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PYL or O.

Pyrrolysine is a naturally-occurring, genetically encoded amino acid, sometimes called the "22nd amino acid". Its 3-letter code is PYL and its single letter code is O. For more information, please see Pyrrolysine in Wikipedia.


In January, 2022, seven wwPDB entries contain PYL. (Prior to remediation, pyrrolysine was sometimes designated BGX or PYH. Those IDs are no longer in use in January, 2022.)

Some proteins in methanogenic archaea include pyrrolysine. Methanosarcina barkeri monomethylamine methyltransferase (MtmB, 1nth, 1l2q) was the first identified structure containing this amino acid[1]. Chemically modified forms of pyrrolysine are present in the MtmB structures 1tv2, 1tv3, and 1tv4[2]. The wwPDB also contains several structures for pyrrolysyl-tRNA synthetase, including 2zce which includes pyrrolysine.


  1. Hao B, Gong W, Ferguson TK, James CM, Krzycki JA, Chan MK. A new UAG-encoded residue in the structure of a methanogen methyltransferase. Science. 2002 May 24;296(5572):1462-6. PMID:12029132 doi:10.1126/science.1069556
  2. Hao B, Zhao G, Kang PT, Soares JA, Ferguson TK, Gallucci J, Krzycki JA, Chan MK. Reactivity and chemical synthesis of L-pyrrolysine- the 22(nd) genetically encoded amino acid. Chem Biol. 2004 Sep;11(9):1317-24. PMID:15380192 doi:10.1016/j.chembiol.2004.07.011

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