Organohalide chemistry underpins many industrial and agricultural processes, and a large proportion of environmental pollutants are organohalides1. Nevertheless, organohalide chemistry is not exclusively of anthropogenic origin, with natural abiotic and biological processes contributing to the global halide cycle2–3. Reductive dehalogenases are responsible for biological dehalogenation in organohalide respiring bacteria4–5, with substrates including the notorious polychlorinated biphenyls (PCBs) or dioxins6–7. These proteins form a distinct subfamily of cobalamin (B12) dependent enzymes that are usually membrane-associated and oxygen-sensitive, hindering detailed studies8–12. We report the characterisation of a soluble, oxygen-tolerant reductive dehalogenase and, by combining structure determination with EPR spectroscopy and simulation, show that a direct interaction between the cobalamin cobalt and the substrate halogen underpins catalysis. In contrast to the carbon-Co bond chemistry catalyzed by the other cobalamin-dependent subfamilies13 we propose that reductive dehalogenases achieve reduction of the organohalide substrate via halogen-Co bond formation. This presents a new paradigm in both organohalide and cobalamin (bio)chemistry that will guide future exploitation of these enzymes in bioremediation or biocatalysis.

Reductive dehalogenases are corrinoid and iron-sulfur cluster-dependent enzymes that mostly act as the terminal oxidoreductases in the bacterial organohalide respiration process. This process often leads to detoxification of recalcitrant organohalide pollutants. While low cell yields and oxygen sensitivity hamper the study of many reductive dehalogenases, this is not the case for the nonrespiratory reductive dehalogenase NpRdhA from Nitratireductor pacificus. We here report in vitro and in vivo reconstitution of an NADPH-dependent reducing system for NpRdhA. Surprisingly, NpRdhA mediated organohalide reduction could not be supported using N. pacificus ferredoxin-NAD(P)H oxidoreductase and associated ferredoxins. Instead, we found a nonphysiological system comprised of the Escherichia coli flavodoxin reductase (EcFldr) in combination with spinach ferredoxin (SpFd) was able to support NADPH-dependent organohalide reduction by NpRdhA. Using this system, organohalide reduction can be performed under both anaerobic and aerobic conditions, with 1.1 ± 0.1 and 3.5 ± 0.3 equiv of NADPH consumed per product produced, respectively. No significant enzyme inactivation under aerobic conditions was observed, suggesting a Co(I) species is unlikely to be present under steady state conditions. Furthermore, reduction of the Co(II) resting state was not observed in the absence of substrate. Only the coexpression of EcFldr, SpFd, and NpRdhA in Bacillus megaterium conferred the latter with the ability to reduce brominated NpRdhA substrates in vivo, in agreement with our in vitro observations. Our work provides new insights into biological reductive dehalogenase reduction and establishes a blueprint for the minimal functional organohalide reduction module required for bioremediation in situ.