L-Arginine deiminase from Pseudomonas aeruginosa (PaADI) catalyzes the hydrolysis of arginine to citrulline and ammonia. PaADI belongs to the guanidino group-modifying enzyme superfamily (GMSF), which conserves backbone fold and a Cys-, His-, and Asp-based catalytic core. In this paper the contributions made by the PaADI core residues Cys406, His278, and Asp166 and the contribution from the neighboring Asp280 (conserved in most but not all GMSF members) to catalysis of the formation and hydrolysis of the Cys406-alkyluronium intermediate were accessed by kinetic analysis of site-directed mutants. In addition, solution hydrolysis in a chemical model of the S-alkylthiouronium intermediate was examined to reveal the importance of general base catalysis in the enzymatic reaction. Substitutions of the active site gating residue Arg401, the L-arginine C R NH 3 + (COO -) binding residues, Arg185, Arg243, and Asn160, or the His278 hydrogen bond partner, Glu224, were found to cause dramatic reductions in the enzyme turnover rate. These results are interpreted to suggest that electrostatic interactions play a dominant role in PaADI catalysis. Structural variations observed in P. aeruginosa GMSF enzymes PaADI, agmatine deiminase (PaAgDI), and N ω ,N ω -dimethylarginine dimethylaminohydrolase (PaDDAH) indicate an early divergence of the encoding genes. Arginine analogues that are known substrates for PaAgDI and PaDDAH were tested with PaADI to define clear boundaries of biochemical function in the three hydrolases. The conservation of a catalytic core associated with the common chemical function and the divergence of substrate-binding residues (as well as one key catalytic residue) to expand the substrate range provide insight into the evolution of the catalysts that form the GMSF.The guanidino-modifying enzyme superfamily (hereafter referred to as the GMSF) 1 is a large family of R/ -propellerfold enzymes that catalyze nucleophilic substitution reactions at the guanidinium carbon atom of L-arginine or an L-arginine derivative (for an excellent review, see ref 1). The family is divided into the hydrolase branch, in which an external guanidino NH 2 group is displaced, and the transferase branch, in which the ornithine moiety is displaced. The most remarkable family member, N-succinylarginine dihydrolase (2), catalyzes two consecutive substitution reactions, thus combining the elements of both branches. Aside from N-succinylarginine dihydrolase (3), the genome of Pseudomonas aeruginosa encodes three chemical homologues of the hydrolase branch of the GMSF: arginine deiminase (accession number P13981; locus name PA5171) (PaADI), agmatine deiminase (accession number Q9I6J9; locus name PA0292) (PaAgDI), and N ω ,N ω -dimethylarginine dimethylaminohydrolase (accession number Q9I4E3; locus name PA1195) (PaDDAH). A comparison of the published PaADI (4) and PaDDAH (5) X-ray structures with the recently deposited structure of the AgADI from Arabidopsis thaliana (PDB code 1VKP) (6) demonstrates that despite their low pairwise ...