Reductive dehaholagenses ( EC 1.97.1.8) are a group of

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

s utilized in organohalide respiring bacteria. These enzymes are mostly attached to the periplasmic side of the cytoplasmic membrane and play a central role in energy-conserving respiratory process for organohalide respiring bacteria by reducing organohalides. During such reductive dehalogenation reaction, organohalides are used as terminal

electron acceptors An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxid ...

. They catalyze the following general reactions: :R-X + 2 e⁻ + 2 H⁺ → R-H + H-X :X-RR-X + 2 e⁻ + 2 H⁺ → R=R + 2X⁻ These membrane-associated enzymes have attracted great interest for the detoxification of organohalide pollutants. Organohalide pollution is a serious global environmental issue affecting soil and groundwater; and reductive dehalogenases offer a promising natural tool for bioremediation.

Structure and mechanism

Reductive dehalogenases are related to the cobamide (or

vitamin B12 Vitamin B12, also known as cobalamin, is a water-soluble vitamin involved in metabolism. It is one of eight B vitamins. It is required by animals, which use it as a cofactor in DNA synthesis, in both fatty acid and amino acid metabolism. It ...

) family of enzymes. They contain a

cobalamin Vitamin B12, also known as cobalamin, is a water-soluble vitamin involved in metabolism. It is one of eight B vitamins. It is required by animals, which use it as a cofactor in DNA synthesis, in both fatty acid and amino acid metabolism. It ...

at its catalytic active site, where actual reductive reaction occurs. They also harbor iron− sulfur clusters that supply the reducing equivalents. All membrane-associated dehalogenases harbor a N-terminal twin-arginine (TAT) signal sequence (RRXFXK), which is a conserved signal peptide for membrane protein translocation.

Monomer In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification ...

ic as well as dimeric forms were previously reported. Enzymatic mechanism is still understudied; however, several studies reported various mechanisms involving an organocobalt

adduct An adduct (from the Latin ''adductus'', "drawn toward" alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all co ...

, a single-electron transfer, and a halogen–cobalt bond.

Common reductive dehalogenases studied

Reductive dehalogenases from ''Dehalobacter'' species

Chloroform Chloroform, or trichloromethane, is an organic compound with formula C H Cl3 and a common organic solvent. It is a colorless, strong-smelling, dense liquid produced on a large scale as a precursor to PTFE. It is also a precursor to various re ...

reductive dehalogenases: TmrA and CfrA

Reductive dehalogenases from ''Dehalococcoides'' species

Vinyl chloride Vinyl chloride is an organochloride with the formula H2C=CHCl. It is also called vinyl chloride monomer (VCM) or chloroethene. This colorless compound is an important industrial chemical chiefly used to produce the polymer polyvinyl chloride (PVC ...

reducing VcrA * Hexachlorobenzene degrading CbrA

Reductive dehalogenases from ''Desulfitobacterium'' species

Perchloroethene Tetrachloroethylene, also known under the systematic name tetrachloroethene, or perchloroethylene, and many other names (and abbreviations such as "perc" or "PERC", and "PCE"), is a chlorocarbon with the formula Cl2C=CCl2 . It is a colorless liq ...

and trichloroethene degrading PceA

Production methods

Native enzymes

The examples are those that can dechlorinate

chloroform Chloroform, or trichloromethane, is an organic compound with formula C H Cl3 and a common organic solvent. It is a colorless, strong-smelling, dense liquid produced on a large scale as a precursor to PTFE. It is also a precursor to various re ...

(TmrA), PCE (PceA), TCE (TceA), and VC (VcrA). Purification of such enzymes in native forms are reportedly difficult; however, a few such enzymes were purified to near homogeneity. Ultracentrifugation, membrane solubilization and a series of

liquid chromatography In chemical analysis, chromatography is a laboratory technique for the separation of a mixture into its components. The mixture is dissolved in a fluid solvent (gas or liquid) called the ''mobile phase'', which carries it through a system (a ...

are the commonly employed techniques to the isolation and purification. A chloroform reducing dehalogenase is the latest reductive dehalogenase that was successfully produced and purified.

Heterologous expressions

The researchers in the field had turned their interest to heterologous expression of the same enzymes due to difficulties in obtaining these enzymes in the native form. Only have recently a few recombinant reductive dehalogenases been functionally expressed, bringing the dehalogenase research into next levels. Those successful efforts facilitate further investigations on their biochemical and structural properties. The first membrane-associated respiratory reductive dehalogenase was heterologously expressed in a soluble and active form and purified using '' Bacillus megaterium''.

Uses in bioremediation

In recent years, research on reductive dehalogenases have attracted great interest from both academic and industrial researchers for their potential application in

bioremediation Bioremediation broadly refers to any process wherein a biological system (typically bacteria, microalgae, fungi, and plants), living or dead, is employed for removing environmental pollutants from air, water, soil, flue gasses, industrial effluent ...

of organohalide contamination.

References

{{reflist EC 1.97.1