hCG is the abbreviation for human chorionic gonadotropin. It is not a short synthetic peptide but a large glycoprotein built from two protein chains plus attached carbohydrate. This page covers the chemistry and measurement of that molecule: how it is assembled, why its label carries International Units instead of milligrams, and which methods are used to characterize it. It says nothing about what the substance does, and it offers no instructions on preparing, handling or measuring out amounts.
The two subunits of hCG
hCG is a heterodimer: two different polypeptide chains, named alpha and beta, held together by noncovalent forces rather than by a covalent bond between them.
- The alpha subunit is 92 amino acid residues long. Its polypeptide sequence is the same one used by three related glycoproteins: luteinizing hormone (LH), follicle-stimulating hormone (FSH) and thyroid-stimulating hormone (TSH).
- The beta subunit is 145 residues long and is what makes hCG distinct from those three. Its final stretch, the C-terminal peptide (CTP), is a serine- and proline-rich tail that the closely related LH beta chain does not have.
Each chain is cross-linked internally by disulfide bonds, five in alpha and six in beta, and both belong to the cystine-knot structural family. Because the two subunits are not covalently joined, denaturing conditions can pull them apart, so free alpha and free beta chains are possible species in any hCG preparation alongside the intact dimer.
How glycosylation makes hCG heterogeneous
hCG carries two kinds of sugar attachment. N-linked glycans are attached to asparagine side chains: two on the alpha subunit and two on the beta subunit. O-linked glycans are attached to serine side chains, and on hCG they sit on the beta CTP, which carries four of them.
These chains are not encoded the way amino acids are; enzymes assemble them step by step, with no single fixed endpoint. Branching varies, the number of terminal sialic acid residues varies, and a given site may carry a fuller or sparser structure from one molecule to the next. The material in a vial is therefore a population of glycoforms: molecules that share one polypeptide sequence but differ in their sugar. Because sialic acid carries a negative charge, glycoforms also differ in net charge and spread into a ladder of bands under isoelectric focusing.
Why hCG has no single molecular mass
A synthetic peptide has one formula and a mass that can be calculated to a fraction of a dalton. hCG does not. The polypeptide backbones are fixed by sequence, but the carbohydrate adds a variable amount on top, so an intact-mass measurement returns a distribution rather than one sharp value.
Any single figure quoted for hCG is an average tied to a particular source material and a particular method. Apparent mass from gel electrophoresis is especially unreliable, since glycans bind less detergent than protein and distort migration. For that reason this article does not quote a molecular mass or a carbohydrate percentage: neither is a constant of the molecule.
What “5,000 IU” on an hCG vial means
With no fixed mass per molecule, milligrams say little about a preparation, so hCG is quantified in International Units (IU). The unit is defined by a WHO International Standard for chorionic gonadotropin, a stable reference material held under international custody with an assigned number of units per ampoule. A test sample is compared against that standard in a defined activity assay, and its content is expressed in the same units. Our guide to reference standards and traceability covers how that chain of comparison works.
So “5,000 IU” states the amount of activity assigned to the vial contents relative to that international reference. It does not state the protein mass, the glycoform profile, the proportion of intact dimer versus free subunits, or the chromatographic purity. Because activity per milligram shifts with glycosylation, two preparations labeled with identical IU can hold different masses of protein. Compare what the mg on a peptide vial means, where the figure is a mass.
How hCG is characterized analytically
Characterization combines several techniques:
- SDS-PAGE. The detergent dissociates the dimer, so alpha and beta appear as separate bands, typically broad and diffuse because each band is a spread of glycoforms.
- Size-exclusion chromatography. Run under native conditions, it separates intact dimer from aggregates and from free subunits.
- Reversed-phase HPLC on wide-pore columns. A molecule this size needs pores of around 300 Å, usually with C4 or C8 chemistry; the narrower pores used for short peptides exclude much of it. Peaks are broader than peptide peaks for the same glycoform reason.
- Immunoassay. ELISA-type methods measure quantity, but results depend on which epitopes the antibodies recognize, for example intact dimer only versus beta subunit in any form. See our note on ELISA quantification.
- Glycan analysis. N-glycans are released enzymatically with PNGase F, labeled and profiled; O-glycans need chemical release. Sialic acid content is often measured separately.
- Mass spectrometry. Intact hCG gives a broad, crowded envelope. After enzymatic deglycosylation the spectrum collapses toward the polypeptide masses and becomes far easier to match to sequence. Peptide mapping with glycopeptide analysis then assigns individual sites. PNGase F converts each glycosylated asparagine to aspartate, a +0.984 Da change covered in our deamidation article.
Why a peptide-style purity figure says less about hCG
For a short synthetic peptide, area-% purity at 214 nm works because the target elutes as one sharp peak and impurities resolve around it. With hCG, the main peak is itself a composite of glycoforms, so where the integration window starts and stops strongly shapes the reported percentage. Free subunits may co-elute with the dimer or appear as separate peaks depending on conditions, and the method may not preserve the noncovalent pairing at all. An area-% value for hCG is therefore a profile under one set of conditions, not proof that the material is intact, correctly paired, fully glycosylated dimer. Our walkthrough on how to read an HPLC chromatogram shows what to check in the trace itself.
How hCG 5,000 IU is supplied here
HCG 5000 IU comes in a sealed vial. An independent reverse-phase HPLC result for it is posted on the product page and should be read with the limits described above. Vials are not printed with lot or production codes; instead, the crimp and cap color on each vial ties it to the published test.
Frequently asked questions
Is hCG a peptide?
Not in the sense used for short synthetic peptides. It is a glycoprotein of two polypeptide chains, 92 and 145 residues, carrying both N-linked and O-linked carbohydrate.
Why is hCG labeled in IU rather than milligrams?
Because glycoform variation means there is no single molecular mass and no fixed link between mass and activity. IU values are assigned by comparison with a WHO International Standard.
Does 5,000 IU of hCG correspond to a fixed mass of protein?
No. Activity per milligram depends on the glycoform mix, so the protein mass behind a given IU figure can differ between preparations.
What does the hCG alpha subunit share with LH, FSH and TSH?
Its 92-residue polypeptide sequence. The beta subunit differs among the four molecules, and the hCG beta chain alone carries the O-glycosylated C-terminal peptide.
Research use only. All products supplied by Battle Born Peptides are laboratory reference materials for in-vitro research and analytical use by qualified professionals. They are not drugs, foods, dietary supplements, cosmetics or medical devices; they are not approved by the FDA or any other regulator for use in humans or animals; and they are not intended to diagnose, treat, cure, mitigate or prevent any disease, or to affect the structure or any function of the body of humans or animals. Nothing in this article is preparation, handling or dosing guidance. See our full research-use terms.