Diversity of alpha-halocarboxylic acid dehalogenases in bacteria isolated from a pristine soil after enrichment and selection on the herbicide 2,2-dichloropropionic acid (Dalapon)

l-2-Haloacid dehalogenase (DehL) from Rhizobium sp. RC1

l-2-Haloacid dehalogenase (DehL) from Rhizobium sp. RC1 is a stereospecific enzyme that acts exclusively on l-isomers of 2-chloropropionate and dichloroacetate. The amino acid sequence of this enzyme is substantially different from those of other l-specific dehalogenases produced by other organisms. DehL has not been crystallised, and hence its three-dimensional structure is unavailable. Herein, we review what is known concerning DehL and tentatively identify the amino acid residues important for catalysis based on a comparative structural and sequence analysis with well-characterised l-specific dehalogenases.

Biodegradation of 4-chloroaniline by bacteria enriched from soil

4-Chloroaniline has been released into the environment due to extensive use in chemical industries and intensive agriculture; hence, it becomes one of the hazardous pollutants in the priority pollutant list. In this study, three gram-negative bacteria were enriched and isolated from agricultural soil as 4-chloroaniline-degrading bacteria. They were identified as Acinetobacter baumannii CA2, Pseudomonas putida CA16 and Klebsiella sp. CA17. They were able to utilize 4-chloroaniline as a sole carbon and nitrogen source without stimulation or cocultivation with aniline or another cosubstrate. The biodegradation in these bacteria was occurred via a modified ortho-cleavage pathway of which the activity of chlorocatechol 1, 2-dioxygenase was markedly induced. They grew well on 0.2-mM 4-chloroaniline exhibiting a 60-75% degradation efficiency and equimolar liberation of chloride. The isolates were able to survive in the presence of 4-chloroaniline at higher concentrations (up to 1.2 mM). 2-Chloroaniline, 3-chloroaniline and aniline, but not 3, 4-dichloroaniline, were also growth substrates for these isolates. The results of cosubstrate supplementation illustrated the suitable conditions of each isolate to improve growth rate and 4-chloroaniline biodegradation efficiency. These results suggest that these isolates have a potential use for bioremediation of the site contaminated with 4-chloroaniline.

Structure and mechanism of bacterial dehalogenases: different ways to cleave a carbon–halogen bond

The dehalogenases make use of fundamentally different strategies to cleave carbon-halogen bonds. The structurally characterized haloalkane dehalogenases, haloacid dehalogenases and 4-chlorobenzoate-coenzyme A dehalogenases use substitution mechanisms that proceed via a covalent aspartyl intermediate. Recent X-ray crystallographic analysis of a haloalcohol dehalogenase and a trans-3-chloroacrylic acid dehalogenase has provided detailed insight into a different intramolecular substitution mechanism and a hydratase-like mechanism, respectively. The available information on the various dehalogenases supports different views on the possible evolutionary origins of their activities.