Pyrrolysine is an amino acid that is used in the biosynthesis of proteins in some methanogenic archaea and bacteria; it is not present in humans.
Pyrrolysine was discovered in 2002 at the active site of methyltransferase 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.
Unlike posttranslational modifications of lysine such as hydroxylysine, methyllysine, and hypusine, pyrrolysine is incorporated during translation (protein synthesis) as directed by the genetic code, just like the standard amino acids.
Source:
https://en.wikipedia.org/wiki/Pyrrolysine
