Substrate distortion by a lichenase highlights the different conformational itineraries harnessed by related glycoside hydrolases.

From the beginning of the past century, halogenated hydrocarbons have been extensively applied in industry and agriculture. Decades after the start of their widespread use, evidence started to accumulate that some of these xenobiotic halogenated compounds are persistent and highly toxic, stimulating investigations how they could be degraded. It appeared that specific bacterial enzymes exist, dehalogenases, which can degrade halogenated compounds. These enzymes make use of a variety of distinctly different catalytic mechanisms to cleave carbon-halogen bonds. X-ray structures of haloalkane dehalogenases, haloacid dehalogenases, and 4-chlorobenzoyl-CoA dehalogenase demonstrated the power of substitution mechanisms that proceed via a covalent aspartyl intermediate. Structural characterizations of haloalcohol dehalogenases revealed the details of another elegant catalytic strategy, exploiting the presence of a vicinal hydroxyl group in the substrate. Finally, 3-chloroacrylic acid dehalogenases function in the bacterial degradation of 1,3-dichloropropene, a compound used in agriculture to kill plant-parasitic nematodes. Crystal structures of these enzymes showed that they function as hydratases to remove the halogen atom. Glu-52 is positioned to function as the water-activating base for the addition of a hydroxyl group to the C-3 atom of 3-chloroacrylate, while the nearby Pro-1 is positioned to provide a proton to C-2. Two arginine residues, αArg-8 and αArg-11, interact with the C-1 carboxylate groups, thereby polarizing the α,β-unsaturated acids. The resulting product is an unstable halohydrin, 3-chloro-3-hydroxypropanoate, which decomposes into the products malonate semialdehyde and HCl.