A detailed study of the L2beta long-loop region of human chorionic gonadotrophin suggests it to be spatially close to, but not part of, the receptor-binding site

An in-depth study of the L2beta long-loop region of human chorionic gonadotrophin (hCG), earlier identified to be a conformational bioneutralization epitope and receptor-binding site of the hormone, was carried out. The linear 38-57 hCGbeta peptide and the corresponding cyclic disulphide peptide were synthesized and antipeptide antibodies developed. Binding studies with antibodies to the linear peptide, and with hCGbeta, hCG and human LH suggest that part of the region is buried at the alpha/beta interface and part exposed in hCG. Observation of the surface exposure of residues 47-53 from the crystal structure of hCG was confirmed by epitope mapping studies of the region. The region is not unique to hCG as a majority of the antibodies to both the linear and cyclic peptides did not exhibit the required specificity. Competitive inhibition studies with the linear and cyclic peptides failed to show inhibition of radiolabelled hCG binding to its receptors. However, both the antipeptide antibodies were able to bioneutralize the hormone in an in vivo assay. Taken together, these results seem to indicate that the L2beta long-loop region is not a receptor-binding site of hCG but spatially close to it.