The two principal gonadotropins in vertebrates are luteinizing hormone (LH) and follicle-stimulating hormone (FSH), although primates produce a third gonadotropin called chorionic gonadotropin (CG). LH and FSH are heterodimers consisting of two peptide chains, an alpha chain and a beta chain. LH and FSH share nearly identical alpha chains (about 100 amino acids long), whereas the beta chain provides specificity for receptor interactions. These subunits are heavily modified by glycosylation.
The alpha subunit is common to each protein dimer (well conserved within species, but differing between them), and a unique beta subunit, which confers biological specificity. The alpha chains are highly conserved proteins of about 100 amino acid residues which contain ten conserved cysteines all involved in disulfide bonds, as shown in the following schematic representation.
'C': conserved cysteine involved in a disulphide bond.
Intracellular levels of free alpha subunits are greater than those of the mature glycoprotein, implying that hormone assembly is limited by the appearance of the specific beta subunits, and hence that synthesis of alpha and beta is independently regulated.
Gonadotropin receptors are embedded in the surface of the target cell membranes and coupled to the G-protein system. Signals triggered by binding to the receptor are relayed within the cells by the cyclic AMP second messenger system.
There are various preparations of gonadotropins for therapeutic use, mainly as fertility medication. For example, the so-called menotropins (also called human menopausal gonadotropins) consist of LH and FSH extracted from human urine from menopausal women. There are also recombinant variants.
^ abGodine JE, Chin WW, Habener JF (1982). "alpha Subunit of rat pituitary glycoprotein hormones. Primary structure of the precursor determined from the nucleotide sequence of cloned cDNAs". J. Biol. Chem.257 (14): 8368–8371. PMID6177696.