Indian Journal of Expcrimcntal Biology Vol. 40, April 2002, pp. 448-455 Human chorionic gonadotropin: An update on its receptor binding regions K Sankaranarayana Jyer *, Ashwini Kumar Mishra t , Smita D. Maha le & Dh ana shree D. Jagt ap Peptide and Protein Research Laboratory, In stitute for Research in Reproduc ti on (ICMR), Mumbai 400012, Indi a Human chorionic go nadotropin (hCG), an important member of the glycoprotein hormone family, plays a cr ucial role in the establishment and maintenance of pregnancy. Glycoprote in hor mones are all heterodimers co mposed of a common a- subunit and a hormone-specific f)-subunit noncovalently linked. These hormones exert their action by binding to specific re- ceptors on the target cells. As both the subunits are involved in receptor binding, th e binding sites on the hormones have to be topographical extending across both subunit s. This article attempts to critically review the work on th e receptor binding regions of hCG in the a- and f)-subunits. A variety of approaches like chemical modification, s it e-directed mutagenesi s, hormone chimeras and synthe ti c peptides have been used to map the receptor binding regions of hCG. There is agreement on th e in vo lveme nt of a number of regions in th e a- and f)- subunits in receptor binding but, there are also some contrasting obser va ti ons. The regions 30-45 and 81-92 in the a-subunit appea r to be invo lv ed in receptor bin di ng whereas 8-22, 85-95, 93-100 a nd 100-110 are the consensus receptor binding regions in th e f)-subunit. Using disulphide peptides of the f)-subunit as probes, recently the regions around Cys(9-57) and Cys(23-72) di sulphide bonds have been identified to be important fo r receptor bindin g. After sifting through the available data two potential receptor binding s it es in the f)-subunit havc been pro- posed. Human chorionic gonadotropin (hCG), a glycoprotein hormone secreted by the placenta, is essential for the establishment of pregnancy and for its maintenance especially in the early stages. It belongs to the family of glycoprotein hormones, luteinizing hormone (LH), follicle stimulating hormone (FSH) and thyroid stimu- lating hormone (TSH) being the other members. The glycoprotein hormones are all heterodimers com- prised, within a species, a common a-subunit non- covalently associated with a hormone-specific f3-subunit. In human glycoprotein hormones, the common a- subunit contains 92 amino acids, with 10 half-cystine residues which form five intramolecular disulphide linkages (Fig. 1). The vary in size from 111 residues in FSH to 145 in CG (Fig. 2) and contain 12 half-cystines which form six conserved disulphide bridges. There is a high degree of sequence similarity (85%) in the first 114 amino acids between the subunits of hCG and hLH but, differs in that it has a carboxy terminus extension rich in serine and proline. The homology between hCG and hLH re- flects a common biological function, as both proteins *Correspondent author : Tele No. : 4132111-12-16-17 Fax No. : 4139412 e-mail: peptidelab@rediffmail.com tBiotech Two , Suite liS , Molec ul ar Medicine, University of Mas- sachusetts Medical Schoo l, 373, Plantation Street, Worcestor, MA 01605, USA bind the same receptor. In addition to the N-linked GOmplex carbohydrates common to all the glycopro- tein hormones, hCG has four additional O-linked car- bohydrates on the serine-rich carboxy terminal exten- sion of the The crystal structure of deglycosylated hCG' ·2 re- veals that each of its two different subunits has similar topology, with three disulphide bonds forming a cystine knot with a long loop and two hairpin loops on either side of the cystine knot. During formation of the heterodimer, the cystine knots of the subunits come together with the long loop of one of the sub- units aligning with the hair pin loops of the other sub- unit giving an asymmetric structure to the molecule. The heterodimer is stabilized by a segment of the subunit which wraps around the a-subunit and is co- valently linked like a seat belt by the disulphide Cys (26-110). The noncovalent association of a common a- subunit with a hormone-specific to generate the hormonal activity raises a number of interesting questions: i) What is the nature of the surfaces of the subunits that are involved in association to form the het- erodimer? ii) Are both subunits involved in receptor binding? iii) Does the individual subunits have biological activity?