Non-proteinogenic amino acids

All 22 proteinogenic amino acids are biosynthesised by organisms and some, but not all, of them also are abiotic (found in prebiotic experiments and meteorites).

Some natural amino acids, such as norleucine, are misincorporated translationally into proteins due to infidelity of the protein-synthesis process.

Many amino acids, such as ornithine, are metabolic intermediates produced biosynthetically, but not incorporated translationally into proteins.

[10] An initial speculation on the deleterious properties of β-amino acids in terms of secondary structure[10] turned out to be incorrect.

[11] Some amino acids contain the opposite absolute chirality, chemicals that are not available from normal ribosomal translation and transcription machinery.

Most bacterial cells walls are formed by peptidoglycan, a polymer composed of amino sugars crosslinked with short oligopeptides bridged between each other.

The oligopeptide is non-ribosomally synthesised and contains several peculiarities including D-amino acids, generally D-alanine and D-glutamate.

Replacement of the remaining hydrogen with a larger substituent, such as a methyl group, distorts the protein backbone.

A similar compound is lanthionine, which can be seen as two alanine molecules joined via a thioether bond and is found in various organisms.

Similarly, djenkolic acid, a plant toxin from jengkol beans, is composed of two cysteines connected by a methylene group.

The genetic code has been described as a frozen accident and the reasons why there is only one standard amino acid with a straight chain, alanine, could simply be redundancy with valine, leucine and isoleucine.

These are formed by post-translational modification of the side chains of standard amino acids present in the target protein.

These modifications are often essential for the function or regulation of a protein; for example, in γ-carboxyglutamate the carboxylation of glutamate allows for better binding of calcium cations,[18] and in hydroxyproline the hydroxylation of proline is critical for maintaining connective tissues.

The osmolytes, sarcosine and glycine betaine are derived from amino acids, but have a secondary and quaternary amine respectively.