Pyrrolysine
Pyrrolysine (abbreviated as Pyl or O; encoded by the 'amber' stop codon UAG) is an ɑ-amino acid that is used in the biosynthesis of proteins in some methanogenic archaea and bacterium; it is not present in humans. It contains an α-amino group (which is in the protonated –+NH3 form under biological conditions), a carboxylic acid group (which is in the deprotonated –COO- form under biological conditions). Its pyrroline side-chain is similar to that of lysine in being basic and positively charged at at neutral pH.
Genetics
Nearly all proteins are made using only 20 standard amino acid building blocks. Two unusual genetically-encoded amino acids are selenocysteine and pyrrolysine. Pyrrolysine was discovered in 2002 at the active site of methyl-transferase enzyme from a methane-producing archeon, Methanosarcina barkeri. This amino acid is encoded by UAG (normally a stop codon), and its synthesis and incorporation into protein is mediated via the biological machinery encoded by the pylTSBCD cluster of genes.
