1,1,1-Trichloroethane (TCA) is a synthetic organic solvent widely used in industrial processes and is a major environmental pollutant commonly found in soil, groundwater, and the atmosphere. TCA is present in at least 696 of the 1430 National Priorities List sites identified by the U.S. Environmental Protection Agency (EPA). Because of TCA’s adverse effects on human health, the EPA has set a maximum contaminant level of 200 ppb in drinking water. TCA is also listed as an ozone-depleting substance by the United Nations Environment Programme. Even when released into soil or leached into groundwater, the primary environmental fate of TCA is volatilization to the atmosphere, where it interacts with ozone and contributes to the erosion of the ozone layer.
TCA can be reductively dechlorinated under anaerobic conditions to 1,1,-dichloroethane (DCA) by Desulfobacterium autotrophicum (C. Egli, R. Scholtz, A. M. Cook, & T. Leisinger (1987) FEMS Microbiol. Lett. 43:257-261.) and a Clostridium sp. (Galli and McCarty, 1989). Reductive dechlorination of TCA to DCA and chloroethane (CA) has been documented in methanogenic consortia (J. H. de Best, A. Hage, H. J. Doddema, D. B. Janssen, & W. Harder (1999) Appl. Environ. Microbiol. 51:277-283.) and a putative Dehalobacter sp. (designated strain TCA1) (Sun et al., 2002). Chen et al. (1999) reported the complete dehalogenation of TCA, DCA, and CA to ethane by an anaerobic consortium in sewage sludge, but ethane was a minor product in all cases (< 5%, < 3%, and < 1% of the initial substrate degraded to ethane, respectively). The enzymes that catalyze the reductive dehalogenation of TCA have not been identified.
TCA can be aerobically transformed to 2,2,2-trichloroethanol by the methanotroph Methylosinus trichosporium OB3b expressing a soluble methane monooxygenase (Oldenhuis et al., 1989), and by ethane-utilizing Mycobacterium spp. isolated from soil (Yagi et al., 1999). Further study revealed that one of the Mycobacterium strains produced trichloroacetic acid and dichloroacetic acid as minor products (12.7% and 1.2%, respectively, vs. 85.5% of TCA converted to 2,2,2-trichloroethanol) during the aerobic transformation of TCA (Hashimoto et al., 2002). Oldenhuis et al (1989) found that M. trichosporium expressing soluble methane monooxygenase completely dechlorinated DCA, but degradation intermediates were not identified. Kim et al. (2000) observed partial dechlorination of DCA (~37% of initial solvent) by an aerobic butane-grown mixed culture. In Nitrosomonas europea, an ammonia monooxygenase catalyzed the oxidation of CA to acetaldehyde, with 2-Chloroethanol produced as a minor product (1-2% 2-chloroethanol vs. 98% acetaldehyde) (Rasche et al., 1990).
The following is a text-format 1,1,1-Trichloroethane pathway map. Organisms which can initiate the pathway are given, but the 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 (8k) format.
1,1,1-Trichloroethane 1,1,1-Trichloroethane Anaerobic consortia Methylosinus trichosporium OB3b Dehalobacter sp. strain TCA1 Mycobacterium spp. | | | 1,1,1-trichloroethane | soluble methane | reductive dehalogenase | monooxygenase | | v v 1,1-Dichloroethane 2,2,2-Trichloroethanol | | | 1,1-dichloroethane | | reductive dehalogenase | | | v v Chloroethane to the | Trichloroethylene | ammonia Pathway | monooxygenase | v [1-Chloroethanol] | | | | v Acetaldehyde | | | | v Intermediary Metabolism (KEGG)
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