Reductive dehalogenases

Reductive dehaholagenses (EC 1.97.1.8) are a group of enzymes utilized in organohalide respiring bacteria.^[1][2] 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. 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) family of enzymes. They contain a cobalamin at its catalytic active site, where actual reductive reaction occurs. They also harbor iron− sulfur clusters that supply the reducing equivalents.^[1][3] All membrane-associated dehalogenases harbor a N-terminal twin-arginine (TAT) signal sequence (RRXFXK), which is a conserved signal peptide for membrane protein translocation. Monomeric as well as dimeric forms were previously reported.

Enzymatic mechanism is still understudied; however, several studies reported various mechanisms involving an organocobalt adduct, a single-electron transfer, and a halogen–cobalt bond.^[1]

Common reductive dehalogenases studied[]

Reductive dehalogenases from Dehalobacter species[]

• Chloroform reductive dehalogenases: TmrA^[4][5] and CfrA^[6]

Reductive dehalogenases from Dehalococcoides species[]

• Vinyl chloride reducing VcrA^[7]
• Hexachlorobenzene degrading CbrA^[8]

Reductive dehalogenases from Desulfitobacterium species[]

• Perchloroethene and trichloroethene degrading PceA^[9]

Production methods[]

Native enzymes[]

The examples are those that can dechlorinate chloroform (TmrA), PCE (PceA), TCE (TceA), and VC (VcrA).^[2] Purification of such enzymes in native forms are reportedly difficult; however, a few such enzymes were purified to near homogeneity.^[10][11] Ultracentrifugation, membrane solubilization and a series of liquid chromatography 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.^[4]

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.^{[12][7][3][5]} 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.^[5]

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 of organohalide contamination.

References[]