In enzymology, a protein-arginine deiminase () is an enzyme that catalyzes a form of post translational modification called arginine de-imination or citrullination: protein L-arginine + H2O protein L-citrulline + NH3 Thus, the two substrates of this enzyme are protein L-arginine (arginine residue inside a protein) and H2O, whereas its two products are protein L-citrulline and NH3: This enzyme belongs to the family of hydrolases, those acting on carbon-nitrogen bonds other than peptide bonds, specifically in linear amidines. The systematic name of this enzyme class is protein-L-arginine iminohydrolase. This enzyme is also called peptidylarginine deiminase. Structural studies As of late 2007, seven structures have been solved for this class of enzymes, with PDB accession codes , , , , , , and . Mammalian proteins Mammals have 5 protein-arginine deiminases, with symbols PADI1, PADI2, PADI3, PADI4, PADI6 except for rodents, there the letter case is different: Padi1, Padi2, Padi3, Padi4, Padi6 The different case is just a historical artifact. It doesn't indicate that the rodent proteins are special. References EC 3.5.3 Enzymes of known structure