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L-2-Haloacid dehalogenase (EC 3.8.1.2) catalyzes the hydrolytic dehalogenation of L-2-haloacids to produce the corresponding D-2-hydroxy acids. We have analyzed the reaction mechanism of the enzyme from Pseudomonas sp. YL and found that Asp10 is the active site nucleophile. When the multiple turnover enzyme reaction was carried out in H2(18)O with L-2-chloropropionate as a substrate, lactate produced was labeled with 18O. However, when the single turnover enzyme reaction was carried out by use of a large excess of the enzyme, the product was not labeled. This suggests that an oxygen atom of the solvent water is first incorporated into the enzyme and then transferred to the product. After the multiple turnover reaction in H2(18)O, the enzyme was digested with lysyl endopeptidase, and the molecular masses of the peptide fragments formed were measured by an ionspray mass spectrometer. Two 18O atoms were shown to be incorporated into a hexapeptide, Gly6-Lys11. Tandem mass spectrometric analysis of this peptide revealed that Asp10 was labeled with two 18O atoms. Our previous site-directed mutagenesis experiment showed that the replacement of Asp10 led to a significant loss in the enzyme activity. These results indicate that Asp10 acts as a nucleophile on the alpha-carbon of the substrate leading to the formation of an ester intermediate, which is hydrolyzed by nucleophilic attack of a water molecule on the carbonyl carbon atom.

Asp 10 of L-2-haloacid dehalogenase from Pseudomonas sp. YL was proposed to act as a nucleophile to attack the ␣-carbon of L-2-haloalkanoic acids to form an ester intermediate, which is hydrolyzed by nucleophilic attack of a water molecule on the carbonyl carbon (Liu, J.-Q, Kurihara, T., Miyagi, M., Esaki, N., and Soda, K. (1995) J. Biol. Chem. 270, 18309 -18312). We have found that the enzyme is paracatalytically inactivated by hydroxylamine in the presence of the substrates monochloroacetate and L-2-chloropropionate. Ion spray mass spectrometry demonstrated that the molecular mass of the enzyme inactivated by hydroxylamine during the dechlorination of monochloroacetate is about 74 Da greater than that of the native enzyme. To determine the increase of the molecular mass more precisely, we digested the inactivated enzyme with lysyl endopeptidase and measured the molecular masses of the peptide fragments. The molecular mass of the hexapeptide Gly 6 -Lys 11 was shown to increase by 73 Da. Tandem mass spectrometric analysis of this peptide revealed that the increase is due to a modification of Asp 10 . When the enzyme was paracatalytically inactivated by hydroxylamine during the dechlorination of L-2-chloropropionate, the molecular mass of the hexapeptide was 87 Da higher. Hydroxylamine is proposed to attack the carbonyl carbon of the ester intermediate and form a stable aspartate ␤-hydroxamate carboxyalkyl ester residue in the inactivated enzyme.Paracatalytic enzyme modification is a catalysis-linked and substrate-dependent enzyme modification (1). It involves a direct chemical reaction between an enzyme-activated substrate and an extrinsic reagent. The catalytic effect of an enzyme can increase the reactivity of a substrate with extrinsic reagents that are not constituents of the normal enzyme-substrate system. The reactive intermediates formed may thus react with extrinsic reagents to branch off from the normal catalytic pathway. Consequently, the enzyme active site may be specifically and irreversibly modified.L-2-Haloacid dehalogenase (EC 3.8.1.2) catalyzes the hydrolytic dehalogenation of L-2-haloalkanoic acids to produce the corresponding D-2-hydroxyalkanoic acids (2-4). Our recent 18 O incorporation experiment showed that the reaction of L-2-haloacid dehalogenase from Pseudomonas sp. YL (L-DEX YL) 1 proceeds through the mechanism shown in Fig. 1 (11) and found that L-DEX YL is paracatalytically inactivated by hydroxylamine. Tandem MS/MS spectrometric analysis revealed that the active site Asp 10 was specifically labeled. Hydroxylamine is thus useful to probe the active site carboxylate group, which constitutes an enzyme-substrate ester intermediate. EXPERIMENTAL PROCEDURES MaterialsLeu 11 , Ser 176 , and Arg 185 were replaced by Lys by site-directed mutagenesis, and the resultant mutant enzyme, L-DEX T15, yields a small peptide fragment containing active site Asp 10 of L-DEX YL by lysyl endopeptidase digestion. (5). Catalytic properties of L-DEX T15 such as the specific activity for L-2-chloropropionate an...

2-Haloacid dehalogenases (2-haloacid halidohydrolase; EC class: 3.8.1.2) catalyze the hydrolytlc dehalogenation of 2-haloalkanoic acids to produce the corresponding 2-hydroxyalkanoic acids. Only the DL-2-haloacid dehalogenase (retention type) reaction proceeds with retention of the C2-configuration of the substrate, and its reaction mechanism is probably different from those of other 2-haloacid dehalogenases.,