens and not for peptides. However, the shape of the surface of the antigen binding loops resembles that of an anti-peptide antibody.