Reductive Dechlorination Catalyzed by Bacterial Transition-Metal Coenzymes

The bacterial transition-metal coenzymes vitamin B₁₂ (Co), coenzyme F₄₃₀ (Ni), and hematin (Fe) catalyzed the reductive dechlorination of polychlorinated ethylenes and benzenes, whereas the electron-transfer proteins four-iron ferredoxin, two-iron ferredoxin, and azurin (Cu) did not. For vitamin B₁₂ and coenzyme F₄₃₀, reductive dechlorination rates for different classes of perchlorinated compounds had the following order: carbon tetrachloride > tetrachloroethylene > hexachlorobenzene. For hematin, the order of reductive dechlorination rates was carbon tetrachloride > hexachlorobenzene > tetrachloroethylene. Within each class of compounds, rates of dechlorination decreased with decreasing chlorine content. Regio- and stereospecificity were observed in these reactions. In the reductive dechlorination of trichloroethylene, cis-l,2-dichloroethylene was the predominant product formed with vitamin B₁₂, coenzyme F₄₃₀, and hematin. Pentachlorobenzene and pentachlorophenol were each dechlorinated by vitamin B₁₂ to yield two out of three possible isomeric tetrachlorobenzenes. Similar relative kinetics and dechlorination products have been observed in anaerobic cultures, suggesting a possible role of transition-metal coenzymes in the reductive dechlorination of polychlorinated compounds in natural and engineered environments.