What Is Tirzepatide? Structure and Chemistry

Tirzepatide is a synthetic lipidated peptide of 39 amino acid residues, with the molecular formula C225H348N48O68 and an average molecular weight of 4813.45 g/mol. Two of its residues are the non-coded amino acid α-aminoisobutyric acid (Aib), the chain ends in a C-terminal amide, and the side chain of lysine 20 carries a C20 fatty diacid attached through a short hydrophilic linker. At the molecular level it is a GIP and GLP-1 receptor agonist peptide: a single chain that binds two different receptors.

The 39-residue sequence

Tirzepatide is a linear chain with a free N-terminal amine on Tyr1 and an amidated C-terminal serine. The table sets the sequence out in blocks of ten in three-letter code, with the modified positions marked.

In one-letter code, with Aib written out, the chain reads Y-Aib-EGTFTSDYSI-Aib-LDKIAQKAFVQWLIAGGPSSGAPPPS-NH2. It has two lysines: Lys16 is unmodified and Lys20 carries the lipidated side chain. Zhao et al. (2022) describe the sequence as primarily based on that of the GIP receptor's native peptide ligand. The last ten residues, Gly30 to the C-terminal serine amide, form a proline-rich tail with three consecutive prolines at positions 36–38.

If you are new to these conventions, peptide sequence notation covers three-letter and one-letter codes and how terminal groups are written.

Two non-coded residues: Aib2 and Aib13

α-Aminoisobutyric acid (Aib, also called 2-methylalanine) is not one of the twenty amino acids encoded by the genetic code. It is alanine with a second methyl group on the α-carbon. That makes it achiral, since the α-carbon carries two identical methyl groups, and it leaves no α-hydrogen.

The two methyl groups narrow the backbone torsion angles the residue can adopt, and Aib is well known in peptide chemistry as a strongly helix-favouring residue. The extra methyl group also crowds the neighbouring peptide bonds sterically.

In tirzepatide, Aib sits at position 2, directly after the N-terminal tyrosine, and at position 13 in the middle of the chain. At both positions the parent sequence has alanine, so each substitution adds exactly one methyl group. In the receptor structures described below, the N-terminal segment Tyr1–Aib2–Glu3 sits deep in the transmembrane core.

The lipidated side chain on lysine 20

The distinctive feature of tirzepatide is the side chain on Lys20. Its ε-amino group is acylated by a linker of three units, and the linker ends in a long-chain fatty diacid.

Each junction is an amide bond. Together, the spacers, the γGlu unit and the diacid make tirzepatide an amphiphilic molecule: a peptide chain carrying a long hydrocarbon tail that ends in a carboxylic acid.

In both published cryo-EM studies, the linker and diacid were not resolved in the density maps, which the authors read as high conformational flexibility. Sun et al. (2022) built the missing chain into their models and examined it with molecular dynamics simulations instead.

Putting the side chain on Lys20 but not on Lys16 is a regioselectivity problem. In solid-phase synthesis it is normally solved by giving the lysine to be modified an orthogonal side-chain protecting group, removed on the resin before the linker units are coupled.

Formula, mass and expected ions

The formula C225H348N48O68 describes the complete neutral molecule: peptide chain, linker and diacid together. Building it up from the 39 residues, the two spacers, the γGlu unit, the diacid and the C-terminal amide reproduces the formula PubChem lists. PubChem rounds the average molecular weight to 4813 g/mol and gives the monoisotopic mass as 4810.5249 Da.

Calculated average masses differ in the second decimal place with the atomic-weight table used: 4813.45 g/mol with the older standard atomic weights (C 12.0107, H 1.00794, N 14.0067, O 15.9994) and 4813.53 g/mol with the current IUPAC abridged values (C 12.011, H 1.008, N 14.007, O 15.999). The difference is under 0.01% and reflects the reference table, not the molecule.

At almost 4.8 kDa, tirzepatide appears in electrospray ionisation mass spectrometry as a series of multiply protonated ions rather than one singly charged ion. Calculated from the average mass, the main charge states fall at:

For a molecule this size the isotope envelope is broad and the monoisotopic peak is not the most abundant one. That is why deconvoluted spectra from lower-resolution instruments are usually compared with the average mass. Mass spectrometry for peptide identity explains charge states and deconvolution in more detail.

Receptor binding at the molecular level

Tirzepatide is a GIP and GLP-1 receptor agonist peptide: one chain that binds and activates two different receptors. Both belong to class B1 of the G protein-coupled receptors (GPCRs), the secretin-like family. Its members combine a large N-terminal extracellular domain (ECD) with the seven-helix transmembrane domain (TMD) common to all GPCRs, and they bind peptide ligands through both domains.

Two cryo-EM studies published in 2022 captured tirzepatide bound to each receptor in the active state, coupled to the heterotrimeric Gs protein:

• Sun et al. (2022) report structures at 3.08 Å (PDB 7RBT) and 2.9 Å (PDB 7RGP), with peptide residues 1–32 resolved.
• Zhao et al. (2022) report a second pair of structures at 3.4 Å each (PDB 7FIY and 7FIM).

In both receptors, tirzepatide adopts an α-helical conformation, and its N-terminal residues reach deep into the transmembrane core. Zhao et al. found that residues 1–10 occupy closely overlapping positions in the two receptors. In the GLP-1 receptor complex, the middle of the helix packs against the extracellular domain and extracellular loops through a polar network involving Ser11, Asp15 and Gln19 and a nonpolar network involving Ser11, Aib13, Phe22, Trp25 and Leu26.

Sun et al. also report radioligand competition binding on membranes from cells expressing each receptor. Tirzepatide's affinity for the GIP receptor equalled that of the receptor's native peptide ligand. At the GLP-1 receptor, its binding was about fivefold weaker than that receptor's native ligand.

How tirzepatide is characterised analytically

Two complementary methods establish the identity and purity of a lipidated peptide like tirzepatide.

• Reversed-phase HPLC separates the main peptide from related substances and reports purity as the main peak's share of total peak area, usually with UV detection at 214–220 nm, where the peptide bond absorbs. The C20 diacid adds a long hydrocarbon chain, so a lipidated peptide is retained more strongly on C8 or C18 columns than a non-lipidated chain of similar length. HPLC and peptide purity covers the method.
• Mass spectrometry confirms identity by matching the observed mass, deconvoluted from the charge states above, to the expected mass.

The impurities these methods look for are those typical of solid-phase synthesis: deletion sequences missing a residue, truncated chains, incompletely deprotected species and, for a lipidated peptide, chains carrying an incomplete linker or no side chain at all. Other lines on a certificate of analysis describe the solid rather than the molecule, for example water content by Karl Fischer titration, counter-ion content and net peptide content.

For our tirzepatide, purity is specified as ≥99% (HPLC, supplier specification). Certificate status is shown on every product page, certificates of analysis are published in the COA Library where available, and a batch certificate is available on request.

Form and storage of the sealed vial

Tirzepatide is supplied as a white to off-white lyophilised powder in a sealed vial, in 10 mg, 20 mg and 40 mg presentations. Identifiers and the certificate status are listed on the tirzepatide product page.

Store the sealed vial at 2–8 °C for short-term storage, or at −20 °C and below for long term. Protect it from light and avoid repeated freeze-thaw cycles. Handle the material as a laboratory chemical, following your institution's chemical-safety procedures. Storing lyophilised peptides explains why cold, dark and dry storage matters for a freeze-dried solid.

In our catalogue, tirzepatide sits in the Incretin & Amylin Analogues research area, which groups lipidated peptide analogues by molecular class. Terms used here, such as lyophilisation, counter-ion and monoisotopic mass, are defined in the glossary.