HGH 191AA is recombinant human somatropin — a 191-amino-acid protein whose primary sequence is identical to the mature 22 kDa isoform of endogenous human growth hormone (UniProt P01241, residues 27–217 of the 217-residue precursor, after signal peptide cleavage). It folds into a four-helix bundle stabilised by two intramolecular disulfide bridges and acts as an agonist at the growth hormone receptor, dimerising it to initiate JAK2/STAT5 signalling. It is produced recombinantly, commonly in E. coli or mammalian expression systems. The '191AA' designation distinguishes it from somatrem, an older 192-residue methionyl variant that carries an extra N-terminal methionine.

Two classification points that this catalogue takes seriously. First, this is a protein, not a peptide: at roughly 22 kDa and 191 residues with defined tertiary structure and disulfide topology, it belongs to a different analytical and stability regime than the synthetic peptides listed elsewhere, and higher-order structure, aggregation state and host-cell protein content are meaningful quality attributes for it in a way they are not for a 29-mer. Second, somatropin potency is assigned biologically in International Units against a WHO International Standard rather than by peptide content, which is why licensed presentations are labelled in IU and why a mass figure alone does not specify what is in a vial.

Somatropin is a genuinely approved medicine, registered in many jurisdictions under numerous brand names for paediatric and adult growth hormone deficiency and several other indications, with decades of regulated clinical use behind it. The honest limitation is that none of that transfers to research-grade material. An unlicensed preparation has no potency traceable to the WHO standard, no verified aggregation, deamidation or oxidation profile, and no characterised host-cell protein or endotoxin burden — and for a recombinant protein those attributes are the principal determinants of immunogenicity. Sequence identity with an approved product is not equivalence to it.