hCG Explained: Alpha and Beta Subunit Structure, Glycosylation, and Recombinant vs Urinary-Derived Preparations

What is human chorionic gonadotropin?

Human chorionic gonadotropin (hCG) is a glycoprotein hormone produced by the syncytiotrophoblast of the placenta, structurally a heterodimer of two non-covalently associated subunits with a combined molecular weight of roughly 36 to 40 kDa depending on glycosylation. It is catalogued under CAS number 96827-07-5. Unlike most compounds in the research catalog, hCG is not a synthetic peptide: it is a large, heavily glycosylated protein produced either by extraction from urine or by recombinant expression, and its structure, analysis, and units of measure all follow from that distinction. This article covers the structure and the preparations; it does not cover clinical use.

What is the alpha subunit?

The alpha subunit of hCG is a 92-amino-acid polypeptide encoded by the CGA gene and shared, with identical sequence, by luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH). It carries two N-linked glycosylation sites at asparagine 52 and asparagine 78 and is stabilized by five internal disulfide bonds. Because the alpha subunit is common to all four glycoprotein hormones, it contributes nothing to receptor specificity on its own; the free alpha subunit circulates without hormonal activity. Its role is structural, forming half of the cystine-knot heterodimer interface that the beta subunit wraps around.

What is the beta subunit?

The beta subunit is the hormone-specific chain. hCG beta is 145 amino acids, encoded by a cluster of CGB genes on chromosome 19, and shares roughly 80% sequence identity with LH beta across the first 114 residues. The defining structural difference is a 24-residue C-terminal peptide extension (CTP), residues 122 to 145, that LH beta lacks. The CTP carries four O-linked glycosylation sites, and the beta subunit also has two N-linked sites at asparagine 13 and asparagine 30, for a total of eight glycosylation sites across the heterodimer. Six disulfide bonds stabilize the beta chain. The beta subunit determines receptor binding specificity, and the CTP is the reason hCG differs from LH in circulating half-life despite acting at the same receptor. The unique beta sequence is also what immunoassays target to distinguish hCG from LH.

Why does glycosylation dominate hCG's behavior?

Carbohydrate accounts for roughly 25 to 30% of hCG's mass, and the glycan structures, particularly the sialic acid residues terminating them, govern the hormone's clearance. Desialylated hCG is cleared from circulation within minutes by hepatic asialoglycoprotein receptors, while fully sialylated hCG persists with a terminal half-life on the order of 24 to 36 hours, against roughly 20 minutes for LH. The four O-linked glycans on the CTP are the largest single contributor to that difference. Glycosylation also varies by source and by physiological state: hCG isolated from early pregnancy urine, from later pregnancy, and from certain tumors differs in glycan branching and sialylation, and these are referred to as isoforms or glycoforms of the same protein backbone. Hyperglycosylated hCG (hCG-H), carrying larger, more branched glycans, is a distinct variant studied in early implantation and in trophoblastic disease, and it is one reason a single "hCG" measurement can mean different things in different assays.

What receptor does hCG act on?

hCG binds the luteinizing hormone/choriogonadotropin receptor (LHCGR), a class A G-protein-coupled receptor with a large leucine-rich extracellular domain, and signals predominantly through Gs and cyclic AMP, with additional coupling to Gq and other pathways reported in cell models. hCG and LH act on the same receptor, which is the basis for hCG's use as an LH surrogate in research; the beta subunit's CTP does not change receptor identity but extends the duration of receptor engagement. Published structural work has resolved the hormone-receptor complex and described how the heterodimer's cystine-knot fold docks into the receptor's extracellular domain. The glossary entry on receptor binding covers the general principles.

How do urinary-derived and recombinant hCG differ?

Urinary hCG is extracted and purified from the urine of pregnant donors and contains a mixture of intact hCG, nicked hCG (with a cleaved beta subunit), free beta subunit, beta core fragment, and residual urinary proteins, in proportions that vary by lot and manufacturer. Recombinant hCG, known as choriogonadotropin alfa, is expressed in Chinese hamster ovary cells from the cloned alpha and beta genes, purified from culture supernatant, and contains predominantly intact heterodimer with a more consistent glycoform profile and no urinary protein contaminants. The recombinant product's glycans are CHO-type rather than human-type, which changes the sialylation pattern slightly and produces a somewhat different isoform distribution. Batch-to-batch consistency, absence of co-purified proteins, and defined specific activity are the reasons recombinant preparations are preferred where reproducibility matters. Research-market hCG listings should state which type they are; many do not.

Why is hCG measured in international units rather than milligrams?

hCG is measured in international units (IU) because it is a glycoprotein whose biological activity depends on glycoform composition, so a given mass does not correspond to a fixed activity. The IU is defined by bioassay against a World Health Organization International Standard, a reference preparation of purified urinary hCG whose activity is assigned by convention. A preparation's specific activity, in IU per milligram of protein, varies with its isoform profile and purity. This is the reason questions like "how many IU in a milligram of hCG" have no single answer: the conversion depends on the preparation. The same logic applies to other glycoprotein hormones and to insulin, and it is why the unit converter on this site treats IU as preparation-specific rather than a universal mass equivalent. For synthetic peptides with a single defined structure, mass is the natural unit; for hCG it is not.

How is hCG identity and purity assessed?

Because hCG is a glycoprotein, the reverse-phase HPLC and electrospray mass spectrometry used for synthetic peptides do not apply directly. Identity and integrity are assessed by SDS-PAGE and western blot (resolving the alpha and beta subunits and detecting nicked or free forms), size-exclusion chromatography (intact dimer versus dissociated subunits), isoelectric focusing or capillary electrophoresis (isoform profile by sialylation), glycan analysis by released-glycan mass spectrometry, and immunoassay against the beta subunit. Bioactivity is confirmed by an in vitro LHCGR cAMP assay or an in vivo bioassay against the international standard. A certificate of analysis for hCG that reports only "purity by HPLC" without a subunit gel, an isoform profile, or a bioactivity figure has not characterized the material in the way its structure requires. The reading a certificate of analysis article covers the synthetic-peptide case; hCG is the clearest example of where that framework needs adjusting.

How does hCG relate to other compounds in the hormonal category?

hCG sits alongside gonadorelin and kisspeptin in the research catalog, but at a different level of the axis. Kisspeptin acts on hypothalamic GnRH neurons; gonadorelin is GnRH itself, acting on pituitary gonadotrophs; and hCG acts downstream at the gonadal LH receptor. The three are small synthetic peptides at the top of the axis and a large native glycoprotein at the bottom, which is why their structures, half-lives, and analytical methods differ so sharply.

This article describes the structure and characterization of human chorionic gonadotropin for educational purposes and does not describe or endorse any use of the compound. It is not medical advice.