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

Abstract

An in-depth study of the L2 long-loop region of human chorionic gonadotrophin (hCG), earlier identified to be a conformational bioneutralization epitope and receptorbinding site of the hormone, was carried out. The linear 38-57 hCG peptide and the corresponding cyclic disulphide peptide were synthesized and antipeptide antibodies developed. Binding studies with antibodies to the linear peptide, and with hCG , hCG and human LH suggest that part of the region is buried at the / 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 L2 long-loop region is not a receptor-binding site of hCG but spatially close to it.

Attempt to map the receptor binding sites of human follicle‐stimulating hormone using disulfide peptides of its β‐subunit indicates major involvement of the regions around disulfide bonds Cys28–Cys82 and Cys32–Cys84 in receptor binding of the hormone

“…The other major difference, as per our data, is in the involvement of the β L2 loop region in receptor binding. Our studies on hFSH clearly indicate that β L2 loop is part of the receptor binding site whereas the corresponding region in hCG is close to but not part of the receptor site of hCG (37). It appears, therefore, that there are major differences in the receptor binding sites of hFSH and hCG.…”