 Pyrrolysine

Pyrrolysine (symbol Pyl or O; encoded by the 'amber' stop codon UAG) is an α-amino acid that is used in the biosynthesis of proteins in some methanogenic archaea and bacteria; it is not present in humans.It contains an α-amino group (which is in the protonated – form under biological conditions), a carboxylic acid group (which is in the deprotonated –COO− form under biological conditions).