Proteinogenic amino acids are those 20 amino acids that are found in proteins and that are coded for in the standard genetic code. Proteinogenic literally means protein building.

Non-proteinogenic amino acids are either not found in proteins (like carnitine, GABA, or L-DOPA), or not coded for in the standard genetic code (like hydroxyproline and selenomethionine). The latter often result from posttranslational modification of proteins.

Some non-proteinogenic amino acids, such as ornithine and homoserine have clear etiological reasons for biology's failure to incorporate them into proteins; both of these amino acids will cyclize against the peptide backbone and fragment the protein with relatively short half-lives.

Proteinogenic amino acids