Terminology Guide

What Does 191AA Mean?

191AA means 191 amino acids, the length of human growth hormone’s main chain — where that count comes from, and what it cannot tell you about a vial.

Updated 29 September 2026

[Research Use Only] This guide is for controlled laboratory research workflows only. It is not for human or veterinary use and does not provide applied-use guidance.

A count of residues, not a grade

191AA stands for 191 amino acids. It is the length of the principal form of human growth hormone — the protein described in medicines as somatropin — counted residue by residue along a single chain. The number says which molecule is meant. It is not a purity figure, a strength or a quality mark, and on a label it describes what is claimed to be inside, not evidence that it is.

Where the 191 comes from

The GH1 gene encodes a precursor of 217 amino acids. The first 26 form a signal peptide, removed as the hormone is secreted, and what remains is the 191-residue chain, held in shape by two disulfide bridges. As a mass it is the "22 kDa" form: an FDA-approved label gives about 22,124 daltons, and an EMA assessment approximately 22,125.

Feature	Value	Why it matters
Precursor encoded by GH1	217 residues	A sequence of this length still includes the signal peptide
Signal peptide	26 residues	Removed on secretion; not part of the hormone
Mature chain	191 residues	The number in 191AA
Disulfide bridges	Cys53–Cys165 and Cys182–Cys189	Mature-chain positions; UniProt numbers them 79–191 and 208–215 on the precursor
Glycosylation	None in recombinant somatropin	The mass is the chain’s alone
Molecular weight	About 22,124 Da	The source of the "22 kDa" name

Two counts come from one gene and describe different things: 217 is the precursor, 191 the hormone. A sequence that does not say which it is has not said what it describes.

One gene, several molecules

Human growth hormone is a family rather than a single substance. The GH1 transcript is spliced in two ways, giving the 22 kDa principal pituitary form and a 20 kDa form lacking residues 32 to 46. A second gene, GH2, encodes a variant made in the placenta. The chain can also be acylated, deamidated or glycosylated, and it forms dimers and larger oligomers of up to at least five chains. "191AA" picks one member out of this family.

Form	Residues	What distinguishes it
22 kDa growth hormone	191	The principal pituitary form, and the sequence of recombinant somatropin
20 kDa growth hormone	176	Alternative splicing removes residues 32–46
Methionyl growth hormone	192	The first recombinant product: the native sequence plus an N-terminal methionine
Deamidated and acylated forms	191	The same chain, chemically modified
Oligomers	191 per chain	Two or more chains, disulfide-linked or non-covalently associated

Each of these is growth hormone in some usage. The residue count separates the first three; it cannot separate the last two from the first.

Recombinant production and the 192nd residue

Growth hormone for medical use was extracted from human pituitary glands until 1985. The gene had been expressed in Escherichia coli in 1979, and the first recombinant product, introduced in 1985, had 192 residues: the native sequence with a methionine added at the N-terminus, which the bacterial process initially required. Current recombinant somatropin has the identical 191-residue sequence of the pituitary hormone. Regulatory descriptions use those terms. An FDA-approved label describes 191 residues, a sequence identical to growth hormone of pituitary origin, and synthesis in a modified strain of E. coli; an EMA assessment describes fermentation in E. coli followed by chromatographic purification.

What the number on a label is claiming

In authorised medicines the count sits in the description of the substance, beside the molecular weight and the statement that the sequence matches pituitary growth hormone. Printed on a label, "191AA" compresses three claims into one: the full-length chain rather than a fragment, the native sequence rather than the 192-residue methionyl form, and the 22 kDa hormone rather than the 20 kDa isoform. None of the three is verified by the label that makes it.

What 191AA cannot tell you

A residue count describes a sequence on paper. Whether a vial holds that sequence, correctly folded and in the stated amount, is a question for analysis — and several ways a protein of this size can change leave its length untouched.

Whether the sequence is the stated one: a substituted residue leaves the count at 191
Whether both disulfide bridges are correctly formed
Whether the chain has deamidated; UniProt records deamidation of a glutamine and an asparagine arising by deterioration
Whether related proteins have formed; in one study assessed by the EMA, high levels were produced deliberately by storage at 40 °C rather than 2–8 °C
Whether chains have associated into dimers or larger oligomers
How much protein the vial holds, and what else it contains

Research Checklist

Confirm batch identity and records.
Document all preparation inputs.
Keep use within controlled laboratory workflows.
Do not infer applied-use suitability from guide content.

Frequently Asked Questions

Is 191AA the same as HGH?

It is the main form of it. Human growth hormone covers a family: the 22 kDa and 20 kDa splice products, a placental variant, and modified and oligomeric forms. 191AA specifies the 191-residue, 22 kDa chain, which is also the sequence recombinant somatropin reproduces.

Is 191AA related to fragment names such as 176–191?

They share a numbering but mean different things. In 191AA the number is a count of residues. In a name like 176–191 the numbers are positions on the chain, and the fragment they describe is the last sixteen residues at its C-terminal end: a small part of the hormone, not the hormone.

What does 22 kDa mean?

22 kilodaltons: the chain’s molecular weight, given as about 22,124 daltons in an FDA-approved label and approximately 22,125 in an EMA assessment. Both documents state the figure as approximate, and both describe the same 191-residue sequence.

Does 191AA on a label confirm what is in the vial?

No. It states which molecule the supplier says it has supplied. Confirmation comes from analysis of the batch — a measured mass consistent with the 22 kDa chain, and separations able to resolve modified and aggregated forms — reported against the batch number on the vial.

Sources

The technical statements in this guide are drawn from the following. Where a definition is contested or a figure depends on method, the guide says so rather than picking one.

UniProtKB P01241: Somatotropin (SOMA_HUMAN)UniProt ConsortiumUsed for the 217-residue GH1 precursor, the 26-residue signal peptide, the 191-residue mature chain, the two disulfide bonds and their positions, the 20 kDa isoform lacking precursor residues 58–72 (32–46 of the mature chain), deamidation of a glutamine and an asparagine by deterioration, and the monomeric to pentameric forms.
Zomacton (somatropin) for injection: prescribing informationU.S. Food and Drug AdministrationUsed for the Description section only: a polypeptide of 191 amino acid residues with a molecular weight of about 22,124 daltons, a sequence identical to growth hormone of pituitary origin, and production in Escherichia coli modified by insertion of the human growth hormone gene.
Omnitrope: EPAR scientific discussionEuropean Medicines AgencyUsed for the non-glycosylated 191-residue protein of approximately 22,125 daltons, production by fermentation in E. coli with chromatographic purification, chemical identity with the major component of pituitary growth hormone, and related proteins generated by storage at 40 °C rather than 2–8 °C.
Somatotropin in the treatment of growth hormone deficiency and Turner syndrome in pediatric patients: a reviewClinical Pharmacology: Advances and Applications, via PMC, National Library of MedicineUsed for history and structure only: the end of pituitary-derived hormone in 1985, expression of the gene in E. coli in 1979, the 192 residues of the first recombinant product and its added N-terminal methionine, the identical 191-residue sequence of current recombinant hormone, and the 22 and 20 kDa isoforms, with residues 32–46 absent from the 20 kDa form.
Growth hormone isoforms (Baumann GP, 2009; PMID 19467614)Growth Hormone & IGF Research, via PubMedUsed for the heterogeneity of human growth hormone: the GH1 and GH2 genes, the 22K and 20K splice products with 22K as the principal pituitary form, the placental variant, and acylated, deamidated, glycosylated and oligomeric forms up to at least pentamers.