What is Protein-arginine deiminase?

In enzymology, a protein-arginine deiminase 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 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.