The 3-dimensional structure of Human Chorionic Gonadotrophin

Amino acid colour codes: ASP, GLU, CYS, MET, LYS, ARG, SER, THR, PHE, TYR, ASN, GLN, GLY, LEU, VAL, ILE , ALA, TRP, HIS and PRO.
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:

Click to see/ hide a list of beta chain residues:

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).
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*, **: These options sacrifice detail to emphasise the coiling of the polypeptide chain including alpha helix and beta pleated sheets
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Colour chains

Glycosylation sites:

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