In enzymology, a protein-arginine deiminase (EC 3.5.3.15) is an enzyme that catalyzes a form of post translational modification called arginine de-imination or citrullination: 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.

As of late 2007, seven structures have been solved for this class of enzymes, with PDB accession codes 1WD8, 1WD9, 1WDA, 2DEW, 2DEX, 2DEY, and 2DW5.

Mammals have 5 protein-arginine deiminases, with symbols except for rodents, there the letter case is different: The different case is just a historical artifact.

It doesn't indicate that the rodent proteins are special.