amino-acid biosynthesis proteinogenic amino-acid biosynthesis L-pyrrolysine biosynthesis PMID:21455182 PMID:17204561 MetaCyc:PWY-6994 GO:0071524 PMID:21550296 UPa:UPA01028 pathway L-pyrrolysine (abbreviated as Pyl or O) is a naturally occurring, genetically coded amino acid used by some methanogenic archaea and bacterium in enzymes that are part of their methane-producing metabolism. It forms part of an unusual genetic code in these organisms, and is considered the 22nd proteinogenic amino acid. It is necessary for all of the known pathways by which methane is formed from methylamines [PMID:21550296]. L-pyrrolysine is similar to lysine, but with an added pyrroline ring linked to the end of the lysine side chain. Pyrrolysine is synthesized in vivo by joining two molecules of L-lysine. One molecule of lysine is first converted to (3R)-3-Methyl-D-ornithine, which is then ligated to a second lysine. An amino group is eliminated, followed by cyclization and dehydration step to yield L-pyrrolysine.