Non-proteinogenic amino acids

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) 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:

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, and chloroplasts, but is often removed posttranslationally.

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