Reference
Peptide or protein? The 40-amino-acid line
Chemistry has no sharp line between a peptide and a protein. United States law does, and it sits at exactly forty amino acids.
Ask a chemist where a peptide ends and a protein begins and you will get a range, not a number. Chains of up to about fifty amino acids are usually called peptides, longer ones proteins, and plenty of textbooks admit the boundary is a convention. A regulator cannot work with a convention. It needs a line it can apply to a product on a specific day.
The line the FDA drew
The US Food and Drug Administration defines a protein as “any alpha amino acid polymer with a specific, defined sequence that is greater than 40 amino acids in size”. Forty or fewer, and the molecule is a peptide. That sentence sits in the US federal regulations, and the FDA said plainly why it chose forty: it looked for a scientific consensus on where proteins begin, did not find one, and picked the figure it judged best supported by the literature. (Federal Register, 2018.)
The number matters because the two sides of it follow different rulebooks. A peptide is regulated as a drug. A protein is regulated as a biologic, the category that covers vaccines and antibodies, with its own approval pathway and its own rules for copies.
What moved across it in 2020
On 23 March 2020, products that had been approved as drugs but met the protein definition were transferred to the biologics side. Insulin, at 51 amino acids, is the best-known example: nothing about the molecule changed, but from that date it was legally a biologic. Glucagon, at 29 amino acids, stayed a drug.
Tesamorelin, at 44 amino acids, is the compound in this range that sits on the far side. It was approved in 2010 and is now handled as a biologic. Its molecule is the same one it always was.
The two that sit one short
Tirzepatide and Retatrutide are both 39 amino acids long, as was exenatide, the first drug of their type, taken from a peptide in Gila monster venom. All three sit one amino acid under the line.
That has fed a rumour worth correcting. You will read that a peptide “can’t be patented” above 40 amino acids, or that a manufacturer lengthened retatrutide to 42 to get around the rule. The forty-amino-acid line has nothing to do with patents, which depend on whether an invention is new and not obvious, not on its length. And we can find no record of retatrutide being anything other than 39 amino acids. The line decides which rulebook a molecule is approved under. That is all it decides.
Where this range sits
Every compound here with a settled amino-acid count, shortest first. Everything above the line would be a peptide in US law. Only one sits below it.
| Amino acids | Compound |
|---|---|
| 3 | GHK-Cu, KPV, Glutathione |
| 4 | Epithalon, SS-31 |
| 5 | Ipamorelin |
| 7 | Semax, Selank, Melanotan II, PT-141 |
| 8 | SNAP-8 |
| 9 | DSIP, Oxytocin Acetate |
| 10 | Kisspeptin-10 |
| 13 | Melanotan I |
| 15 | BPC-157 |
| 16 | MOTS-c |
| 28 | Thymosin Alpha-1 |
| 29 | CJC-1295 |
| 31 | Semaglutide |
| 39 | Tirzepatide, Retatrutide |
| The line: more than 40 amino acids is a protein | |
| 44 | Tesamorelin |
TB-500 is left out of the count on purpose. It is a seven-amino-acid fragment, but its parent molecule, thymosin beta-4, is 43, which puts the parent on the protein side and the fragment far below it. The GLP-1 analogues also carry fatty-acid chains, and some of these molecules use amino acids that do not occur in nature. Neither changes the count: the definition is about the length of the amino-acid chain.
What the line is not
- Not chemistry. A 40-residue chain and a 41-residue chain are made the same way, and nothing about the 41st amino acid changes what the molecule is.
- Not worldwide. It is a US definition. Other regulators classify medicines in their own ways.
- Not about quality. Being a peptide or a protein says nothing about how well a particular batch was made. That is what an analytical report is for.
