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In biochemistry, non-coded, non-proteinogenic, or "unnatural" amino acids are those not naturally encoded or found in the genetic code of any organisms. Despite the use of only 23 amino acids (21 in eukaryotes〔plus formylmethionine in eukaryotes with prokaryote organelles like mitochondria〕) by the translational machinery to assemble proteins (the proteinogenic amino acids), over 140 natural amino acids are known and thousands of more combinations are possible. Several non-proteinogenic amino acids are noteworthy because they are: * intermediates in biosynthesis * post-translationally incorporated into protein * possess a physiological role (e.g. components of bacterial cell walls, neurotransmitters and toxins) * natural and man-made pharmacological compounds * present in meteorites and in prebiotic experiments (e.g. Miller–Urey experiment) ==Definition by negation== Technically, any organic compound with an amine (-NH2) and a carboxylic acid (-COOH) functional group is an amino acid. The proteinogenic amino acids are small subset of this group that possess central carbon atom (α- or 2-) bearing an amino group, a carboxyl group, a side chain and an α-hydrogen levo conformation, with the exception of glycine, which is achiral, and proline, whose amine group is a secondary amine and is consequently frequently referred to as an imino acid for traditional reasons, albeit not an imino. The genetic code encodes 20 standard amino acids. However, there are three extra proteinogenic amino acids: selenocysteine, pyrrolysine and ''N''-formylmethionine. The former two do not have a dedicated codon, but are added in place of a stop codon when a specific sequence is present, UGA codon and SECIS element for selenocysteine, UAG PYLIS downstream sequence for pyrrolysine. Formylmethionine is an amino acid encoded by the start codon AUG in bacteria, mitochondria and chloroplasts, but is often removed posttranslationally. (S)-N-Formylmethionine V.1.svg|Formylmethionine. This amino acid is a methionine whose amino group has been protected by a formyl group L-selenocysteine-2D-skeletal.png|Selenocysteine. This amino acid contains a selenol group on its β-carbon Pyrrolysine.svg|Pyrrolysine. This amino acid is formed by joining to the ε-amino group of lysine a carboxylated pyrroline ring There are various groups of amino acids: * 20 standard amino acids * 23 proteinogenic amino acids * over 80 amino acids created abiotically in high concentrations * about 900 are produced by natural pathways * over 118 engineered amino acids have been placed into protein These groups overlap, but are not identical. All 23 proteinogenic amino acids are biosynthesised by organisms, but not all of them are abiotic (found in prebiotic experiments and meteorites), such as histidine. Many amino acids, such as ornithine, are metabolic intermediates produced biotically, but not coded. Others are only metabolic intermediates, such as citrulline. Others are solely found in abiotic mixes, such as α-methylnorvaline. Over 30 unnatural amino acids have been translationally inserted into protein in engineered systems, yet are not biosynthetic.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Non-proteinogenic amino acids」の詳細全文を読む スポンサード リンク
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