Protein
https://en.wikipedia.org/wiki/Amino_acid
Over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 appear in the genetic code of life.
Essential amino acid
https://en.wikipedia.org/wiki/Essential_amino_acid
| Essential | Conditionally essential | Non-essential |
|---|---|---|
| Histidine (H) | Arginine (R) | Alanine (A) |
| Isoleucine (I) | Cysteine (C) | Aspartic acid (D) |
| Leucine (L) | Glutamine (Q) | Asparagine (N) |
| Lysine (K) | Glycine (G) | Glutamic acid (E) |
| Methionine (M) | Proline (P) | Serine (S) |
| Phenylalanine (F) | Tyrosine (Y) | |
| Threonine (T) | Selenocysteine (U) | |
| Tryptophan (W) | ||
| Valine (V) | Pyrrolysine* (O) |
*: Pyrrolysine, sometimes considered the 22nd amino acid, is not used by the human body.