Editing Arginine (section)

===Proteins===
Arginine's side chain is [[Amphiphile|amphipathic]], because at physiological pH it contains a positively charged guanidinium group, which is highly polar, at the end of a hydrophobic [[Aliphatic compound|aliphatic]] hydrocarbon chain. Because globular proteins have hydrophobic interiors and hydrophilic surfaces,<ref>{{Cite book |title=Biochemistry|url=https://archive.org/details/biochemistry0003math|url-access=registration| vauthors = Mathews CK, Van Holde KE, Ahern KG |date=2000|publisher=Benjamin Cummings|isbn=978-0805330663|edition=3rd|location=San Francisco, Calif.|pages=[https://archive.org/details/biochemistry0003math/page/180 180]|oclc=42290721}}</ref> arginine is typically found on the outside of the protein, where the hydrophilic head group can interact with the polar environment, for example taking part in [[hydrogen bond]]ing and [[Salt bridge (protein and supramolecular)|salt bridges.]]<ref name=":4">{{Cite book|url=https://books.google.com/books?id=CN9sYPJdXs4C&pg=PA326|title=Bioinformatics for Geneticists: A Bioinformatics Primer for the Analysis of Genetic Data| vauthors = Barnes MR |date=2007-04-16|publisher=John Wiley & Sons|isbn=9780470026199 |pages=326 }}</ref> For this reason, it is frequently found at the interface between two proteins.<ref>{{Cite book|url=https://books.google.com/books?id=hbd8dlG7zkIC&pg=PA13|title=Protein-protein Recognition| vauthors = Kleanthous C |date=2000|publisher=Oxford University Press|isbn=9780199637607|pages=13|language=en}}</ref> The aliphatic part of the side chain sometimes remains below the surface of the protein.<ref name=":4" />

Arginine residues in proteins can be deiminated by PAD enzymes to form citrulline, in a [[post-translational modification]] process called [[citrullination]].This is important in fetal development, is part of the normal immune process, as well as the control of gene expression, but is also significant in [[autoimmune disease]]s.{{sfn|Griffiths|Unwin|2016|p=275}} Another post-translational modification of arginine involves [[methylation]] by protein [[methyltransferase]]s.{{sfn|Griffiths|Unwin|2016|p=176}}