Single, dual and triple receptor agonists: how semaglutide, tirzepatide and retatrutide differ
A structural and receptor-target comparison of three incretin research peptides — GLP-1, GIP and glucagon receptor engagement, molecular weights, CAS numbers and what the acylation is doing.
Three peptides that get grouped together in catalogs are structurally and pharmacologically distinct in a way the grouping obscures. Semaglutide engages one receptor. Tirzepatide engages two. Retatrutide engages three. Understanding which receptors, and why the molecules were built the way they were, is the difference between treating them as interchangeable and treating them as separate research tools.
The receptors involved
All three sit in the incretin family — class B G protein-coupled receptors that respond to gut-derived peptide hormones. Three receptors matter here:
- —GLP-1R — the glucagon-like peptide-1 receptor.
- —GIP-R — the glucose-dependent insulinotropic polypeptide receptor.
- —GCGR — the glucagon receptor.
These receptors share structural homology, which is precisely what makes multi-receptor agonists chemically feasible: a single peptide backbone can be engineered to present binding determinants that satisfy more than one of them.
Side by side
| Compound | Receptor targets | Molecular formula | Molecular weight | CAS |
|---|---|---|---|---|
| Semaglutide | GLP-1R | C187H291N45O59 | 4113.6 g/mol | 910463-68-2 |
| Tirzepatide | GIP-R, GLP-1R | C225H348N48O68 | 4813.5 g/mol | 2023788-19-2 |
| Retatrutide | GIP-R, GLP-1R, GCGR | C221H342N46O68 | 4731.3 g/mol | 2381089-83-2 |
A detail worth noticing: retatrutide engages one more receptor than tirzepatide while having a slightly lower molecular weight. Receptor breadth is not a function of size. It is a function of which residues occupy which positions, and how the molecule presents them.
Backbone origins
Semaglutide
A 31-amino-acid analog built on the native GLP-1 backbone, modified at the positions that otherwise make GLP-1 extremely short-lived. Native GLP-1 has a circulating half-life measured in a couple of minutes, largely because dipeptidyl peptidase-4 cleaves it near the N-terminus. Substituting the residue at that cleavage site removes the vulnerability.
Tirzepatide
A 39-amino-acid peptide based on the GIP sequence rather than the GLP-1 sequence, engineered so that it also satisfies GLP-1R. It is described as an imbalanced dual agonist: engagement is not equal across the two receptors, and that asymmetry is a deliberate design property rather than an artifact.
Retatrutide
Also 39 amino acids, engineered to engage all three receptors. Adding GCGR to the profile is a meaningfully different proposition, because glucagon receptor signaling opposes several of the effects associated with GLP-1R signaling. The molecule is balancing pathways that pull in different directions, which is why the ratio of engagement across the three receptors is the interesting variable rather than any single affinity figure.
What the fatty acid chain is doing
All three carry a fatty-diacid moiety attached to the peptide backbone — C18 on semaglutide, C20 on tirzepatide and retatrutide. This is not incidental decoration; it is the half-life engineering.
The lipid chain promotes reversible association with serum albumin. Albumin-bound peptide is protected from renal clearance and from proteolytic degradation, and it is released gradually as free peptide is consumed. The effect is a circulating reservoir. This is why these molecules persist on a timescale of days where the native hormones persist on a timescale of minutes.
Acylation has consequences for handling. The lipid chain increases the tendency of these peptides to adsorb to surfaces and to associate with one another, which is relevant to how a sample behaves in solution, how it should be handled during preparation, and how consistently it recovers from a container.
Reading the specification, not the category
Because these compounds are often listed adjacently, it is easy to compare them on price per milligram alone. That comparison is close to meaningless across different receptor profiles — a single-receptor agonist and a triple-receptor agonist are not substitutes, and milligram-for-milligram equivalence between them does not exist in any useful sense.
The comparisons that do carry information are the ones on the certificate: observed mass against theoretical mass, chromatographic purity, and whether the lot number on the certificate matches the vial.
This article describes receptor targets and molecular structure only. It makes no claim about physiological outcomes, and these compounds are supplied strictly for laboratory and in-vitro research use — not for human or animal use. No dosing or administration guidance is provided.