Amino Acid Codes: One-Letter and Three-Letter Chart

Amino acid codes are short abbreviations for the 20 standard amino acids. Each one has a three-letter code, such as Gly for glycine, and a one-letter code, such as G. Scientists use them to write peptide and protein sequences quickly, so a 15-amino-acid peptide like BPC-157 can be written as the 15-letter string GEPPPGKPADDAGLV. Below you will find a full amino acid codes chart, why some letters look odd, and how to read a peptide sequence.

Amino acid codes chart: all 20 amino acids

Here are the 20 standard amino acids with their three-letter and one-letter codes, plus the side-chain group, which tells you a lot about how each one behaves.

Both systems are set by an international committee, the IUPAC-IUB Joint Commission on Biochemical Nomenclature, so they are the same in every textbook, database and lab report. The full rules are set out in its 1983 recommendations on nomenclature and symbolism for amino acids and peptides (European Journal of Biochemistry, 1984).

Extra codes you might see

A few extra letters turn up in sequence databases and older papers:

• U (Sec) is selenocysteine, sometimes called the 21st amino acid.
• O (Pyl) is pyrrolysine, the 22nd genetically encoded amino acid.
• B (Asx) means aspartic acid or asparagine, when a test cannot tell them apart.
• Z (Glx) means glutamic acid or glutamine.
• X (Xaa) means any or unknown amino acid.

Three-letter vs one-letter amino acid codes

The three-letter codes came first. They are easy to remember because most are simply the first three letters of the name: Ala, Gly, Val, Leu and so on. A few break the pattern, such as Asn for asparagine, Gln for glutamine, Ile for isoleucine and Trp for tryptophan.

The one-letter code was introduced in the 1960s for long protein sequences, where three letters per amino acid quickly became unwieldy. It was popularised by the protein scientist Margaret Dayhoff and adopted by the IUPAC-IUB commission in its 1968 rules for a one-letter notation for amino acid sequences.

As a rule of thumb, three-letter codes are clearer for short peptides in papers and certificates, while one-letter codes are standard in databases and for anything long.

Why some one-letter codes look strange

There are 20 amino acids but several share a first letter, so not all of them could have their initial. The committee followed a simple order:

1. Unique first letters: cysteine (C), histidine (H), isoleucine (I), methionine (M), serine (S) and valine (V) had their first letter to themselves.
2. Most common wins: where names clashed, the amino acid found most often in proteins got the first letter. So alanine is A, glycine is G, leucine is L, proline is P and threonine is T.
3. Sound-alikes: others got a letter that sounds like their name: R for “aRginine”, F for “Fenylalanine”, N for “asparagiNe”, Q for “Q-tamine” (glutamine) and Y for “tYrosine”. W was given to tryptophan, often remembered as “tWyptophan” or by its double-ringed shape.
4. Leftovers: D and E went to the two acidic amino acids, aspartic acid and glutamic acid, and lysine got K, the letter just before L.

How to read a peptide sequence using amino acid codes

A peptide is a chain of amino acids joined by peptide bonds. A sequence simply lists those amino acids in order. Here is how to read one.

1. Read left to right, from N-terminus to C-terminus. The first letter is the amino acid with a free amine group at the start of the chain. The last letter is the one with the free acid group at the end.
2. Count the letters to get the length. One letter is one amino acid. A chain of n amino acids has n minus 1 peptide bonds.
3. Look for hyphens in three-letter code. Gly-His-Lys means glycine joined to histidine joined to lysine. In one-letter code the same peptide is just GHK.
4. Check the ends for modifications. “Ac-” at the start means the N-terminus is acetylated. “-NH2” at the end means the C-terminus is amidated.

Our guide to the different types of peptides explains how peptides are then grouped by length, from dipeptides to polypeptides.

Worked examples from research peptides

Short research peptides are a good way to practise reading amino acid codes:

• GHK (Gly-His-Lys): a tripeptide of glycine, histidine and lysine. Bound to copper it forms GHK-Cu, which Peptides UK supplies as research-grade GHK-Cu.
• KPV (Lys-Pro-Val): three amino acids, lysine, proline and valine. Note that lysine is K, not L.
• Oxytocin (CYIQNCPLG-NH2): nine amino acids with an amidated C-terminus. The two cysteines (C) link together through a disulfide bridge to form a small ring.
• BPC-157 (GEPPPGKPADDAGLV): 15 amino acids, including four prolines. Our breakdown of the BPC-157 amino acid sequence counts every position and explains what the mix of residues tells you.

How many amino acids are coded for?

DNA is read in three-letter “words” called codons. There are 64 possible codons. Of these, 61 code for the 20 standard amino acids and three are stop signals. Most amino acids have more than one codon. For example, GTG, GTT, GTC and GTA all code for valine.

Nature has two exceptions. In some organisms, a stop codon is recoded to insert selenocysteine. A review from the University of Munich describes how this led researchers to call it the 21st amino acid (Böck and colleagues, Molecular Microbiology, 1991). In 2002 a team at Ohio State University showed that certain archaea use the UAG codon for pyrrolysine, which they described as the 22nd genetically encoded amino acid (Srinivasan, James and Krzycki, Science, 2002).

Synthetic peptides are not limited to the genetic code. Chemists can add D-amino acids, modified residues and unusual end groups. Our guide to how research peptides are made explains how a sequence is built one amino acid at a time.

Why amino acid codes matter when checking a peptide

For a research lab, the sequence is the identity of a peptide. A single swapped or missing letter gives a different molecule, even if the vial label looks the same. A deleted amino acid, a common synthesis error, shows up as a shorter sequence and a lower mass.

That is why it helps to be able to read amino acid codes on a certificate of analysis (COA). Check that the sequence matches the published one, that the length is right and that the mass spectrometry result agrees with the expected molecular weight. Our guide on how to read a peptide certificate of analysis walks through each section of a COA.

At Peptides UK, our research peptides are HPLC & COA Verified. All of our products are for laboratory research use only and are not intended for human or veterinary use.

Amino acid codes: frequently asked questions