HGH 191AA: Molecular Structure

Molecular Structure and Properties#

HGH 191AA is a peptide whose molecular structure and properties have been characterized through analytical chemistry and structural biology studies.

Amino Acid Sequence#

We describe HGH 191AA (somatropin; GH1) at the level of sequence, covalent connectivity, physicochemical profile, and structure.

Primary structure and processing. The mature circulating hormone is 191 amino acids long, generated from a 217‑residue precursor by removal of a 26‑residue N‑terminal signal peptide. High‑coverage LC‑MS mapping reports the complete 191‑aa sequence with N‑terminal Phe and C‑terminal Phe: FPTIPLSRLFDNAMLRAHRLHQLAFDTYQEFEEAYIPKEQKYSFLQNPQTSLCFSESIPTPSNREETQQKSNLELLRISLLLIQSWLEPVQFLRSVFANSLVYGASDSNVYDLLKDLEEGIQTLMGRLEDGSPRTGQIFKQTYSKFDTNSHNDDALLKNYGLLYCFRKDMDKVETFLRIVQCRSVEGSCGF (Phe1…Phe191). Multiple independent analyses confirm the 191‑aa length of the predominant 22 kDa isoform and the precursor/signal peptide organization (217 total; signal peptide 1–26).

Covalent connectivity. Somatropin contains two intramolecular disulfide bonds, unambiguously assigned by non‑reduced tryptic mapping and MS as Cys53–Cys165 and Cys182–Cys189. These pairings are consistently reported across biochemical and proteomic studies.

Physicochemical properties. Reported molecular mass values for the 191‑aa monomer are approximately 22.1 kDa: theoretical masses reported around 22,129 Da and 22.096 kDa, with an apparent SDS‑PAGE mass ~22 kDa. The isoelectric point is acidic: measured pI ≈ 5.0 by isoelectric focusing, in close agreement with theoretical estimates ~5.1–5.3. Consistent with this pI, the net charge at neutral pH is modestly negative overall; surface charge is distributed with acidic patches on helix faces and basic clusters that contribute to receptor interfaces (qualitative inference from low pI and structural contact mapping).

Three‑dimensional structure and features. HGH adopts a class I cytokine four‑helix bundle fold with up–up–down–down topology and ~50–55% α‑helix content by CD and crystallography. The hormone engages two growth hormone receptor (GHR) extracellular domains via two spatially separated binding sites (site I and site II) to form the signaling 1:2 GH:GHR complex, as seen in the crystal structure (PDB 3HHR). Reported helix spans include helix A (~residues 9–34), helix C (~97–107), and helix D (~158–190); helix B varies in published numbering. The long loop between helices C and D (~134–149) is proteolytically susceptible, and the alternative‑spliced 20 kDa isoform lacks residues 32–46.

Post‑translational and proteoform notes. Pituitary GH proteomics identify phosphorylation at Ser77, Ser132, and Ser176, and deamidation events in some proteoforms; the 22 kDa species is the predominant pituitary isoform.

Embedded reference summary follows.

Stability and Formulation#

Somatropin (recombinant human growth hormone, 191 amino acids) displays stability behavior typical of therapeutic proteins with additional features driven by its sequence and interfacial sensitivity.

pH stability

• Empirical solution studies across pH 6.0–7.0 indicate optimal stability near pH ≈6.25–6.5 when formulated in 10 mM histidine or citrate buffer with preservative; at pH 6.0 rHGH shows substantial monomer loss and increased aggregation, while pH 7.0 decreases chemical degradation but may increase fine particle counts under stress. Commercial products are commonly formulated above the protein’s pI (~5.3) in the pH 6–7 range (Mohammadpanah 2013).
• Interfacial aggregation is pronounced and intrinsically linked to agitation rather than pH per se; nonionic surfactants suppress this pathway though they may slightly reduce thermal unfolding temperatures at higher levels (Katakam 1995).

Temperature sensitivity and environmental stresses

• Storage at 4 °C limits monomer loss and high–molecular weight (HMW) formation; elevated temperatures (25–50 °C) accelerate formation of related proteins, dimers/HMW species, and particles, with acidic pH exacerbating aggregation (Mohammadpanah 2013).
• Short, high‑shear agitation at ambient temperature induces severe precipitation due to air–water interfacial denaturation; surfactants mitigate this effect (Katakam 1995).
• Light promotes oxidation and aggregation in liquid presentations more than in lyophilized product; photostress increased HMW species and acidic/basic charge variants, and raised oxidation at Met14 and Met125 in somatropin, with lesser effects in the lyophilized state (Pritts 2025).

Major degradation pathways

• Deamidation at Asn residues occurs via direct hydrolysis and via cyclic succinimide intermediates, producing Asp/isoAsp; rates increase with pH and temperature, and some sites can accumulate succinimide under mildly acidic conditions. These reactions are well established for GH and peptides/proteins broadly (Bischoff 1994; Bummer 2007; Mohammadpanah 2013).
• Oxidation, particularly of methionine residues, is enhanced by light in solution; somatropin shows increased oxidation at Met14 and Met125 under photostress, correlating with higher HMW species (Pritts 2025).
• Aggregation/adsorption is driven by interfacial denaturation at air–water (agitation) and is mitigated by nonionic surfactants; temperature accelerates noncovalent and sometimes covalent aggregation (Katakam 1995; Mohammadpanah 2013). In the dried state, excipient crystallization can lead to covalent aggregation or clipping.

Formulation considerations

• Buffers/pH: Histidine or citrate (≈10 mM) at pH ~6.25–6.5 showed the best overall stability among pH 6–7 tested; insufficient ionic strength (e.g., 5 mM phosphate) increased monomer loss. Phenol at ~2.5 mg/mL was used as preservative in these studies (Mohammadpanah 2013).
• Surfactants: Nonionic surfactants such as polysorbates (Tween 80), Pluronic F‑68, or Brij 35 effectively suppress agitation‑induced interfacial aggregation; however, at higher levels they lowered the DSC onset of thermal denaturation, indicating a trade‑off (Katakam 1995).
• Lyophilization: Amorphous sugars (trehalose, lactose, cellobiose) and mannitol/sorbitol when remaining amorphous protect secondary structure and reduce aggregation; crystallization of bulking agents (e.g., mannitol) diminishes protection and can enable covalent aggregation/clipping. Proper drying and low‑moisture storage are important (Costantino 1998).
• Alternative stabilization: Complexation with heparin reduced agitation‑induced precipitation and showed pH/ionic‑strength dependent binding/release behavior; maximal precipitation of uncomplexed hGH under agitation occurred near pH ~4.7 in the tested set (Zamiri 2005).

Practical implications

• Target pH near 6.3–6.5 in low‑to‑moderate ionic strength histidine or citrate buffers; include a suitable preservative when needed and a nonionic surfactant to control interfacial aggregation, balancing any impact on thermal margins.
• Store refrigerated to minimize temperature‑driven aggregation and chemical degradation; avoid agitation and protect from light, especially for liquid presentations; prefer lyophilized formats when long light/temperature excursions are expected.
• In the dried state, prioritize amorphous glass‑forming excipients and control residual moisture to prevent excipient crystallization and associated covalent changes.

Pharmacokinetics#

We interpret “HGH 191AA” as unmodified recombinant human growth hormone identical to pituitary GH (somatropin; 191 amino acids, ~22 kDa). Pharmacokinetic properties below refer to standard daily somatropin products unless noted.

Absorption • Clinically used routes: subcutaneous (SC), intramuscular (IM), and intravenous (IV) for PK characterization; intranasal has been studied experimentally. After SC dosing, peak serum concentrations typically occur about 2–4 hours post‑dose in pediatric studies and are similar in adults for daily products (route and timing comparable across brands). • Bioavailability by route: SC absolute bioavailability is approximately 70–81% depending on product; IM is lower at about 63%; intranasal formulations achieve very low systemic bioavailability (~3%) though they can still elevate IGF‑1 in experimental settings.

Distribution • Volume of distribution is small, about 0.07 L/kg, reflecting confinement largely to the vascular and interstitial space. Pediatric PK appears similar to adults on a weight‑normalized basis. Plasma protein binding is not well quantified; circulating GH interacts with GH‑binding protein and its receptor.

Metabolism • Somatropin is cleared predominantly by proteolytic degradation in liver and kidney, with contributions from receptor‑mediated internalization and renal filtration; intact urinary excretion is minimal.

Elimination • Systemic clearance is roughly 0.14 L/h/kg in adults and children for unmodified GH; in extravascular studies, apparent CL/F values around 150–215 mL/h/kg have been reported and vary by brand/formulation.

Half‑life (by route) • Intravenous: terminal half‑life ≈ 0.36 hour, reflecting rapid systemic clearance when absorption is bypassed. • Subcutaneous: terminal half‑life typically ~2.1–4.9 hours depending on product (e.g., Nutropin ≈ 2.1 h; Genotropin ≈ 3.0 h; Humatrope ≈ 4.9 h). In pediatrics, mean SC half‑life ≈ 3.8 hours. • Intramuscular: terminal half‑life ~3.0–3.8 hours (e.g., Humatrope IM ≈ 3.8 h). The longer apparent half‑lives after SC/IM vs IV reflect rate‑limited absorption from the injection site.

Additional notes on absorption/formulation effects • Depot microsphere formulations (e.g., Nutropin Depot) prolong exposure for ~13–15 days after a single SC injection in children, with estimated relative bioavailability ~50% vs daily RHGH (not an unmodified product). Various long‑acting modifications (e.g., PEGylation, albumin binding) markedly reduce clearance and extend half‑life, but these are distinct from standard HGH 191AA.

Key quantitative summary (adults; typical values unless noted) • Tmax after SC: ~2–4 h (pediatric data typical and consistent with adult daily products). • Bioavailability: SC ~70–81% (product‑dependent); IM ~63%; intranasal ~3% (experimental). • Vd: ~0.07 L/kg. • Clearance: ~0.14 L/h/kg (systemic); CL/F after SC/IM ~150–215 mL/h/kg depending on brand. • Half‑life: IV ~0.36 h; SC ~2.1–4.9 h; IM ~3.0–3.8 h; pediatric SC ~3.8 h. • Metabolism/elimination: proteolytic degradation predominantly in liver and kidney; minimal intact urinary excretion.

Embedded summary table of PK properties and ranges is provided below for quick reference.

Related Reading#

• HGH 191AA overview and research guide
• Peptides similar to HGH 191AA
• HGH 191AA research evidence