Pyrrolysine is a naturally occurring, genetically coded amino acid used by some methanogenic archaea and one known bacterium in enzymes that are part of their methane-producing metabolism. It is similar to lysine, but with an added pyrroline ring linked to the end of the lysine side chain. It forms part of an unusual genetic code in these organisms, and is considered the 22nd proteinogenic amino acid. 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 NH2 group is eliminated, followed by cyclization and dehydration step to yield L-pyrrolysine. As determined by X-ray crystallography and MALDI mass spectrometry, pyrrolysine is made up of 4-methylpyrroline-5-carboxylate in amide linkage with the ϵN of lysine.
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