In biochemistry, non-coded or non-proteinogenic amino acids are distinct from the 22 proteinogenic amino acids (21 in eukaryotes), which are naturally encoded in the genome of organisms for the assembly of proteins. However, over 140 non-proteinogenic amino acids occur naturally in proteins (but not included in the genetic code) and thousands more may occur in nature or be synthesized in the laboratory. Chemically synthesized amino acids are often referred to as unnatural or non-canonical amino acids. Unnatural amino acids can be synthetically prepared from their native analogs via modifications such as amine alkylation, side chain substitution, structural bond extension cyclization, and isosteric replacements within the amino acid backbone. Many non-proteinogenic amino acids are important:
- intermediates in biosynthesis,
- in post-translational formation of proteins,
- in a physiological role (e.g. components of bacterial cell walls, neurotransmitters and toxins),
- natural or man-made pharmacological compounds,
- present in meteorites or used in prebiotic experiments (such as the Miller–Urey experiment),
- might be important neurotransmitters, such as γ-aminobutyric acid, and
- can play a crucial role in cellular bioenergetics, such as creatine.