Peptidyl-arginine deiminase (PAD) enzymes catalyse 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 (Pfam:PF03068), 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 homologues) [1]. The predicted catalytic residues in PPAD (Swiss:Q9RQJ2) are Asp130, Asp187, His236, Asp238 and Cys351 [1]. These are absolutely conserved with the exception of Asp187 which is absent in two family members. PPAD is also able to catalyse the deimination of free L-arginine, but has primarily peptidyl-arginine specificity. It may have a FMN cofactor [2].

1: A novel superfamily of enzymes that catalyze the modification of guanidino groups. Shirai H, Blundell TL, Mizuguchi K; Trends Biochem Sci 2001;26:465-468. PMID:11504612

2: Purification, characterization, and sequence analysis of a potential virulence factor from Porphyromonas gingivalis, peptidylarginine deiminase. McGraw WT, Potempa J, Farley D, Travis J; Infect Immun 1999;67:3248-3256. PMID:10377098