Reaction Mechanism of Fluoroacetate Dehalogenase from Moraxella sp. B*

Fluoroacetate dehalogenase (EC 3.8.1.3) catalyzes the dehalogenation of fluoroacetate and other haloacetates. The amino acid sequence of fluoroacetate dehalogenase from Moraxellasp. B is similar to that of haloalkane dehalogenase (EC 3.8.1.5) fromXanthobacter autotrophicus GJ10 in the regions around Asp-105 and His-272, which correspond to the active site nucleophile Asp-124 and the base catalyst His-289 of the haloalkane dehalogenase, respectively (Krooshof, G. H., Kwant, E. M., Damborský, J., Koča, J., and Janssen, D. B. (1997)Biochemistry 36, 9571–9580). After multiple turnovers of the fluoroacetate dehalogenase reaction in H2 18O, the enzyme was digested with trypsin, and the molecular masses of the peptide fragments formed were measured by ion-spray mass spectrometry. Two 18O atoms were shown to be incorporated into the octapeptide, Phe-99–Arg-106. Tandem mass spectrometric analysis of this peptide revealed that Asp-105 was labeled with two 18O atoms. These results indicate that Asp-105 acts as a nucleophile to attack the α-carbon of the substrate, leading to the formation of an ester intermediate, which is subsequently hydrolyzed by the nucleophilic attack of a water molecule on the carbonyl carbon atom. A His-272 → Asn mutant (H272N) showed no activity with either fluoroacetate or chloroacetate. However, ion-spray mass spectrometry revealed that the H272N mutant enzyme was covalently alkylated with the substrate. The reaction of the H272N mutant enzyme with [14C]chloroacetate also showed the incorporation of radioactivity into the enzyme. These results suggest that His-272 probably acts as a base catalyst for the hydrolysis of the covalent ester intermediate.