About Reductive Dehalogenases and the Ortholog Group Classification System

Reductive Dehalogenases (RdhA or RDases if characterized) are enzymes which catalyze the elimination of a halide from a carbon-halogen bond through reductive dehalogenation or dihaloelimination. Reductive dehalogenases are oxygen-sensitive and require reducing equivalents to function. These enzymes have been isolated from phylogenetically distant anaerobic microbes who dehalogenate as part of a facultative or obligate metabolism. Obligate organohalide respiring bacteria (OHRB) conserve energy for growth from using halogenated organic compounds as terminal electron acceptors in respiration and H as an electron donor. These OHRB express respiratory reductive dehalogenases which can be categorized as part of a quinone-dependent or quinone-independent electron transport chain. Facultative OHRB can use different electron donors (such as small organic compounds) and can use electron acceptors (such as Fe(III), SO₄^2- or NO₃^-) other than organohalides for growth.

Obligate OHRB genomes typically contain more copies of reductive dehalogenase genes than non-obligate bacteria. Some of these rdhA genes code for enzymes which catalyze the reductive dehalogenation of chlorinated and brominated pollutants in anaerobic subsurface environments, providing a valuable ecosystem service in the detoxification of these compounds. Natural attenuation, biostimulation, and bioaugmentation using mixed microbial consortia capable of reductive dechlorination are now commonly used to remediate sites contaminated with chlorinated or brominated pollutants. The success of bioremediation is largely due to the discovery of highly substrate-specific OHRB that link dehalogenation with growth, and thus whose populations increase as dehalogenation proceeds.

Several decades of study of obligate and non-obligate OHRB has documented thousands of reductive dehalogenase sequences, of which less than 1% have been biochemically characterized to any degree and even fewer with an available crystal structure. In 2013, Hug et al. developed a classification system for all reductive dehalogenases based on orthology that had been inferred from sequence similarity. An Ortholog Group contains reductive dehalogenases whose amino acid sequences have 90% or greater full-length pair-wise identity. This was done in order to unify and organize new sequences, especially for sequences with unknown function. Due to high sequence similarity, Ortholog Groups are also likely to share some related functions. We developed this website in order to aid in the classification of new sequences, and to store and share a manually curated list of reductive dehalogenases with links to functional information, where available. We hope that this project will continue to grow with the growing database in order to eventually predict function and structure of new dehalogenase sequences. We welcome suggestions from community members on how to improve this resource via our contact page.