2,4-Dichlorophenoxyacetic acid (2,4-D), a chlorinated phenoxy compound, functions as a systemic herbicide and is used to control many types of broadleaf weeds. It is used in cultivated agriculture and in pasture and rangeland applications, forest management, home and garden situations and for the control of aquatic vegetation.
The wide use of this compound has prompted interest in its biodegradation. Alcaligenes eutrophus and other bacteria can degrade 2,4-D through 2,4-dichlorophenol, 3,5-dichlorocatechol, and ortho cleavage of this catechol (Fukumori & Hausinger, 1993). Alternatively, Axotobacter chroococcum first removes the chloride in the 2 position to produce 4-chlorophenoxyacetate, 4-chlorophenol, and 4-chlorocatechol, again with ortho cleavage of this catechol (Balajee and Mahadevan, 1993). 2,4-D biodegradation by this pathway branch may produce a byproduct antibiotic protoanemonin, which can be degraded to cis-acetylacrylate by a dienelactone hydrolase of Pseudomonas sp. strain B13 (Brückmann et al., 1998). Comamonas testosteroni JH5 can cleave 4-chlorocatechol by a meta pathway, forming 5-chloro-2-hydroxymuonate semialdehyde which can be further transformed by a 2-hydroxymuconic semialdehyde dehydrogenase or a 2-hydroxymuconic semialdehyde hydrolase. Dehydrogenation is the major route, however the hydrolase step is also of physiological significance. Both branches lead to 5-chloro-2-oxopent-4-eneoate. This does not accumulate, and stoichiometric chloride is quickly released, but the exact route of chloride elimination is not known (Hollender et al., 1997). Pseudomonas cepacia P166 can move the chloride in 5-chloro-2-oxopent-4-eneoate through 5-chloro-4-hydroxy-2-oxopentanate to chloroacetate. Chloroacetate accumulates transiently, and stoichiometric dehalogenation is observed (Arensdorf & Focht, 1995).
The following is a text-format 2,4-D degradation 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.
Graphical Map (12k) | Graphical Map (18k) 2,4-Dichloro- 2,4-Dichloro- phenoxyacetate (2,4-D) phenoxyacetate (2,4-D) Alcaligenes eutrophus Azotobacter chroococcum Ralstonia eutropha Flavobacterium sp. | Arthrobacter sp. | | | | 2,4-D/alpha- | 2,4-D dehalogenase | ketoglutarate | | dioxygenase | | | from the |\ v Nitrofen | \ 4-Chlorophenoxyacetate Pathway | \ | | | \ | | | \ | 4-chlorophenoxy- +---+ | \ /| acetate monooxygenase | | \ / | | | \ / | v v \ / v 2,4-Dichlorophenol \ / 4-Chlorophenol | \ / | | Glyoxylate | | | | | 2,4-dichlorophenol | | 2,4-dichlorophenol | hydroxylase | | hydroxylase | v | | Intermediary | | Metabolism | | (KEGG) | v v 3,5-Dichlorocatechol 4-Chlorocatechol <------------- from the | / \ 2,4-Dichloroaniline Pathway | / \ | | C. testosteroni JH5 | | | | 3,5-dichlorocatechol | | 4-chlorocatechol | 1,2-dioxygenase | | 2,3-dioxygenase | | | | | | 2-hydroxymuconic semialdehyde v | v dehydrogenase 2,4-Dichloro-cis, from the | 5-Chloro-2-hydroxymuconic --------------------> 5-Chloro-2-hydroxy- cis-muconate 1,4-Dichloro- | semialdehyde muconate | benzene Pathway | | | | | | | 2-hydroxymuconic | | dichloromuconate | | 4-chlorocatechol | semialdehyde | | cycloisomerase | | 1,2-dioxygenase | hydrolase | | | | | 4-oxalocrotonate | v | v v decarboxylase | trans-2-Chloro- <----------+ 3-Chloro-cis, 5-Chloro-2-oxo- <----------------------------------+ dienelactone +------ cis-muconate pent-4-enoate | | | | | | | Ps. cepacia P166 | | muconate | | | 2-chlorodiene- | cyclo- | | 5-chloro-2-oxopent-4-enoate | lactone | isomerase | | hydratase | isomerase | | | | v | v | Protoanemonin | 5-Chloro-4-hydroxy- | Pseudomonas sp. | 2-oxopentanate | strain B13 | | | | | | 5-chloro-4-hydroxy-2-oxopentanate v | diene- | | aldolase cis-2-Chloro- | lactone | | dienelactone | hydrolase | | | v | v | cis-Acetyl- | Chloroacetaldehyde + Pyruvate | acrylate | | | | chlorodiene- | chloromuconate | | | lactone | cycloisomerase | | | hydrolase | | | | | | | v v v v 2-Chloromaleylacetate cis-4-Carboxymethylene- to the Intermediary \ but-2-en-4-olide 1,2-Dichloroethane Metabolism \ | Pathway (KEGG) \ | \ maleylacetate | dienelactone \ reductase | hydrolase \ | \ | \ | \ v Malelyacetate | | | maleylacetate | reductase | | v 3-Oxoadipate | | | | | v Intermediary Metabolism (KEGG)
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