Editing Amino acid (section)

==== Special case side chains ====

Several side chains are not described well by the charged, polar and hydrophobic categories. [[Glycine]] (Gly, G) could be considered a polar amino acid since its small size means that its solubility is largely determined by the amino and carboxylate groups. However, the lack of any side chain provides glycine with a unique flexibility among amino acids with large ramifications to protein folding.<ref name="Garrett-2010" /> [[Cysteine]] (Cys, C) can also form hydrogen bonds readily, which would place it in the polar amino acid category, though it can often be found in protein structures forming covalent bonds, called [[disulphide bonds]], with other cysteines. These bonds influence the folding and stability of proteins, and are essential in the formation of [[Antibody#Structure|antibodies]]. [[Proline]] (Pro, P) has an alkyl side chain and could be considered hydrophobic, but because the side chain joins back onto the alpha amino group it becomes particularly inflexible when incorporated into proteins. Similar to glycine this influences protein structure in a way unique among amino acids. [[Selenocysteine]] (Sec, U) is a rare amino acid not directly encoded by DNA, but is incorporated into proteins via the ribosome. Selenocysteine has a lower redox potential compared to the similar cysteine, and participates in several unique enzymatic reactions.<ref>{{cite journal | vauthors = Papp LV, Lu J, Holmgren A, Khanna KK | title = From selenium to selenoproteins: synthesis, identity, and their role in human health | journal = Antioxidants & Redox Signaling | volume = 9 | issue = 7 | pages = 775–806 | date = July 2007 | pmid = 17508906 | doi = 10.1089/ars.2007.1528 }}</ref> [[Pyrrolysine]] (Pyl, O) is another amino acid not encoded in DNA, but synthesized into protein by ribosomes.<ref>{{cite journal | vauthors = Hao B, Gong W, Ferguson TK, James CM, Krzycki JA, Chan MK | title = A new UAG-encoded residue in the structure of a methanogen methyltransferase | journal = Science | volume = 296 | issue = 5572 | pages = 1462–1466 | date = May 2002 | pmid = 12029132 | doi = 10.1126/science.1069556 | s2cid = 35519996 | bibcode = 2002Sci...296.1462H }}</ref> It is found in archaeal species where it participates in the catalytic activity of several methyltransferases.