Amino Acids Chart — The 20 Standard Amino Acids
There are 20 standard amino acids encoded by the genetic code, each with a one-letter and three-letter abbreviation. This chart lists every amino acid with its side-chain class, whether it is essential in the human diet, its average residue mass, Kyte–Doolittle hydropathy and the codons that encode it.
| Amino acid | 3-letter | 1-letter | Class | Essential | Mass (Da) | Hydropathy | Codons |
|---|---|---|---|---|---|---|---|
| Alanine | Ala | A | Nonpolar | No | 71.1 | +1.8 | GCT, GCC, GCA, GCG |
| Arginine | Arg | R | Basic | No | 156.2 | -4.5 | CGT, CGC, CGA, CGG, AGA, AGG |
| Asparagine | Asn | N | Polar | No | 114.1 | -3.5 | AAT, AAC |
| Aspartic acid | Asp | D | Acidic | No | 115.1 | -3.5 | GAT, GAC |
| Cysteine | Cys | C | Polar | No | 103.1 | +2.5 | TGT, TGC |
| Glutamic acid | Glu | E | Acidic | No | 129.1 | -3.5 | GAA, GAG |
| Glutamine | Gln | Q | Polar | No | 128.1 | -3.5 | CAA, CAG |
| Glycine | Gly | G | Nonpolar | No | 57.1 | -0.4 | GGT, GGC, GGA, GGG |
| Histidine | His | H | Basic | Yes | 137.1 | -3.2 | CAT, CAC |
| Isoleucine | Ile | I | Nonpolar | Yes | 113.2 | +4.5 | ATT, ATC, ATA |
| Leucine | Leu | L | Nonpolar | Yes | 113.2 | +3.8 | TTA, TTG, CTT, CTC, CTA, CTG |
| Lysine | Lys | K | Basic | Yes | 128.2 | -3.9 | AAA, AAG |
| Methionine | Met | M | Nonpolar | Yes | 131.2 | +1.9 | ATG |
| Phenylalanine | Phe | F | Nonpolar | Yes | 147.2 | +2.8 | TTT, TTC |
| Proline | Pro | P | Nonpolar | No | 97.1 | -1.6 | CCT, CCC, CCA, CCG |
| Serine | Ser | S | Polar | No | 87.1 | -0.8 | TCT, TCC, TCA, TCG, AGT, AGC |
| Threonine | Thr | T | Polar | Yes | 101.1 | -0.7 | ACT, ACC, ACA, ACG |
| Tryptophan | Trp | W | Nonpolar | Yes | 186.2 | -0.9 | TGG |
| Tyrosine | Tyr | Y | Polar | No | 163.2 | -1.3 | TAT, TAC |
| Valine | Val | V | Nonpolar | Yes | 99.1 | +4.2 | GTT, GTC, GTA, GTG |
NonpolarPolarAcidicBasic· Mass = average residue mass · Hydropathy = Kyte–Doolittle
Essential vs non-essential amino acids
Nine of the twenty amino acids are essential: the human body cannot synthesise them, so they must come from the diet — histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. The remaining eleven are non-essential (or conditionally essential) and can be made from other metabolites.
Side-chain classes
Amino acids are grouped by the chemistry of their side chain. Nonpolarside chains are hydrophobic and tend to pack into a protein's core. Polar side chains are uncharged but hydrophilic. Acidic (Asp, Glu) carry a negative charge at neutral pH and basic(Lys, Arg, His) carry a positive charge — together they determine a protein's isoelectric point and net charge.
Frequently asked questions
- How many amino acids are there?
- There are 20 standard (proteinogenic) amino acids encoded directly by the genetic code. Two more — selenocysteine and pyrrolysine — are incorporated by special mechanisms in some organisms.
- What are the essential amino acids?
- The nine amino acids the human body cannot make and must get from food are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.
- What is the difference between the one-letter and three-letter codes?
- Both name the same amino acid. Three-letter codes (e.g. Ala, Gly) are easier to read; one-letter codes (e.g. A, G) are compact and used in sequence files and most software, including SeqBench.
- What do nonpolar, polar, acidic and basic mean?
- They classify the amino acid's side chain. Nonpolar side chains are hydrophobic; polar ones are uncharged but hydrophilic; acidic side chains carry a negative charge at neutral pH; basic ones carry a positive charge.
- What is hydropathy?
- Hydropathy (here the Kyte–Doolittle scale) measures how hydrophobic an amino acid is. Positive values are hydrophobic (water-avoiding), negative values hydrophilic. Averaged over a protein it gives the GRAVY score.