Mapping the receptor binding regions of human chorionic gonadotropin (hCG) using disulfide peptides of its β‐subunit: possible involvement of the disulfide bonds Cys9−Cys57 and Cys23−Cys72 in receptor binding of the hormone

Abstract: Human chorionic gonadotropin (hCG) is a heterodimeric glycoprotein hormone essential for the establishment and maintenance of pregnancy. The α‐ and β‐subunits of hCG are highly cross‐linked internally by disulfide bonds which seem to stabilize the tertiary structures required for the noncovalent association of the subunits to generate hormonal activity. The purpose of this study was to delineate the role of the disulfide bonds of hCGβ in receptor binding of the hormone. Six disulfide peptides incorporating each of the six disulfide bonds of hCGβ were synthesized and screened, along with their linear counterparts, for their ability to competitively inhibit the binding of [¹²⁵I] hCG to sheep ovarian corpora luteal LH/CG receptor. Disulfide peptide Cys (9?57) was found to be ≈ 4‐fold more potent than the most active of its linear counterparts in inhibiting radiolabeled hCG from binding to its receptor. Similarly, disulfide peptide Cys (23?72) exhibited receptor binding inhibition activity, whereas the constituent linear peptides were found to be inactive. The results suggest the involvement of the disulfide bonds Cys⁹?Cys⁵⁷ and Cys²³?Cys⁷² of the β‐subunit of hCG in receptor binding of the hormone. This study is the first of its kind to use disulfide peptides rather than linear peptides to map the receptor binding regions of hCG.