Peptidyl-arginine deiminase (PAD) enzymes catalyze the deimination of the guanidino group from carboxy-terminal arginine residues of various peptides to produce ammonia. PAD from Porphyromonas gingivalis (PPAD) appears to be evolutionarily unrelated to mammalian PAD (pfam03068), which is a metalloenzyme. PPAD is thought to belong to the same superfamily as aminotransferase and arginine deiminase, and to form an alpha/beta propeller structure. This family has previously been named PPADH (Porphyromonas peptidyl-arginine deiminase homologs). The predicted catalytic residues in PPAD are Asp130, Asp187, His236, Asp238 and Cys351. These are absolutely conserved with the exception of Asp187 which is absent in two family members. PPAD is also able to catalyze the deimination of free L-arginine, but has primarily peptidyl-arginine specificity. It may have a FMN cofactor.