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1H -3-hydroxy-4-oxoquinoline 2,4-dioxygenase (Qdo) fromPseudomonas putida 33/1 and 1H -3-hydroxy-4-oxoquinaldine 2,4-dioxygenase (Hod) fromArthrobacter ilicis Rü61a catalyze an N-heterocyclic-ring cleavage reaction, generatingN -formylanthranilate andN -acetylanthranilate, respectively, and carbon monoxide. Amino acid sequence comparisons between Qdo, Hod, and a number of proteins belonging to the α/β hydrolase-fold superfamily of enzymes and analysis of the similarity between the predicted secondary structures of the 2,4-dioxygenases and the known secondary structure of haloalkane dehalogenase fromXanthobacter autotrophicus GJ10 strongly suggested that Qdo and Hod are structurally related to the α/β hydrolase-fold enzymes. The residues S95 and H244 of Qdo were found to be arranged like the catalytic nucleophilic residue and the catalytic histidine, respectively, of the α/β hydrolase-fold enzymes. Investigation of the potential functional significance of these and other residues of Qdo through site-directed mutagenesis supported the hypothesis that Qdo is structurally as well as functionally related to serine hydrolases, with S95 being a possible catalytic nucleophile and H244 being a possible catalytic base. A hypothetical reaction mechanism for Qdo-catalyzed 2,4-dioxygenolysis, involving formation of an ester bond between the catalytic serine residue and the carbonyl carbon of the substrate and subsequent dioxygenolysis of the covalently bound anionic intermediate, is discussed.

Bacterial 2,4-Dioxygenases: New Members of the α/β Hydrolase-Fold Superfamily of Enzymes Functionally Related to Serine Hydrolases