Antibody Recognition of a Human Chorionic Gonadotropin Epitope (hCG beta(66-80)) Depends on Local Structure Retained in the Free Peptide

Human chorionic gonadotropin (hCG) is an important biomarker in pregnancy and oncology, where it is routinely detected and quantified by specific immunoassays. Intelligent epitope selection is essential to achieving the required assay performance. We present binding affinity measurements demonstrating that a typical beta 3-loop-specific monoclonal antibody (8G5) is highly selective in competitive immunoassays and distinguishes between hCG beta(66-80) and the closely related luteinizing hormone (LH) fragment LH beta(86-100), which differ only by a single amino acid residue. A combination of optical spectroscopic measurements and atomistic computer simulations on these free peptides reveals differences in turn type stabilized by specific hydrogen bonding motifs. We propose that these structural differences are the basis for the observed selectivity in the full protein.