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Proteinogenic amino acids, also known as standard, normal, or primary 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.
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