Gonadorelin: Molecular Structure

Molecular Structure and Properties#

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

Amino Acid Sequence#

We summarize the primary structure and physicochemical profile of Gonadorelin (GnRH1), including sequence, charge characteristics, isoelectric point, and structural features, and embed a concise reference table.

Molecular structure and amino acid sequence Gonadorelin is the synthetic form of the endogenous GnRH1 decapeptide with blocked termini: an N-terminal pyroglutamate (pGlu) and a C-terminal amide. The sequence is pGlu–His–Trp–Ser–Tyr–Gly–Leu–Arg–Pro–Gly–NH2, often written as pE1–H2–W3–S4–Y5–G6–L7–R8–P9–G10–NH2 (equivalently pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2).

Physicochemical properties

• Molecular weight: 1182.33 Da (reported for the amidated decapeptide).
• Isoelectric point: reported in the alkaline range, approximately pI ≈ 9.
• Ionizable groups and typical pKa: With the N-terminus cyclized to pyroglutamate and the C-terminus amidated, the principal ionizable side chains are His (H2, pKa ~6), Tyr (Y5, pKa ~10.1), and Arg (R8, pKa ~12.5). Thus at neutral pH, Arg is fully protonated, His is partially protonated depending on microenvironment, and Tyr is uncharged.
• Net charge at physiological pH (7.4) and charge distribution: The peptide carries approximately +1 net charge at pH 7.4, dominated by R8; H2 contributes marginal positive charge near neutrality, and other residues are neutral due to side chains or blocked termini. Positively charged density is localized at R8, with pH-contingent contribution from H2.
• UV absorbance: Strong absorbance at 280 nm arises from Tyr and Trp in the sequence.
• Typical salt forms/formulations: Common pharmaceutical salts include acetate/diacetate tetrahydrate and hydrochloride forms used in marketed products.

Structural features

• Turn motif: A β-turn is suggested at the C-terminal Pro–Gly region, consistent with structure visualizations and hydrogen-bonding patterns in small-peptide turns.
• Receptor-binding considerations: Computational docking and mutational analyses emphasize the role of R8 in forming contacts with acidic residues in the GnRH1 receptor binding pocket; binding-mode selection in simulation pipelines highlights these interactions as features of native-like poses. The sequence and terminal modifications (pGlu N-terminus, amide C-terminus) are conserved and important for receptor activation.

Notes and limitations

• Exact elemental formula and monoisotopic mass are not explicitly reported in the retrieved excerpts; however, the reported molecular weight (1182.33 Da) and alkaline pI (≈9) are provided. Charge assignment at pH 7.4 follows standard residue pKa values combined with the evidenced blocked termini and side-chain identities. Where relevant, formulation salts may influence apparent pH/pI in solution conditions but do not change the intrinsic sequence or modifications.

Stability and Formulation#

We summarize the stability of gonadorelin (GnRH; pGlu‑His‑Trp‑Ser‑Tyr‑Gly‑Leu‑Arg‑Pro‑Gly‑NH2) across pH and temperature, its principal degradation pathways, and formulation considerations for aqueous solutions and injections.

Key quantitative points

pH of maximal stability: ~5–5.5 Half-life at 70 °C near pH 5: ≈70 days Apparent activation energy, pH 2.1: ≈76 kJ/mol Apparent activation energy, pH 5.1: ≈88 kJ/mol Apparent activation energy, pH 8.4: ≈120 kJ/mol Key mechanisms: acid-catalyzed deamidation/hydrolysis; solvent-catalyzed hydrolysis (pH ~5 minimum); base-catalyzed epimerization and C-terminal deamidation Oxidation sensitivity noted; experiments run under N2 to avoid oxidation — consider O2 exclusion and light protection

Blockquote: Concise, quantitative stability highlights for gonadorelin (pH optimum, high-temperature half-life, activation energies, main degradation mechanisms, and oxidation risk) with source citations for rapid reference.

Gonadorelin displays a characteristic pH–log kobs profile with three regions: (i) proton-catalyzed degradation at low pH, (ii) a solvent-catalyzed minimum in the near‑acidic range, and (iii) base-catalyzed reactions at high pH. The peptide is most stable near pH 5–5.5; in kinetic studies a half-life of roughly 70 days at 70 °C was observed near this pH, indicating a pronounced stability optimum under mildly acidic conditions (reflecting the solvent-catalyzed minimum). Review summaries align, noting acid‑catalyzed deamidation at pH 1–3, increased backbone cleavage possibilities near pH 5–6 (adjacent to Ser), and base‑catalyzed epimerization/deamidation above pH 7.

Temperature sensitivity

Degradation follows Arrhenius behavior; apparent activation energies increase with pH, reported as about 76 kJ/mol at pH 2.1, 88 kJ/mol at pH 5.1, and 120 kJ/mol at pH 8.4. A solvent microconstant on the order of 10⁻7 s⁻1 was estimated at the stability minimum, underscoring slow, solvent‑mediated decay there. These data indicate accelerated degradation at elevated temperature across the pH range, with particularly high sensitivity under basic conditions.

Degradation pathways

• Acidic region (pH ~1–3): Proton‑catalyzed deamidation/hydrolysis dominates; the C‑terminal amide is susceptible under strong acid.
• Near‑neutral/mildly acidic (pH ~5–6): Overall slowest decay via solvent‑catalyzed hydrolysis; backbone cleavage adjacent to Ser has been observed for related GnRH peptides in this window.
• Basic region (pH >7): Parallel reactions occur, including C‑terminal amide deamidation (Gly10) and epimerization at Ser4 to the D‑isomer; racemization/epimerization pathways are base‑catalyzed and become prominent at alkaline pH.
• Oxidation risk: Experimental kinetic studies were conducted under nitrogen to avoid oxidation, implying sensitivity to oxygen/oxidation (Trp/Tyr/His are potential oxidation sites); light‑induced and metal‑catalyzed oxidation are general peptide risks to be controlled in formulation.

Buffer and ionic‑strength effects

Near the stability minimum, acetate or phosphate buffers exert little effect on kobs, whereas borate/carbonate at higher pH increase degradation rates. Ionic strength produced modest, condition‑dependent effects (e.g., increased kobs at pH 2, decreased at pH 9).

Formulation considerations for injections/aqueous solutions

• Target pH and buffer: Formulations should evaluate pH 3–6 and often select around pH ~5, which corresponds to the kinetic stability minimum; acetate or phosphate buffers are common choices, avoiding borate/carbonate that can accelerate base-catalyzed pathways.
• Excipients and tonicity: Mannitol is frequently used as a tonicity agent/stabilizer in LHRH peptide injections; for example, an approved GnRH antagonist injection (ganirelix) contains mannitol and acetic acid adjusted to pH ~5.0, illustrating a practical composition in this class. Research formulations sometimes used sodium azide as preservative, but this is not for human parenteral use (k.2008amyloidasa pages 10-10).
• Oxidation and light: Minimize dissolved oxygen (nitrogen overlay or de‑aeration), protect from light, and consider antioxidants or metal chelators when justified by susceptibility of residues and impurity profiles; manage transition‑metal contaminants and peroxides from excipients.
• Lyophilization vs solution: Lyophilized gonadorelin products are expected to be more chemically stable than aqueous solutions, reducing hydrolytic pathways; however, when solution stability is sufficient near pH ~5 with appropriate controls, aqueous presentations are feasible and operationally simpler.
• Adsorption/aggregation: As with many peptides, adsorption to container/contact surfaces and aggregation can occur, influenced by concentration, pH, interfaces, and stress. Mitigation includes suitable container/stopper materials, minimizing air–liquid interfaces, adding surfactants or viscosity enhancers when compatible, and controlling storage/handling stresses.
• Preservatives: Some LHRH analog injections (e.g., deslorelin) have been reported with benzyl alcohol as preservative; applicability to specific gonadorelin products depends on labeling and compatibility assessments. Direct prescribing‑information/USP specifics for gonadorelin were not retrieved in this evidence set and thus cannot be quoted here.

Collectively, primary kinetic data establish that gonadorelin is most stable in mildly acidic solution (pH ~5–5.5), with solvent‑catalyzed degradation slowest there; both strong acid and base accelerate distinct pathways (acid‑catalyzed deamidation/hydrolysis vs base‑catalyzed epimerization/deamidation). Temperature accelerates degradation across pH with higher apparent activation energies under basic conditions. Formulations commonly leverage pH ~5 with acetate/phosphate buffers, control oxygen and light, use benign excipients such as mannitol, and consider lyophilization if long‑term stability in solution is insufficient; adsorption/aggregation risks should be mitigated by container and excipient choices. Where specific label guidance is needed (storage temperatures, in‑use stability, reconstitution), consult product‑specific prescribing information or pharmacopeial monographs, which were not available in the present evidence set.

Comprehensive summary table

Pharmacokinetics#

Objective summary. We sought human pharmacokinetic data for native gonadorelin (GnRH) covering absorption, distribution, metabolism, elimination, half-life, and bioavailability. Our searches retrieved one source with human PK parameters for a GnRH analogue (buserelin) and one methodological/overview source citing stability/degradation literature for gonadorelin, but no primary human PK study or product label for native gonadorelin. Accordingly, quantitative values below are limited to buserelin where explicitly reported, and we indicate gaps for native gonadorelin.

For native gonadorelin, our retrieved material did not contain numeric human PK parameters for tmax, half-life, protein binding, volume of distribution, clearance, or absolute bioavailability by route. The overview source primarily listed analytical and stability references and did not provide human PK values. Therefore, we cannot provide validated numeric PK parameters for native gonadorelin from the current evidence set.

Limitations and next steps. To fully answer the question for native gonadorelin, the key sources to retrieve would be: (1) the prescribing information for Factrel (gonadorelin hydrochloride) or equivalent labels (e.g., Lutrepulse SmPC) and (2) classic human PK studies in the Journal of Clinical Endocrinology & Metabolism from the 1980s comparing IV versus SC administration. These were not available in the retrieved evidence for this response.

Related Reading#

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