Pyrrolysine

Pyrrolysine (Pyl or O) is a rare amino acid present in some enzymes. It is considered to be the 22^nd genetically coded amino acid.

Structure of pyrrolysine

Like all of the amino acids, pyrrolysine is formed from the COOH group and the NH₂group which are involved in peptide bonds. It has a very complex side chain which in particular includes a pyrrole ring.

Function of pyrrolysine

Pyrrolysine is bound to a specific transfer RNA whose anticodon pairs with the messenger RNA UAG codon. This codon is usually a stop codon which causes arrest in protein synthesis when it is read by the ribosome. Like selenocysteine, a specific mechanism incorporates pyrrolysine in place of the stop codon.