At least six amino acids can be acetylated in biological systems: lysine, serine, threonine, tyrosine, cysteine, and whichever residue sits at a protein’s N-terminus. Lysine is by far the most studied and most common target, but acetylation on other residues plays important and increasingly recognized roles in cell signaling, metabolism, and gene expression.
Three Types of Protein Acetylation
Acetylation falls into three broad categories based on where the acetyl group attaches. Each involves different chemistry, different enzymes, and different biological consequences.
N-terminal (Nα) acetylation adds an acetyl group to the very first amino acid of a protein chain. This is irreversible and extremely common, affecting roughly 50 to 80 percent of all proteins in human cells. The first two to four amino acids of a protein determine whether it gets this modification. A family of enzymes called N-terminal acetyltransferases (NATs) handles the job, with the largest one, NatA, responsible for modifying about 40 percent of the human proteome on its own. NatB covers another 21 percent, and a group of three others (NatC, NatE, and NatF) together handle the remaining 21 percent. Some of these enzymes work while the protein is still being built on the ribosome, while others modify proteins after synthesis is complete. N-terminal acetylation influences protein folding, where a protein ends up inside the cell, how it interacts with other proteins, and how quickly it gets broken down.
Lysine (Nε) acetylation targets the side chain of lysine residues anywhere along a protein. Unlike N-terminal acetylation, this is reversible. Enzymes called acetyltransferases (HATs) add the acetyl group, and deacetylases (HDACs) remove it. Large-scale studies in mammals have identified thousands of lysine acetylation sites across thousands of proteins, putting the scale of lysine acetylation in the same range as phosphorylation, the most widely studied protein modification.
O-acetylation attaches an acetyl group to the oxygen-containing side chains of serine, threonine, or tyrosine. This category is less well characterized but is gaining attention, particularly because serine and threonine acetylation can directly compete with phosphorylation at the same sites, blocking key signaling pathways.
Lysine: The Primary Target
Lysine acetylation was first discovered on histone proteins in 1968, though the enzymes responsible weren’t identified until 1995. The process uses a molecule called acetyl-CoA as the donor, transferring an acetyl group onto the nitrogen in lysine’s long, flexible side chain. This neutralizes the positive charge that lysine normally carries, which can dramatically change how a protein behaves.
On histones (the spool-like proteins that DNA wraps around), acetylation loosens the grip between DNA and the histone, making genes more accessible for reading. Histone H4 is commonly acetylated at lysines 5 and 12 shortly after it’s made, which is critical for assembling new stretches of packaged DNA. Histone H3 has five key acetylation sites on its tail (at positions 9, 14, 18, 23, and 27) plus an important site deeper in its structure at lysine 56. These modifications aren’t just about gene activation. They also play roles in DNA repair signaling, with some sites required for the cell’s response to DNA breaks.
But lysine acetylation extends far beyond histones. Close to 400 non-histone proteins have been functionally characterized as acetylation targets in mammals. One of the best-studied is p53, a tumor suppressor protein that acts as a cellular emergency brake. Acetylation increases p53’s stability, enhances its ability to bind DNA, and even determines whether a damaged cell pauses to repair itself or self-destructs. For example, acetylation at one specific lysine (position 120) steers p53 toward activating cell-death genes rather than cell-cycle-arrest genes, effectively deciding the cell’s fate.
Serine and Threonine: Competing With Phosphorylation
Serine and threonine acetylation was first identified in the context of bacterial infection. A virulence factor produced by the plague bacterium Yersinia specifically acetylates a serine residue in human cells that would normally be phosphorylated by a signaling enzyme. By occupying that site, the bacterial protein blocks the host’s immune signaling cascade. This makes O-acetylation a chemical antagonist to phosphorylation, one of the cell’s most important communication tools.
Beyond bacterial hijacking, proteomics studies have detected serine and threonine O-acetylation on non-histone proteins in higher organisms, including a handful of yeast enzymes involved in controlling cell shape and division. Histone H3 serine 10, a well-known phosphorylation site linked to gene activation and chromosome condensation, has also been found to carry O-acetylation. The full scope of this modification in healthy human cells is still being mapped.
Cysteine: A Metabolic Regulator
Cysteine residues can undergo S-acetylation, where the acetyl group attaches to the sulfur atom in cysteine’s side chain. Non-targeted mass spectrometry of mouse liver tissue revealed hundreds of cysteine acetylation sites. Researchers confirmed the modification was genuinely sulfur-linked by showing it could be removed with one type of chemical reducing agent (DTT) but not another (TCEP), a signature of sulfur-based bonds.
Pathway analysis of the proteins carrying this modification showed a strong metabolic signature. The most enriched categories were amino acid breakdown and metabolism, suggesting cysteine S-acetylation plays a regulatory role in how cells process nutrients.
How Acetylation Gets Removed
For lysine acetylation, removal is handled by two mechanistically distinct families of deacetylases. The classical HDACs (classes I, II, and IV) rely on a zinc ion to strip the acetyl group. Sirtuins (class III) use a completely different mechanism that depends on NAD+, a molecule tied to the cell’s energy state. This NAD+ dependence makes sirtuins uniquely sensitive to metabolic conditions.
The seven mammalian sirtuins regulate metabolism at multiple levels. SIRT1, the most studied, deacetylates transcription regulators that control fat metabolism, glucose production, and bile acid signaling. During fasting, SIRT1 first promotes glucose production in the liver by activating certain gene regulators, then later suppresses it by triggering the breakdown of others. SIRT3 operates inside mitochondria, activating enzymes involved in fatty acid burning, the citric acid cycle, and amino acid metabolism. SIRT5 regulates an enzyme in the urea cycle that handles ammonia disposal.
This metabolic wiring means acetylation isn’t just a static mark on proteins. It’s a dynamic system that responds to what you eat, how much energy your cells have, and whether you’re in a fed or fasted state.
The Full List at a Glance
- Lysine: Reversible Nε-acetylation on the side chain. The most abundant and best-characterized form. Thousands of sites identified across the proteome.
- N-terminal residue (any): Irreversible Nα-acetylation on the first amino acid of a protein. Affects 50 to 80 percent of eukaryotic proteins.
- Serine: O-acetylation on the hydroxyl group. Can block phosphorylation at the same site.
- Threonine: O-acetylation on the hydroxyl group. Same chemistry as serine acetylation.
- Tyrosine: O-acetylation on the hydroxyl group. Less characterized than serine or threonine.
- Cysteine: S-acetylation on the sulfur atom. Linked to metabolic regulation, with hundreds of sites identified in liver tissue.