During the production of the pesticide Lindane (gamma-hexachlorocyclohexane (HCH)), about 85% of the product consisted of isomers such as alpha-, beta- and delta-HCH. They were discarded into the environment, causing serious soil pollution. Aerobic biodegradation mineralizes alpha-, gamma-, and delta-HCH, whereas beta-HCH persists. Therefore, the degradation of beta-HCH and the other isomers was studied under anaerobic conditions by Peter Middeldorp (Environmental Science and Technology (1996) 30(7):2345-2349), in order to develop an anaerobic soil treatment process. beta-HCH was found to anaerobically biodegrade to benzene and chlorobenzene, both of which can undergo further degradation aerobically. The pathway from beta-HCH to benzene and chlorobenzene is the first anaerobic pathway in the EAWAG-BBD. This pathway can be carried out by a Dehalobacter sp. in coculture with a Sedimentibacter sp. (van Doesburg et al., 2005).
A second, aerobic, beta-HCH metabolic pathway is mediated by haloalkane dehalogenase LinB in Sphingomonas paucimobilis UT26. The stereochemistry of the 2,3,4,5,6-pentachlorocyclohexanol has not been determined; the stereoisomer shown is based on beta-HCH. The pentachlorocyclohexane is not further metabolized (Nagata et al., 2004).
The biodegradation pathway for gamma-hexachlorocyclohexane is found elsewhere.
The following is a text-format beta-1,2,3,4,5,6-hexachlorocyclohexane pathway map. Follow the links for more information on compounds or reactions. This map is also available in graphic (13k) format.
beta-1,2,3,4,5,6-Hexachlorocyclohexane beta-1,2,3,4,5,6-Hexachlorocyclohexane Dehalobacter sp. Sphingomonas paucimobilis UT26 Sedimentibacter sp. | | | haloalkane | A | dehalogenase | | v v +--- delta-3,4,5,6-Tetrachlorocyclohexene ---+ 2,3,4,5,6-Pentachlorocyclohexanol | | | B | C | | v v [5,6-Dichloro- [5,6-Dichloro- 1,3-cyclohexadiene] 1,3-cyclohexadiene] | | | | from the v v Organomercury ----> Benzene Chlorobenzene and | | Triphenyltin | benzene 1,2-dioxygenase | Pathways | | | v v to the cis-Dihydrobenzenediol Chlorobenzene Pathway | | cis-1,2-dihydrobenzene- | 1,2-diol dehydrogenase | v Catechol | | v to the Nitrobenzene Pathway
Page Author(s): Peter Middeldorp, Ryan McLeish, Meaghan Fitzgerald and Michael Turnbull
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