Carbon tetrachloride (CCl4) is a toxic, carcinogenic compound which is used as a general solvent in industrial degreasing operations. It is also used as grain fumigant and a chemical intermediate in the production of refrigerants. Carbon tetrachloride is a widespread environmental pollutant due to improper disposal.
Carbon tetrachloride can be degraded under anaerobic conditions via three different pathways. No enzyme is involved in these pathways; rather, reactions are catalyzed by cofactors present in microorganisms. The left-most pathway is a sequential two-electron reduction process. Carbon tetrachloride is degraded to chloroform, dichoromethane, chloromethane and ultimately methane by hydrogenolytic dechlorinations. Many anaerobic bacteria can catalyze the first two reactions of this pathway using the cofactors heme, factor F430, and corrinoids, such as aquocobalamin and methylcobalamin.
These corrinoids can also convert CCl4 to carbon monoxide (Krone et al., 1991). This is part of the middle pathway, a simple two-electron reduction process in which chloroform is only a minor product. Carbon tetrachloride, in the right-most pathway, can be degraded via sulfur and oxygen substitution in one-electron reduction reactions.
Chang-Ho Lee et al. (1999) report that iron-limited cultures of Pseudomonas stutzeri strain KC can transform CCl4 to CO2 and non-volatile products using the catalyst pyridine-2,6-bis(thiocarboxylate) via the latter two pathways.
The following is a text-format anaerobic carbon tetrachloride pathway map. Organisms which can initiate the pathway are given, but other organisms may also carry out later steps. Follow the links for more information on compounds or reactions. This map is also available in graphic (14k) format.
Carbon Tetrachloride Carbon Tetrachloride Carbon Tetrachloride Pseudomonas stutzeri KC | | Methanosarcina barkeri | | Desulfobacterium autotrophicum | | Moorella thermoacetica | | Methanobacterium thermoautotrophicum +--------+--------+ +-----+-----+ | | | | | | v B2 v B3 C1 v v C2 | carbon tetrachloride | | | | | reductive dehalogenase | | | | | v v v v v Carbon Formate [Thiophosgene] [Phosgene] Chloroform monoxide | | | | | | | | | | | | | | chloroform | | +----+ +----+ | reductive dehalogenase | | | | | v v | | v to the to the v v Dichloromethane Thiocynate C1 Metabolic Carbon | Pathway Cycle Pathway dioxide | | dichloromethane | reductive dehalogenase | v Methyl chloride | | | methyl chloride | reductive dehalogenase | v Methane | | | | | v to the C1 Metabolic Cycle pathway
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