Natural Types and Subunit Structure
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.
+---------------------------+ +----------+| +-------------|--+ | || | | | xxxxCxCxxxxxxCxCCxxxxxxxxxxxxxCCxxxxxxxxxxCxCxxCx | | | | +------|-----------------+ | | | +----------------------------+'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.
Another human gonadotropin is human chorionic gonadotropin (hCG), produced by the placenta during pregnancy.
Read more about this topic: Gonadotropin
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