HCG: Molecular Structure

Molecular Structure and Properties#

HCG is a peptide whose molecular structure and properties have been characterized through analytical chemistry and structural biology studies.

Amino Acid Sequence#

Human chorionic gonadotropin (hCG) is a heterodimeric glycoprotein hormone composed of a common α subunit (92 amino acids) and an hCG-specific β subunit (145 amino acids). Each subunit adopts a cystine‑knot fold, stabilized by multiple disulfide bonds, and the heterodimer forms an elongated, banana‑shaped architecture with a substantial buried interface. The β subunit has a unique O‑glycosylated C‑terminal peptide (CTP; residues 115–145) that is disordered in the crystal structure and is a major determinant of charge heterogeneity and half‑life.

Subunit composition, disulfides, and structural features

• α (CGA): 92 aa; five intrachain disulfides forming a cystine‑knot (Cys7–31, Cys10–60, Cys28–82, Cys34–84, Cys59–87). N‑glycosylation at Asn52 and Asn78.
• β (CGB): 145 aa; six intrachain disulfides (cystine‑knot core Cys9–57, Cys34–88, Cys38–90; additional Cys23–72, Cys26–110, Cys93–100). N‑glycosylation at Asn13 and Asn30; O‑glycosylation on the CTP at Ser121, Ser127, Ser132, Ser138. The CTP (115–145) is heavily O‑glycosylated and flexible/disordered.
• Quaternary structure: the heterodimer buries ~3,860 Å2 at the interface; each subunit presents protruding cystine‑knot loops that shape antigenic and receptor‑binding surfaces.

Amino acid sequence information

• Mature chain lengths are α 92 aa and β 145 aa. The β CTP corresponds to residues 115–145 and is an O‑glycan–rich extension not required for receptor binding. While the exact single‑letter sequences are not reproduced in the cited excerpts, boundaries, glycosylation sites, and disulfide connectivities are defined above and in the table artifact.

Glycosylation and isoforms

• N‑glycans: α Asn52/Asn78; β Asn13/Asn30. O‑glycans: β Ser121/127/132/138. Human hCG carries complex‑type N‑glycans and mucin‑type O‑glycans on the CTP; the extent of antennarity, fucosylation, and especially sialylation varies with pregnancy stage and pathology (e.g., early‑pregnancy hyperglycosylated hCG, trophoblastic tumors), driving pronounced physicochemical heterogeneity.
• Isoforms: intact hCG, free subunits, nicked β, β‑core fragment, pituitary hCG, urinary hCG, and hyperglycosylated hCG (hCG‑H). Urinary vs recombinant/pituitary preparations show distinct isoform patterns by intact‑level CE/LC–MS due to glycan differences.

Physicochemical properties

• Molecular masses (deglycosylated peptides and intact): α peptide ~10.2 kDa; β peptide ~15.5 kDa; heterodimer peptide sum ≈25.7–25.9 kDa. With glycans, typical intact masses are ~35.3–37.3 kDa; hyperglycosylated pregnancy/tumor forms can approach ~40 kDa. Carbohydrates contribute ~25–41% of total mass (higher in hyperglycosylated forms).
• Isoelectric point (pI) and charge heterogeneity: intact hCG focuses broadly around pI ~4.0–6.5, reflecting glycan sialylation; α subunit pI ~6.5–8.5; β subunit pI ~4.0–5.5. Highly acidic tumor‑derived variants can be as low as pI ~3.6–3.9. Increased sialic acid content (e.g., in hyperglycosylated and some tumor‑derived hCG) shifts pI to more acidic values; differences between urinary and recombinant/pituitary preparations reflect distinct glycosylation.
• Charge distribution: negative charge is concentrated in the β CTP due to multiple sialylated O‑glycans; N‑glycan sialylation on α Asn52/78 and β Asn13/30 also contributes. Variation in antennarity and sialylation number (reports of up to ~8–15 terminal sialic acids across isoforms) underlies microheterogeneity and broad IEF patterns.

Functional/structural notes

• The cystine‑knot motif defines the subunit core; the β CTP modulates circulatory half‑life and charge without directly mediating receptor binding. Differences in glycosylation (antenna number, fucosylation, sialylation) correlate with bioactivity and pharmacokinetics and are diagnostic in pregnancy and trophoblastic disease.

hCG structure and properties overview

Limitations and sequence detail

• The cited sources delineate lengths, domains, glycosites, and disulfide maps; exact residue‑by‑residue sequences were not contained in the provided excerpts. If desired, curated primary sequences for mature CGA (α) and CGB (β) can be incorporated from sequence databases and cross‑referenced to the structural and glycosylation annotations summarized here.

Stability and Formulation#

We summarize physicochemical stability of human chorionic gonadotropin (hCG) with emphasis on pH, temperature, degradation pathways, and formulation.

1. pH stability — Direct, quantitative pH profile for hCG is limited; hCG is sensitive to acidic/denaturing conditions (low-pH HPLC mobile phases reduce activity) and can dissociate under chaotropes/propionic acid; experimental work typically uses phosphate buffer ~pH 7.3–7.4 for solutions.

2. Temperature sensitivity — Lyophilized hCG reference reagents showed no loss of immunoreactivity after up to 4 months at 37–45 °C in accelerated WHO ampouling studies, but aqueous hCG loses activity rapidly under heat stress: control solutions at 40 °C lost ~58% intact hCG by 2 weeks in accelerated tests.

2b) Excipient effects under heat stress — Specific stabilizers improved retention at 40 °C: 10 mg/cm3 L-lysine retained >52–54% activity after 6–8 weeks; L-glycine (0.2 mg/cm3) and L-methionine (0.2 mg/cm3) preserved ~50% over several weeks; combinations with sucrose or mannitol were also beneficial in reported experiments.

2c) Cold / freeze–thaw — Freezing can concentrate solutes and alter pH, but some chorionic gonadotropin preparations (analogue eCG) showed minimal loss after limited freeze–thaw or refrigeration; extreme heat (e.g., 100 °C for minutes) causes irreversible denaturation.

3. Degradation pathways — Chemical: oxidation and deamidation are documented liabilities (including during production/processing); loss or modification of N-/O-glycans (desialylation) shortens half-life. Physical: subunit dissociation, unfolding, aggregation and proteolytic cleavage have been observed and can reduce bioactivity.

4. Formulation considerations — Lyophilized presentations are preferred for long-term stability; WHO reference reagents were ampouled from phosphate pH 7.4 containing 2 g/L HSA and sealed under N2 (the lyophilization step can incur ~10–20% immunoreactivity loss during processing but the products remain stable).

4b) Liquid formulations and in-use handling — Liquid hCG is less stable and typically requires stabilizers (sugars: sucrose, mannitol; amino acids: glycine, lysine, methionine; antioxidants/metal salts reported in literature). Buffer choice (phosphate ~pH 7.3–7.4), protein carriers (e.g., HSA) and validated preservative/compatibility testing are important; exact product-specific in-use times and preservative guidance were not retrieved here.

Blockquote: A concise, citable summary of key findings on hCG stability organized by pH, temperature, degradation pathways and formulation considerations, with context IDs for source tracking.

pH stability – Direct systematic pH–rate profiles for hCG are limited. Experimental and analytical reports indicate that acidic/denaturing environments reduce activity, and hCG can dissociate under chaotropes or organic acids; investigators therefore use near‑neutral phosphate buffers (~pH 7.3–7.4) for solution work (e.g., Embree’s HPLC work noted loss of activity at low‑pH mobile phases; stabilization studies dissolved hCG at pH 7.34).

Temperature sensitivity – Lyophilized reference reagents: WHO ampouled hCG-related preparations (intact and fragments) in phosphate pH 7.4 + 2 g/L HSA, sealed under nitrogen. Accelerated stability testing showed no loss of immunoreactivity for 4 months even at 37–45 °C; typical 10–20% immunoreactivity loss occurred during the lyophilization/sealing process but did not compromise long-term stability predictions via Arrhenius analysis. – Aqueous solution: In accelerated tests at 40 °C in phosphate buffer pH 7.34, control hCG solutions lost about 58% of intact hormone by 2 weeks, indicating substantial heat lability in solution. Addition of stabilizers mitigated loss: L‑lysine 10 mg/cm3 retained approximately 52–54% over 6–8 weeks; low‑dose glycine (0.2 mg/cm3) or methionine (0.2 mg/cm3) preserved about half the activity over several weeks; combinations with sucrose or mannitol were reported beneficial. – Cold/freeze–thaw: Direct hCG data are sparse; for the close analog equine CG, brief refrigeration and limited freeze–thaw cycles had minimal impact on structure or bioactivity, whereas extreme heat (100 °C, minutes) caused irreversible denaturation (supporting general thermal sensitivity of CGs).

Degradation pathways – Chemical: Analytical characterization highlights oxidation and deamidation as liabilities that can accompany processing and storage; glycan alterations (e.g., desialylation) shorten circulating half‑life. – Physical: hCG is a glycosylated heterodimer stabilized by multiple disulfides; it can dissociate into subunits under denaturing conditions, leading to loss of bioactivity, with potential to reassociate under suitable conditions. Proteolytic cleavage and aggregation/unfolding are additional risks.

Formulation considerations – Lyophilized vs liquid: Lyophilized preparations are consistently more stable than liquids. WHO reference reagents used phosphate buffer at pH 7.4 with human serum albumin as carrier and nitrogen sealing; 10–20% immunoreactivity loss during lyophilization is typical but long‑term stability is high under accelerated conditions. – Buffers/excipients: Solution studies commonly use phosphate near neutral pH (≈7.3–7.4). Amino acids (lysine, glycine, methionine) and sugars/polyols (sucrose, mannitol) can improve thermal stability in solution; literature also notes metal salts (ZnCl2, AlCl3) and proteins/polysaccharides as potential stabilizers. Optimal concentrations are formulation‑specific (e.g., lysine 10 mg/cm3; glycine or methionine at 0.2 mg/cm3 effective in one series), underscoring the need to titrate excipient levels. – Preservatives and carriers: WHO reagents included 2 g/L HSA as a stabilizing carrier. Specific antimicrobial preservative compatibility for hCG was not detailed in the retrieved documents and should be established empirically for any multidose liquid presentation. – Storage and in‑use handling: Labels were not retrieved here; however, the combined evidence supports refrigerated storage for liquids and preference for lyophilized products reconstituted in near‑neutral buffers, with minimal in‑use hold times at ambient temperature. Product‑specific instructions should be followed when available.

Overall, hCG is most stable in the solid (lyophilized) state and near‑neutral pH; it is sensitive to heat in solution, with degradation primarily via chemical modifications (deamidation/oxidation), dissociation, and proteolytic cleavage. Formulations using neutral buffers, protein carriers, and selected amino acids/sugars can mitigate degradation, while long‑term stability in the solid state is robust under accelerated conditions.

Pharmacokinetics#

We summarize human pharmacokinetic properties of human chorionic gonadotropin (hCG), covering absorption, distribution, metabolism, elimination, half-life, and bioavailability, distinguishing urinary highly purified hCG (HP‑hCG) and recombinant choriogonadotropin alfa (r‑hCG), with route and assay context.

Absorption • Subcutaneous (SC): After single SC dosing, serum concentrations rise with median tmax about 16 h for HP‑hCG and about 24 h for r‑hCG in healthy women; prior data cited in the same study noted tmax around 20 h for urinary hCG and similar extent between IM and SC routes (urinary). In Chinese healthy adults, r‑hCG showed median tmax 16.0 h (test) and 19.96 h (reference). • Intramuscular (IM): Prior comparative work cited within Radicioni reports IM and SC urinary hCG have similar extent of exposure (bioavailability by AUC), with mean Cmax ~339 IU/L reached around 20 h and t1/2 ~32–33 h after 10,000 IU; details of the original study were not retrieved here. • Intravenous (IV): Not reported in the retrieved accessible texts; absolute absorption parameters from IV are therefore unavailable in this evidence set.

Distribution • Apparent volume of distribution for SC r‑hCG in healthy Chinese adults was approximately 38 L for the test formulation and 47 L for the reference (means ± SD 38.1 ± 18.4 L and 47.2 ± 20.4 L); in females specifically, 39.2 ± 14.1 L (test) vs 51.7 ± 19.3 L (reference). HP‑hCG Vd was not reported in the accessible texts.

Elimination and half-life • Terminal half-life after SC dosing in healthy women: HP‑hCG 36.8 ± 5.1 h; r‑hCG 38.6 ± 6.1 h. Prior data cited in the same paper report urinary hCG t1/2 ≈32–33 h after SC/IM. • In healthy Chinese adults given SC r‑hCG, mean t1/2 was 31.9 ± 15.9 h (test) and 39.7 ± 11.5 h (reference), with longer mean t1/2 reported in women for the reference product (44.4 ± 11.6 h). • Total body clearance for SC r‑hCG was approximately 0.84–0.90 L/h across formulations, with sex-stratified values in a similar range.

Metabolism and excretion • The accessible studies primarily provide serum PK and do not directly quantify metabolism or excretory fractions. However, both studies note immunoassay-based quantification of intact hCG in serum without describing metabolic pathways. Detailed metabolic fate (e.g., degradation to free β-subunit and β‑core fragments and renal handling) is not provided in the retrieved full texts; thus, route of biotransformation cannot be quantified from this evidence set.

Bioavailability • Relative bioavailability/extent of exposure: In healthy women, after dose-normalization, HP‑hCG exhibited about 20% greater AUC and roughly 50% higher Cmax compared with r‑hCG, with median tmax shorter by ~8 h (16 vs 24 h). Prior work cited indicates IM and SC urinary hCG have similar extent of exposure. • Between two r‑hCG formulations (LZM003 vs Ovidrel) in Chinese adults, bioequivalence criteria were met for AUC and Cmax (GMR 90% CIs within 80–125%), indicating comparable relative bioavailability between those formulations by the SC route. • Absolute bioavailability (vs IV) was not reported in the accessible texts.

Key quantitative summary (selected) • SC HP‑hCG 10,000 IU in healthy women: tmax 16 h (12–36), Cmax 338 ± 101 IU/L, AUC0–∞ 23,779 ± 4,945 IU·h/L, t1/2 36.8 ± 5.1 h. • SC r‑hCG 250 µg (~6,500 IU) in healthy women: tmax 24 h (12–48), Cmax 148.5 ± 36.5 IU/L, AUC0–∞ 12,937 ± 2,723 IU·h/L, t1/2 38.6 ± 6.1 h. • SC r‑hCG (LZM003 vs Ovidrel) in healthy Chinese adults: median tmax 16.0 vs 20.0 h; t1/2 31.9 ± 15.9 vs 39.7 ± 11.5 h; CL 0.867 ± 0.371 vs 0.837 ± 0.327 L/h; Vd 38.1 ± 18.4 vs 47.2 ± 20.4 L; bioequivalent for AUC and Cmax.

Limitations and context • The accessible evidence set lacks IV data to calculate absolute bioavailability and does not report mass-balance or metabolite profiles (e.g., urinary β‑core fragment fractions). Assay differences and dose non-equivalence between HP‑hCG and r‑hCG complicate cross-product comparisons; dose-normalization and assay context are essential.

Embedded summary table

Related Reading#

• HCG overview and research guide
• Peptides similar to HCG
• HCG research evidence