REACTION-MECHANISM OF FLUOROACETATE DEHALOGENASE FROM MORAXELLA SP. B

Fluoroacetate dehalogenase (EC 3.8.1.3) catalyzes the dehalogenation o f fluoroacetate and other haloacetates, The amino acid sequence of flu oroacetate dehalogenase from Moraxella sp, B is similar to that of hal oalkane dehalogenase (EC 3.8.1.5) from Xanthobacter autotrophicus GJ10 in the regions around Asp-105 and His-272, which correspond to the ac tive site nucleophile Asp-124 and the base catalyst His-289 of the hal oalkane dehalogenase, respectively (Krooshof, G, H,, Kwant, E. M,, Dam borsky, J,, Koca, J,, and Janssen, D, B, (1997) Biochemistry 36, 9571- 9580), After multiple turnovers of the fluoroacetate dehalogenase reac tion in (H2O)-O-18, the enzyme was digested with trypsin, and the mole cular masses of the peptide fragments formed were measured by ion-spra y mass spectrometry, Two O-18 atoms were shown to be incorporated into the octapeptide, Phe-99-Arg-106, Tandem mass spectrometric analysis o f this peptide revealed that Asp-105 was labeled with two O-18 atoms. These results indicate that Asp-105 acts as a nucleophile to attack th e alpha-carbon of the substrate, leading to the formation of an eater intermediate, which is subsequently hydrolyzed by the nucleophilic att ack of a water molecule on the carbonyl carbon atom. A His-272 --> Asn mutant (H272N) showed no activity with either fluoroacetate or chloro acetate. However, ion-spray mass spectrometry revealed that the H272N mutant enzyme was covalently alkylated with the substrate. The reactio n of the H272N mutant enzyme with [C-14]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 th e covalent eater intermediate.