Every protein you will meet is built from these twenty. Learn the one-letter codes early, because sequence questions assume you know them.
| Amino acid | 3-letter | 1-letter | Side-chain class | Side-chain pKa | Polarity at pH 7 | Essential |
|---|---|---|---|---|---|---|
| Alanine | Ala | A | Aliphatic | — | Nonpolar | No |
| Arginine | Arg | R | Basic | 12.48 | Positive | Conditional |
| Asparagine | Asn | N | Amide | — | Polar | No |
| Aspartic acid | Asp | D | Acidic | 3.65 | Negative | No |
| Cysteine | Cys | C | Thiol | 8.18 | Polar | Conditional |
| Glutamic acid | Glu | E | Acidic | 4.25 | Negative | No |
| Glutamine | Gln | Q | Amide | — | Polar | Conditional |
| Glycine | Gly | G | Aliphatic | — | Nonpolar | Conditional |
| Histidine | His | H | Basic | 6.00 | Positive | Yes |
| Isoleucine | Ile | I | Aliphatic | — | Nonpolar | Yes |
| Leucine | Leu | L | Aliphatic | — | Nonpolar | Yes |
| Lysine | Lys | K | Basic | 10.53 | Positive | Yes |
| Methionine | Met | M | Thioether | — | Nonpolar | Yes |
| Phenylalanine | Phe | F | Aromatic | — | Nonpolar | Yes |
| Proline | Pro | P | Cyclic imino | — | Nonpolar | Conditional |
| Serine | Ser | S | Hydroxyl | — | Polar | No |
| Threonine | Thr | T | Hydroxyl | — | Polar | Yes |
| Tryptophan | Trp | W | Aromatic | — | Nonpolar | Yes |
| Tyrosine | Tyr | Y | Aromatic | 10.07 | Polar | Conditional |
| Valine | Val | V | Aliphatic | — | Nonpolar | Yes |
Side-chain pKa values are the standard free amino acid values; an em dash means the side chain does not ionise near physiological pH. “Conditional” means the amino acid becomes essential during illness, stress or rapid growth.