Toluene (methylbenzene) is an aromatic hydrocarbon natural product of diagenic origin and an important commercial chemical. It is, for example, commonly used as a paint thinning agent and in other solvent applications. The BTEX mixtures referred to in bioremediation applications contain benzene, ethylbenzene, TOLUENE, and xylenes. The biodegradation of toluene has been well-studied at the molecular level and it, thus, serves as one of the principal models for understanding the mechanisms of bacterial benzene ring metabolism. This page shows only its aerobic pathway. The anaerobic degradation of toluene is documented elsewhere in the EAWAG-BBD.
An alternate pathway for the degradation of 3-hydroxytoluene (m-cresol) by P. alcaligenes and P. putida is presented in the m-Cresol Pathway.
Toluene degradation can be initiated by oxidation of the methyl group (Shaw and Harayama, 1992), ring monooxidation at positions 2, 3, or 4 (Shields et al., 1995, Olsen et al., 1994, and Yen et al., 1991), or ring 2,3-dioxidation (Wackett et al., 1988). The enzyme that starts the ring dioxidation branch of this pathway, toluene 1,2-dioxygenase, has many other catalytic abilities, which are documented in a table of the Reactions of Toluene 1,2-Dioxygenase.
The following is a text-format toluene degradation pathway. 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 (9 K) | Graphical Map (15 K) | Graphical Map (9 K) Toluene Toluene Toluene Toluene Toluene Ps. mendocina Ps. pickettii Ps. putida Ps. mendocina F1 (pWWO) or (pTOL) PK01 KR1 | | | | | | toluene | toluene | toluene | toluene | toluene | side-chain | 2-monooxygenase| 3-monooxygenase| dioxygenase| 4-monooxygenase | monooxygenase| | | | | (xylene | | to and from | | from the |monooxygenase)| | the m-Cresol | | Toluene-4-sulfonate | | | Pathway | | and Trinitrotoluene | | | ^ | | Pathways v v v | v v | Benzyl 2-Hydroxy- 3-Hydroxy- <-+ Toluene- 4-Hydroxy- <-----+ alcohol toluene toluene cis-dihydrodiol toluene | \ | / | | \ | / | | benzyl \ toluene | toluene / toluene | 4-cresol from the | alcohol \ 2-mono- | 3-mono- / cis-dihydrodiol | dehydrogenase Mandelate | dehydrogenase \ oxygenase| oxygenase/ dehydrogenase | (hydroxylating) Pathway | \ | / | | | \ | / | | | from the \ | / from the | | v m-Xylene \ v / 1-Methyl- v +>Benzaldehyde and o-Xylene--->3-Methyl-<---naphthalene 4-Hydroxy- | Pathways catechol Pathway benzaldehyde | | | | | | | benzaldehyde | catechol | 4-hydroxy- | dehydrogenase | 2,3-dioxygenase | benzaldehyde | | | dehydrogenase | | | | | | | | | v v v Benzoate cis,cis-2-Hydroxy- 4-Hydroxy- | 6-oxohepta-2,4-dienoate benzoate | | | | | | v | cis,cis-2-hydroxy- | to the | 6-oxohepta-2,4-dienoate | Benzoate | hydrolase | Pathway | | | | | | from the v v Carbazole and Cinnamate ------> cis-2-Hydroxypenta- to the Pathways 2,4-dienoate Vanillin | Pathway | | cis-2-hydroxypenta- | 2,4-dienoate | hydratase | | | v 4-Hydroxy-2-oxo- valerate | | 4-hydroxy-2-oxo- | valerate aldolase | v Acetaldehyde + Pyruvate | | | | | | | | | | v v Intermediary Intermediary Metabolism Metabolism (KEGG) (KEGG)
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