Fig. (1) Catalytic mechanism of dehalogenation by DhaA and strategy for trapping the covalent intermediate. A. In the first step of catalysis, the nucleophile, Asp¹⁰⁶ attacks the α-carbon of the chloroalkane (shown in red) to produce a covalent, alkyl-enzyme intermediate. His²⁷² catalyzes hydrolysis of the intermediate, the release of products from the active site, and regeneration of enzyme. Glu¹³⁰ provides structure at the active site and stabilizes the positive charge on His²⁷² that forms during hydrolysis. In addition to forming the halide binding site, Trp¹⁰⁷ and Asn⁴¹ stabilize the Cl^- leaving group following hydrolysis [16]. B. The strategy for trapping the covalent intermediate was to replace His²⁷² with a residue (e.g. Phe) that cannot act as a general base, and therefore cannot hydrolyze the alkyl-enzyme intermediate.