Pyrrolysine. Molecular model of the amino acid pyrrolysine. Its chemical formula is C12.H21.N3.O3. Atoms are represented as spheres and are colour- coded: carbon (blue), hydrogen (gold), oxygen (red) and nitrogen (dark blue). Amino acids are the building blocks of proteins. Pyrrolysine is used by methanogenic (methane-producing) archaea (single-celled microorganisms that lack a nucleus), such as Methanosarcina barkeri, which lives on the bottom of freshwater lakes. Pyrrolysine is involved in a specific type of anaerobic respiration in these microorganisms, where carbon dioxide or ethanoic acid is used to make energy and methane is produced as a waste product. To date (as of 2008), pyrrolysine has not been found to be used by any other type of organism.