The 3-dimensional structure of Human Chorionic Gonadotrophin
Amino acid colour codes:
LEU, VAL, ILE ,
Human Chorionic Gonadotrophin, also known as Human Chorionic Gonadotropin, or just hCG, is a hormone produced by the placenta (actually the syncytiotrophoblast) of a developing embryo, and is the basis of pregnancy tests. It is composed of two polypeptide chains, α (alpha) - 92 amino acids - and β (beta) - 145 amino acids , so it shows the quaternary level of protein structure (and it does not consist of 4 sub-units). It is described as heterodimeric because it is composed of two different sub-units.
The primary level of protein structure is shown by the order of the amino acid residues making up the polypeptide chains.
Click to see/ hide a list of alpha chain residues:
ALA PRO ASP THR GLN ASP CYS PRO GLU CYS THR LEU GLN GLU ASN PRO PHE PHE SER GLN PRO GLY ALA PRO ILE LEU GLN CYS MET GLY CYS CYS PHE SER ARG ALA TYR PRO THR PRO LEU ARG SER LYS LYS THR MET LEU VAL GLN LYS ASN VAL THR SER GLU SER THR CYS CYS VAL ALA LYS SER TYR ASN ARG VAL THR VAL MET GLY GLY PHE LYS VAL GLU ASN HIS THR ALA CYS HIS CYS SER THR CYS TYR TYR HIS LYS SER
Click to see/ hide a list of beta chain residues:
SER LYS GLU PRO LEU ARG PRO ARG CYS ARG PRO ILE ASN ALA THR LEU ALA VAL GLU LYS GLU GLY CYS PRO VAL CYS ILE THR VAL ASN THR THR ILE CYS ALA GLY TYR CYS PRO THR MET THR ARG VAL LEU GLN GLY VAL LEU PRO ALA LEU PRO GLN VAL VAL CYS ASN TYR ARG ASP VAL ARG PHE GLU SER ILE ARG LEU PRO GLY CYS PRO ARG GLY VAL ASN PRO VAL VAL SER TYR ALA VAL ALA LEU SER CYS GLN CYS ALA LEU CYS ARG ARG SER THR THR ASP CYS GLY GLY PRO LY ASP HIS PRO LEU THR CYS ASP ASP PRO ARG PHE GLN AS SER SER SER SER LYS ALA PRO PRO PRO SER LEU PRO SER PRO SER ARG LEU PRO GLY PRO SER ASP THR PRO ILE LEU PRO GLN
These names and numbers can be seen on the 3D graphic below.
Interestingly, the α-chain is identical with luteinizing hormone (LH), follicle-stimulating hormone (FSH) and thyroid-stimulating hormone (TSH), all produced in the pituitary gland. Each of these has a unique β-chain.
At the secondary level of protein structure, it can be seen that the polypeptide chains making up the hCG molecule are quite coiled, although there is only one section of true alpha helix and several beta plates, as shown by the cartoon option below. This coiling is stabilised by hydrogen bonds.
The tertiary structure of the molecule is maintained by a number of disulphide bridges between cysteine sidechains, as well as hydrogen bonds between amino acid sidechains, and acid/base interactions.
The quaternary level of protein structure is shown by the fact that there are two polypeptide chains, not linked by strong bonds like the disulphide bridges mentioned above. Interestingly, the genes coding for each are found on different chromosomes, so these two chains must be brought together after being independently synthesised.
HCG and the others mentioned above are all glycoproteins: they have sugars (N-acetyl glucosamine) attached to the polypeptide chains (at asparagine side-chains).
Click on the links below to interact with the molecular model