Cobamide-mediated enzymatic reductive dehalogenation via long-range electron transfer

Cindy Kunze,Wilfred R Hagen,Gabriele Diekert,Torsten Schubert,Marie Uksa,Holger Dobbek,Martin Bommer

doi:10.1038/ncomms15858

Abstract

The capacity of metal-containing porphyrinoids to mediate reductive dehalogenation is implemented in cobamide-containing reductive dehalogenases (RDases), which serve as terminal reductases in organohalide-respiring microbes. RDases allow for the exploitation of halogenated compounds as electron acceptors. Their reaction mechanism is under debate. Here we report on substrate–enzyme interactions in a tetrachloroethene RDase (PceA) that also converts aryl halides. The shape of PceA’s highly apolar active site directs binding of bromophenols at some distance from the cobalt and with the hydroxyl substituent towards the metal. A close cobalt–substrate interaction is not observed by electron paramagnetic resonance spectroscopy. Nonetheless, a halogen substituent para to the hydroxyl group is reductively eliminated and the path of the leaving halide is traced in the structure. Based on these findings, an enzymatic mechanism relying on a long-range electron transfer is concluded, which is without parallel in vitamin B12-dependent biochemistry and represents an effective mode of RDase catalysis.

Highlights

The capacity of metal-containing porphyrinoids to mediate reductive dehalogenation is implemented in cobamide-containing reductive dehalogenases (RDases), which serve as terminal reductases in organohalide-respiring microbes
Comparison of substrate–enzyme complexes allowed for conclusions on the initial attack by the cobamide cofactor
This observation is consistent with predictions derived from the natural bond orbital model based on the partial charges for each substituent[23]

Summary

Introduction

The capacity of metal-containing porphyrinoids to mediate reductive dehalogenation is implemented in cobamide-containing reductive dehalogenases (RDases), which serve as terminal reductases in organohalide-respiring microbes. A halogen substituent para to the hydroxyl group is reductively eliminated and the path of the leaving halide is traced in the structure Based on these findings, an enzymatic mechanism relying on a long-range electron transfer is concluded, which is without parallel in vitamin B12-dependent biochemistry and represents an effective mode of RDase catalysis. Several anaerobic bacteria use organohalides as terminal electron acceptors in their respiratory metabolism These often toxic, hazardous and usually highly persistent compounds, which originate from industrial, biotic or geochemical sources, are reductively dehalogenated by these microbes. Reductive dehalogenase (RDase) enzymes are membrane-bound terminal reductases in organohalide respiration and harbour two Fe–S clusters and a cobamide cofactor Adduct formation with radical traps or chloropropenyl radicals was observed during the conversion of chloropropenes and interpreted as indicative for the generation of substrate radicals[16]

Methods

Results

Conclusion