Bacterial 2-haloacid dehalogenases: structures and reaction mechanisms

Microbial dehalogenases have been attracting a great deal of attention because of their possible application to fine chemical synthesis and bioremediation of halo compound-polluted environment. Dehalogenases employ various different mechanisms to cleave the carbon–halogen bond. 2-Haloacid dehalogenases catalyze the hydrolytic dehalogenation of 2-haloalkanoic acids to produce the corresponding 2-hydroxyalkanoic acids. The reaction mechanism of l-2-haloacid dehalogenase from Pseudomonas sp. YL has been clarified by 18O incorporation experiment, site-directed mutagenesis and X-ray crystallographic analysis. The carboxylate group of Asp10 performs a nucleophilic attack on the α-carbon atom of the substrate to displace the halogen atom and produce the ester intermediate, which is subsequently hydrolyzed to produce the corresponding d-2-hydroxyalkanoic acid and regenerate the Asp10 residue. The reaction catalyzed by fluoroacetate dehalogenase from Moraxella sp. B similarly proceeds in two steps: the carboxylate group of Asp105 performs a nucleophilic attack on the substrate α-carbon atom to form an ester intermediate, and the intermediate is hydrolyzed by a water molecule activated by His272. In contrast with these two enzymes, a water molecule directly attacks the substrate to displace the halogen atom and produce 2-hydroxyalkanoic acid in the reaction catalyzed by dl-2-haloacid dehalogenase from Pseudomonas sp. 113.