structurally, haloalkane dehalogenases belong to the alpha/beta-hydrolase superfamily. their active site is buried in a predominantly hydrophobic cavity at the interface of the alpha/beta-hydrolase core domain and the helical cap domain, and is connected to the bulk solvent by access tunnels. the active-site residues that are essential for catalysis are referred to as the catalytic pentad, and comprise a nucleophilic aspartate residue, a basic histidine residue, an aspartic or glutamic acid moiety that serves as a general acid and either two tryptophan residues or a tryptophan-asparagine pair that serve to stabilize the leaving halide ion. the haloalkane dehalogenase family currently includes 14 distinct enzymes with experimentally confirmed dehalogenation activity. an analysis of the sequences and structures of haloalkane dehalogenase and their homologues divided the family into three subfamilies, which differ mainly in the composition of their catalytic pentad and cap domain.