title: The X-ray structure of trans-3-chloroacrylic acid dehalogenase reveals a novel hydration mechanism in the tautomerase superfamily.
abstract: Isomer-specific 3-chloroacrylic acid dehalogenases function in the bacterial degradation of 1,3-dichloropropene, a compound used in agriculture to kill plant-parasitic nematodes. The crystal structure of the heterohexameric trans-3-chloroacrylic acid dehalogenase (CaaD) from Pseudomonas pavonaceae 170 inactivated by 3-bromopropiolate shows that Glu-52 in the alpha-subunit is positioned to function as the water-activating base for the addition of a hydroxyl group to C-3 of 3-chloroacrylate and 3-bromopropiolate, whereas the nearby Pro-1 in the beta-subunit is positioned to provide a proton to C-2. Two arginine residues, alphaArg-8 and alphaArg-11, interact with the C-1 carboxylate groups, thereby polarizing the alpha,beta-unsaturated acids. The reaction with 3-chloroacrylate results in the production of an unstable halohydrin, 3-chloro-3-hydroxypropanoate, which decomposes into the products malonate semialdehyde and HCl. In the inactivation mechanism, however, malonyl bromide is produced, which irreversibly alkylates the betaPro-1. CaaD is related to 4-oxalocrotonate tautomerase, with which it shares an N-terminal proline. However, in 4-oxalocrotonate tautomerase, Pro-1 functions as a base participating in proton transfer within a hydrophobic active site, whereas in CaaD, the acidic proline is stabilized in a hydrophilic active site. The altered active site environment of CaaD thus facilitates a previously unknown reaction in the tautomerase superfamily, the hydration of the alpha,beta-unsaturated bonds of trans-3-chloroacrylate and 3-bromopropiolate. The mechanism for these hydration reactions represents a novel catalytic strategy that results in carbon-halogen bond cleavage.
authors.affiliation: Laboratory of Biophysical Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands.
authors.forename: René M
authors.initials: RM
authors.lastname: de Jong
authors.forename: Wim
authors.initials: W
authors.lastname: Brugman
authors.forename: Gerrit J
authors.initials: GJ
authors.lastname: Poelarends
authors.forename: Christian P
authors.initials: CP
authors.lastname: Whitman
authors.forename: Bauke W
authors.initials: BW
authors.lastname: Dijkstra
mesh_terms: D001665:Binding Sites; D002384:Catalysis; D006867:Hydrolases / Q000737:chemistry*; D008958:Models, Molecular; D011487:Protein Conformation; D011549:Pseudomonas / Q000201:enzymology; D014867:Water / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D014867:Water; D006867:Hydrolases; C077721:trans-3-chloroacrylic acid dehalogenase