Efficient NADPH-dependent dehalogenation afforded by a self-sufficient reductive dehalogenase

Evaluation of alternate hosts for recombinant expression of a reductive dehalogenase

Organohalides are recalcitrant, toxic environmental pollutants. Reductive dehalogenase enzymes (RDases) found in organohalide respiring bacteria (OHRB) utilise organohalides as electron acceptors for cellular energy and growth, producing lesser-halogenated compounds. Consequently, microbial reductive dehalogenation via organohalide respiration represents a promising solution for clean-up of organohalide pollutants. Dehalobacter sp. UNSWDHB is an OHRB capable of respiring highly toxic chloroform (CF) and converting it to dichloromethane (DCM). TmrA has been identified as an RDase responsible for this conversion and different strategies for generation of functional recombinant TmrA is the focus of this article. In this study, TmrA was recovered from inclusion bodies expressed in E. coli and refolded in the presence of FeCl3, Na2S and cobalamin to yield functional enzyme. TmrA has been previously expressed in a soluble and functional form in the corrinoid-producing Bacillus megaterium. Using a fractional experimental design for cultivation and induction combined with purification under anaerobic conditions resulted in substantially higher activity of recombinant and native TmrA than previously reported. TmrA was then expressed in a soluble and active form in Shimwellia blattae. Co-expression with two different putative chaperone proteins from the original host did not increase the level of soluble expression in S. blattae, however activity assays showed that removing the TAT signal from TmrA increases the dechlorination activity compared to when the TAT signal is present. Finally, TmrA was successfully expressed in a soluble and active form in the H2-oxidizing C. necator H16, a novel host for the expression of RDases.

Sustainable B12-Dependent Dehalogenation of Organohalides in E. coli

Microbial bioremediation can provide an environmentally friendly and scalable solution to treat contaminated soil and water. However, microbes have yet to optimize pathways for degrading persistent anthropogenic pollutants, in particular organohalides. In this work, we first expand our repertoire of enzymes useful for bioremediation. By screening a panel of cobalamin (B12)-dependent reductive dehalogenases, we identified previously unreported enzymes that dechlorinate perchloroethene and regioselectively deiodinate the thyroidal disruptor 2,4,6-triiodophenol. One deiodinase, encoded by the animal-associated anaerobe Clostridioides difficile, was demonstrated to dehalogenate the naturally occurring metabolites L-halotyrosines. In cells, several combinations of ferredoxin oxidoreductase and flavodoxin extract and transfer low-potential electrons from pyruvate to drive reductive dehalogenation without artificial reductants and mediators. This work provides new insights into a relatively understudied family of B12-dependent enzymes and sets the stage for engineering synthetic pathways for degrading unnatural small molecule pollutants.