What Is Acetylation? How an N-Terminal Acetyl Cap Changes a Peptide

Published October 2026 · Patriot Labs Research Team · ~16 min read

The short answer

N-terminal acetylation is a chemical change at the start of a peptide in which an acetyl group (CH₃CO–) is attached to the free α-amino group of the first residue, turning a positively charged amine into a neutral amide. It removes the N-terminal charge, is among the most common protein modifications in the cell, and in laboratory synthesis is the last group added before the peptide is cleaved from the resin.

The modification is small — one acetyl group, about 42 daltons — but it changes the molecule in ways that matter. It neutralises a charge that the free amine would otherwise carry, makes the end of the chain slightly more hydrophobic, and blocks the enzymes that trim peptides from the N-terminal end. In the body a dedicated family of enzymes installs it as the protein is being made; in a synthesis laboratory it is added deliberately with acetic anhydride. One point causes endless confusion and is worth settling first: the word “acetylation” covers two different reactions at two different places on a molecule, and this guide is mostly about the one at the chain’s start.

What acetylation is, atom by atom

Acetylation transfers an acetyl group — a two-carbon unit, CH₃C(=O)– — onto an amino group. At a peptide’s N-terminus the target is the free α-amino group of the first residue, –NH₂. Adding the acetyl converts it to an acetamide, –NH–C(=O)–CH₃. In formula terms the peptide gains C₂H₂O, an increase of about 42 daltons, and nothing else about the chain changes.

The consequence that drives everything downstream is charge. A free α-amino group is basic: at the roughly neutral pH of body fluids it picks up a proton and sits as –NH₃⁺, carrying a full positive charge. The acetamide nitrogen is not basic in that range; it stays neutral. So the difference between an acetylated peptide and its free-amine counterpart is not only an atomically small addition, it is the loss of a fixed positive charge at the start of the molecule. That change in net charge affects the peptide’s electrostatics, its behaviour in ion-exchange and reversed-phase chromatography, and often its interactions with other molecules. It is the mirror image, at the opposite end of the chain, of what C-terminal amidation does by removing a negative charge.

N-terminal vs lysine acetylation: two reactions people confuse

Two distinct modifications both go by the name “acetylation,” and conflating them is the single most common error about this topic. They occur at different chemical sites and follow different rules.

N-terminal (Nα) acetylation is the subject of this guide: the acetyl group is placed on the α-amino group at the very start of the chain, there is at most one per protein, it is installed by the N-terminal acetyltransferases (NATs), and no enzyme is known to remove it — it is effectively permanent. Lysine (Nε) acetylation is a different reaction entirely: the acetyl group is placed on the side-chain ε-amino group of an internal lysine residue, a single protein can carry many such marks, they are installed by lysine acetyltransferases (KATs, historically called histone acetyltransferases or HATs), and they are readily removed by lysine deacetylases (KDACs, including the HDAC enzymes and the sirtuins). The acetylation of histone tails that features in epigenetics is the lysine kind; it is dynamic and regulatory. The N-terminal kind is a largely static feature a protein is born with. Both reactions use the same acetyl donor, acetyl-CoA, and both add the same 42 daltons, which is part of why they are so easily mixed up.

How cells acetylate the N-terminus: the NAT enzymes

In cells, N-terminal acetylation is carried out by a family of enzymes called the N-terminal acetyltransferases, or NATs, which transfer the acetyl group from acetyl-CoA onto the protein’s α-amino group. Most of them act co-translationally: they sit at the ribosome and modify the nascent chain as its first residues emerge from the exit tunnel, so the cap is often in place before the protein is even finished.

Which NAT acts, and whether a protein is acetylated at all, is decided by the first one or two residues. Eukaryotic proteins all begin life with an initiator methionine, and a separate set of enzymes, the methionine aminopeptidases, clips that methionine off when the second residue is small (such as alanine, serine, threonine, cysteine, glycine or valine). The identity of the residue then exposed — or the methionine itself, if it was kept — determines which NAT recognises the terminus. This sequence logic was mapped out in detail by Polevoda and Sherman and refined by large proteomic surveys from Arnesen, Van Damme, Gevaert and colleagues.

Seven to eight NATs are recognised, each a catalytic subunit often partnered with auxiliary subunits that anchor it to the ribosome or tune its specificity. The table summarises the main ones.

The chemical route is the same in every case: acetyl-CoA binds, its acetyl group is transferred to the α-amino nitrogen, and coenzyme A is released. What differs is recognition — which N-terminal sequence each enzyme will accept — and that is what makes N-terminal acetylation a programmed feature of the proteome rather than a random decoration.

How common N-terminal acetylation is

N-terminal acetylation is one of the most abundant protein modifications known. Across eukaryotes, estimates place the share of proteins carrying an acetylated N-terminus at roughly 50 to 80 percent, with figures around 80 percent frequently cited for human proteins and NatA alone estimated to account for something like 38 percent of the human proteome.

Those numbers are worth pausing on. A modification found on half to four-fifths of all proteins is not a special case; it is closer to a default setting for how a protein chain is finished. That scale is why N-terminal acetylation is often mentioned in the same breath as phosphorylation when researchers rank the modifications that shape the proteome, and it is why terminal modifications are a standard consideration when a peptide is being characterised. In the context of naturally occurring signalling peptides, a capped terminus is common enough that its presence or absence is treated as a defining part of a given peptide’s identity.

Why it matters: charge, folding, interactions and fate

For a modification so small, N-terminal acetylation touches a surprising range of protein behaviour, and research over the past two decades has moved it from a curiosity to a recognised regulator. Several effects recur.

The first is the charge change already described: removing the N-terminal positive charge and adding a small hydrophobic group alters the electrostatics and surface chemistry at the chain’s start. The second is folding and aggregation. Depleting NatA activity causes widespread protein misfolding in cells, and for specific proteins the cap matters a great deal — the acetylated N-terminus of α-synuclein promotes its folding into an α-helical form and influences how it binds membranes and resists aggregation, a point studied intensively because α-synuclein aggregation is central to Parkinson’s disease. The third is protein–protein interaction and complex assembly: an acetylated N-terminus can serve as a small hydrophobic recognition feature that docks into a partner protein, as in certain ubiquitin-pathway complexes. A fourth is subcellular targeting, where the modified terminus helps direct some proteins to membranes or organelles.

None of this means acetylation does the same thing everywhere. The direction and size of the effect depend on the sequence and the protein involved, and for some proteins the modification is close to neutral. What is reliable is the general lesson: the chemistry of a protein’s first residue is part of the information it carries, and acetylation changes that information.

Acetylation as a degradation signal: the Ac/N-degron

One of the most striking discoveries about N-terminal acetylation is that it can mark a protein for destruction. In 2010 Hwang, Shemorry and Varshavsky showed that an acetylated N-terminus can function as a specific degradation signal — an “Ac/N-degron” — recognised by a dedicated branch of the N-end rule (N-degron) pathway, with the E3 ubiquitin ligase Doa10 identified in yeast as a recogniser of acetylated N-termini.

The twist is that the signal is conditional. Later work showed that an Ac/N-degron is often shielded when the protein folds or joins its proper partner, so a correctly assembled protein hides the degron and survives, while an unpartnered or excess subunit leaves it exposed and is removed. In that way N-terminal acetylation can help a cell police the correct stoichiometry of protein complexes, degrading the leftovers. So acetylation does not simply destabilise or stabilise a protein; depending on context it can do either, which is why it is best described as a mark that the cell’s quality-control machinery can read rather than a fixed verdict. For short synthetic peptides this biology is mostly background; their practical stability story is the aminopeptidase resistance described next.

The two end-caps: acetylation and amidation

N-terminal acetylation is one of a pair of common “capping” modifications, and its partner sits at the opposite end of the chain: C-terminal amidation. Together they are sometimes called a double cap, because each closes one end of the peptide against the exopeptidases that chew inward from that end.

Acetylation blocks aminopeptidases, the enzymes that remove residues one at a time from the N-terminus, because it removes the free α-amino group those enzymes recognise. Amidation, covered in its own guide, blocks carboxypeptidases at the C-terminus in the same way, by replacing the free carboxylate they depend on. A peptide capped at both ends is therefore protected from exopeptidase trimming from either direction — though neither cap does anything to stop endopeptidases, which cut in the middle of the chain. The textbook natural example of a doubly capped peptide is α-melanocyte-stimulating hormone (α-MSH), which is N-acetylated at its serine start and C-amidated at its valine end. The two modifications are chemically independent — a peptide can carry either, both or neither — but they share the same underlying trick: replacing a charged, enzyme-readable terminus with a neutral amide. For the C-terminal half of this story, see What Is Amidation?

Acetylated peptides in nature and research

N-terminal acetylation appears across natural signalling peptides and is a frequently requested modification on synthetic research peptides, where it is signalled by the prefix “Ac–” (or “acetyl-”) in the name. The table gives representative examples.

α-MSH is a particularly instructive case because it is double-capped and because its acetylation is not simply an on-switch: in flounder pigment-cell studies, N-acetylation decreased pigment-dispersing activity in one cell type while increasing it in another, a reminder that a terminal modification changes a molecule’s behaviour in ways that depend on context rather than in a single fixed direction. Argireline illustrates the naming convention cleanly: its full written form, Ac-Glu-Glu-Met-Gln-Arg-Arg-NH₂, spells out an acetyl cap at one end and an amide at the other.

How an N-terminal acetyl is made in the laboratory

In chemical synthesis, acetylation is almost always the last step performed while the peptide is still attached to the resin. This follows from the direction of synthesis. Research peptides are built by solid-phase peptide synthesis (SPPS), the method R. B. Merrifield introduced in 1963, in which the chain is assembled one protected residue at a time on an insoluble bead — and the chain is built from the C-terminus toward the N-terminus, so the N-terminal residue is the very last one added.

Once that final residue is coupled, its temporary Nα-protecting group (the Fmoc group, in the common Fmoc approach) is removed with piperidine to expose the free α-amine. To acetylate it, the chemist treats the resin with acetic anhydride (Ac₂O), usually together with a base such as DIPEA or pyridine — sometimes with a catalytic amount of DMAP — in DMF. The anhydride transfers an acetyl group to the amine, and the peptide is then cleaved from the resin with trifluoroacetic acid, carrying its new acetyl cap with it. Notably, this is the same acetic-anhydride chemistry that peptide chemists use routinely as a “capping” step during chain assembly, where it blocks any chains that failed to couple so they cannot continue growing; deliberate N-terminal acetylation is simply that capping reaction applied on purpose to the finished chain’s last residue.

The contrast with biology is worth noting: a cell uses an enzyme and acetyl-CoA, while a chemist uses acetic anhydride, but both routes deliver the identical –NHCOCH₃ cap. The practical consequence is that “acetylated” is a design decision, and a peptide ordered with an N-terminal acetyl is a genuinely different molecule from the same sequence left with a free amine. For how the rest of the chain is assembled, see What Is Solid-Phase Peptide Synthesis?

How acetylation is detected: the 42-dalton signature

Acetylation is confirmed by mass. Each acetyl group adds 42.0106 daltons (monoisotopic) to the molecule, a shift read directly from the intact mass by mass spectrometry, so an acetylated peptide is about 42 daltons heavier than its free-amine counterpart.

There is one well-known pitfall. Trimethylation — adding three methyl groups — raises the mass by 42.0470 daltons, only about 0.0364 daltons more than an acetyl group. The two are therefore nearly isobaric, and distinguishing them from mass alone requires high resolving power, on the order of 28,000 for a 1,000-dalton peptide. Where that resolution is not available, researchers lean on orthogonal evidence: acetylated peptides produce a diagnostic immonium-type ion at m/z 126 in tandem mass spectrometry, and because the acetyl cap is more hydrophobic than a free amine, an acetylated peptide shifts to a later retention time in reversed-phase HPLC, whereas a methylated peptide does not shift the same way. A certificate of analysis for an acetylated peptide should report a measured mass matching the acetylated calculation, and the expected HPLC behaviour follows from the added hydrophobicity. Patriot Labs reports each lot’s identity and purity from an independent US laboratory by HPLC and mass spectrometry, with the certificate of analysis on the product page; for a capped peptide, that reported mass is what shows the cap is present. For more on reading those documents, see Understanding Peptide COA Testing.

What this guide does not establish

This guide is an explanation of a chemical and biological modification. It is not a claim about any product’s effects, and several boundaries are worth stating plainly.

• Nothing here describes any use in a person or animal. Biology is described to explain how peptides are acetylated and why the modification changes a molecule’s properties, not to suggest that any compound should be administered or that it produces any effect in a living subject.
• Acetylation is a structural fact, not a benefit. That an acetyl cap can influence folding, interactions or one route of degradation in the published research describes the molecule’s chemistry; it is not a statement that any research peptide is safe, effective, or suitable for any purpose.
• Whether a given peptide is acetylated depends on the specific peptide. The examples here are drawn from the scientific literature; they do not indicate the modification state of any particular product, which is defined by that product’s own specification and certificate of analysis.
• Nothing here describes where Patriot Labs’ peptides are synthesized. Patriot Labs is a first-responder-owned, US-based company, and the only quality facts used here are that every lot is tested by an independent US laboratory by HPLC and mass spectrometry, with the COA available on the product page.

Frequently asked questions

What is acetylation in peptides?

In peptides, acetylation most often means N-terminal acetylation: attaching an acetyl group (CH₃CO–) to the free α-amino group at the start of the chain, converting the terminal –NH₂ into an acetamide (–NHCOCH₃). It removes the positive charge the free amine carries at neutral pH, adds about 42 daltons to the mass, and is one of the most common modifications in the cell. A second, separate reaction — acetylation of the side-chain amino group of internal lysine residues — is also called acetylation, which is why the two are often confused.

What is the difference between N-terminal acetylation and lysine acetylation?

They happen at different places on the molecule and behave differently. N-terminal (Nα) acetylation modifies the α-amino group of the very first residue, occurs once per chain, is carried out by N-terminal acetyltransferases (NATs), and has no known enzyme that reverses it. Lysine (Nε) acetylation modifies the side-chain amino group of internal lysines, can occur at many sites, is carried out by lysine acetyltransferases (KATs, also called HATs), and is readily reversed by lysine deacetylases such as HDACs and sirtuins. Histone acetylation in epigenetics is the lysine kind. Both use acetyl-CoA as the acetyl donor and both add about 42 daltons.

What enzymes carry out N-terminal acetylation?

A family of N-terminal acetyltransferases (NATs), named NatA through NatH, each with its own substrate preference set by the first one or two residues of the protein. The best-studied, NatA, pairs a catalytic subunit (Naa10) with a ribosome-anchoring subunit (Naa15) and acts on small residues such as serine, alanine, threonine, valine, cysteine and glycine once the initiator methionine has been removed. NatB and NatC act on methionine-starting N-termini, NatD is specific for certain histones, NatF (Naa60) works on membrane proteins at the Golgi, and NatH (Naa80) acetylates actin. Most act co-translationally, on the nascent chain as it emerges from the ribosome.

Is N-terminal acetylation reversible?

No enzyme is known to remove an N-terminal acetyl group, so N-terminal acetylation is generally treated as irreversible and effectively permanent for the life of the protein. This is a key difference from lysine acetylation, which is dynamically added and removed by opposing enzymes. Because it is laid down co-translationally and not taken off, N-terminal acetylation is better thought of as a fixed property a protein is born with than as a switch that toggles.

How common is N-terminal acetylation?

It is one of the most abundant protein modifications known. Studies estimate that roughly 50 to 80 percent of eukaryotic proteins carry an acetylated N-terminus, with figures around 80 percent commonly cited for human proteins; NatA alone is estimated to handle something like 38 percent of the human proteome. That scale puts N-terminal acetylation in the same league as phosphorylation as a modification that shapes a large fraction of all proteins.

Why does N-terminal acetylation matter for a peptide?

Because the small cap changes the chemistry at the chain's start. It removes the N-terminal positive charge and makes the end slightly more hydrophobic, which can influence folding and resistance to aggregation, help or hinder the assembly of protein complexes, direct some proteins to membranes, and act as a recognition signal that affects how long a protein lasts (the Ac/N-degron pathway). For short synthetic peptides, the most practical consequence is that the acetyl cap blocks aminopeptidases, the enzymes that trim residues from the N-terminal end. The size and direction of these effects depend on the specific sequence.

How is an N-terminal acetyl group added during peptide synthesis?

It is added as the final step while the peptide is still on the resin. In Fmoc solid-phase peptide synthesis the chain is built from the C-terminus toward the N-terminus, so the N-terminal residue is the last one coupled. After its temporary Fmoc protecting group is removed to expose the free α-amine, the resin is treated with acetic anhydride (Ac₂O), usually with a base such as DIPEA or pyridine in DMF, which acetylates that amine. The peptide is then cleaved from the resin, and the acetyl cap stays in place. It is the same acetic-anhydride chemistry used routinely to cap unreacted chains between couplings, applied here on purpose to the final residue.

How much mass does an acetyl group add, and how is it distinguished from trimethylation?

An acetyl group adds 42.0106 daltons (monoisotopic), read directly from the intact mass. The classic trap is trimethylation, which adds 42.0470 daltons — only about 0.0364 daltons more, so the two are nearly isobaric and telling them apart needs high mass-resolving power (on the order of 28,000 for a 1,000-dalton peptide) or an orthogonal clue. Acetylated peptides give a diagnostic immonium-type ion at m/z 126 in tandem mass spectrometry and shift to a later retention time in reversed-phase HPLC because the cap is more hydrophobic, whereas methylation does not shift retention the same way.

Is acetyl hexapeptide-8 (argireline) N-terminally acetylated?

Yes. Acetyl hexapeptide-8, also known by the trade name Argireline, has the sequence Ac-Glu-Glu-Met-Gln-Arg-Arg-NH₂ — the prefix “Ac” marks an N-terminal acetyl cap, and the “NH₂” marks a C-terminal amide, so it is capped at both ends. It is a synthetic hexapeptide modelled on the N-terminal sequence of the synaptic protein SNAP-25 and is studied for its interaction with the SNARE complex. The “acetyl” in its name, like the “acetyl-” prefix on many ingredient peptides, denotes exactly this N-terminal modification.

References

1. McTiernan, N., Kjøsås, I., & Arnesen, T. (2025). Illuminating the impact of N-terminal acetylation: from protein to physiology. Nature Communications, 16. doi:10.1038/s41467-025-55960-5
2. Aksnes, H., Ree, R., & Arnesen, T. (2019). Co-translational, Post-translational, and Non-catalytic Roles of N-Terminal Acetyltransferases. Molecular Cell, 73(6), 1097–1114. doi:10.1016/j.molcel.2019.02.007
3. Ree, R., Varland, S., & Arnesen, T. (2018). Spotlight on protein N-terminal acetylation. Experimental & Molecular Medicine, 50, 1–13. doi:10.1038/s12276-018-0116-z
4. Aksnes, H., Drazic, A., Marie, M., & Arnesen, T. (2016). First Things First: Vital Protein Marks by N-Terminal Acetyltransferases. Trends in Biochemical Sciences, 41(9), 746–760. doi:10.1016/j.tibs.2016.07.005
5. Drazic, A., Myklebust, L. M., Ree, R., & Arnesen, T. (2016). The world of protein acetylation. Biochimica et Biophysica Acta (BBA) – Proteins and Proteomics, 1864(10), 1372–1401. doi:10.1016/j.bbapap.2016.06.007
6. Starheim, K. K., Gevaert, K., & Arnesen, T. (2012). Protein N-terminal acetyltransferases: when the start matters. Trends in Biochemical Sciences, 37(4), 152–161. doi:10.1016/j.tibs.2012.02.003
7. Hwang, C.-S., Shemorry, A., & Varshavsky, A. (2010). N-Terminal Acetylation of Cellular Proteins Creates Specific Degradation Signals. Science, 327(5968), 973–977. doi:10.1126/science.1183147
8. Arnesen, T., Van Damme, P., Polevoda, B., Helsens, K., Evjenth, R., Colaert, N., ... Gevaert, K. (2009). Proteomics analyses reveal the evolutionary conservation and divergence of N-terminal acetyltransferases from yeast and humans. Proceedings of the National Academy of Sciences, 106(20), 8157–8162. doi:10.1073/pnas.0901931106
9. Van Damme, P. (2021). Charting the N-Terminal Acetylome: A Comprehensive Map of Human NatA Substrates. International Journal of Molecular Sciences, 22(19), 10692. doi:10.3390/ijms221910692
10. Polevoda, B., & Sherman, F. (2003). N-terminal Acetyltransferases and Sequence Requirements for N-terminal Acetylation of Eukaryotic Proteins. Journal of Molecular Biology, 325(4), 595–622. doi:10.1016/S0022-2836(02)01269-X
11. Mizusawa, K., & Takahashi, A. (2013). N-acetylation of α-melanocyte-stimulating hormone and its effect on pigment-cell activity. Frontiers in Endocrinology, 4, 143. doi:10.3389/fendo.2013.00143
12. Yang, L., et al. (2009). Differentiation of isobaric acetylation and trimethylation by reversed-phase retention time and high mass accuracy. Analytical Biochemistry. PMCID: PMC2787863
13. Merrifield, R. B. (1963). Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide. Journal of the American Chemical Society, 85(14), 2149–2154. doi:10.1021/ja00897a025
14. N-terminal acetylation. Wikipedia. https://en.wikipedia.org/wiki/N-terminal_acetylation
15. Acetyl hexapeptide-8. Wikipedia. https://en.wikipedia.org/wiki/Acetyl_hexapeptide-8
16. Merck / Sigma-Aldrich. N-Terminal Modification of Peptides: Acetylation (technical article on acetic anhydride capping in Fmoc SPPS). https://www.sigmaaldrich.com/

For related chemistry, see What Is Amidation? for the matching C-terminal cap, What Is a Peptide Bond? for the backbone amide that links residues, What Is Lipidation? and Peptide Half-Life Extension for other ways peptides are modified, and Understanding Peptide COA Testing for how a modification is confirmed on a certificate of analysis.

All Patriot Labs products are sold strictly for in-vitro research and laboratory use only. Not for human or veterinary consumption. This guide is educational and describes acetylation as a chemical and biological modification in general terms; it is not medical advice, does not describe how to use any product, and makes no claim about the composition or modification state of any specific product.