Xylene is the "X" in the BTEX chemicals (Benzene, Toluene, Ethylbenzene, and Xylene) which are volatile organic compounds, common constituents of gasoline, and common groundwater contaminants. Nearly all para-xylene recovered from petroleum in the US is consumed in the manufacture of terephthalic acid (PTA), which is used in the production of polyester fiber, resin, and film. Some para-xylene is used as a solvent and in the manufacture of di-paraxylene and herbicides. United States industry consumed over 6 million pounds of para-xylene in 1995.
The metabolic enzymes in this pathway have been shown to have similar specificty for toluene, para-xylene, and meta-xylene. Davey and Gibson (1974) report that para-xylene is degraded to 4-methylcatechol by the oxidation of a methyl substituent to an alcohol group and the addition of another alcohol group. Murray and Duggleby (1972) report that 4-methylcatechol is degraded by the meta (or alpha-ketoacid) pathway, in which the aromatic ring of 4-methylcatechol is cleaved in the meta-position. Gunther and Schlosser (1994) report that 4-methylcatechol is also degraded by ortho-cleavage to 4-methylmuconolactone. Ralstonia eutropha JMP134 degrades 4-methylcatechol via this route, converting 4-methylmuconolactone to 4-methyl-3-oxoadipate in several steps (Erb et al., 1998). Rhodococcus sp. strain YU6 is able to metabolize o-xylene via direct ring oxidation and meta-cleavage (Jang et al., 2005). The m-xylene and o-xylene pathways are also included in the EAWAG-BBD.
The following is a text-format p-Xylene pathway map. An organism which can initiate the pathway is 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 (16k) format.
p-Xylene p-Xylene Pseudomonas putida Rhodococcus sp. strain YU6 | | | xylene | p-xylene | monooxygenase v dioxygenase | | v v p-Methylbenzyl Alcohol 3,6-Dimethylcatechol | | | benzyl alcohol | 3,6-dimethylcatechol | dehydrogenase | 2,3-dioxygenase | | v v p-Tolualdehyde 2-Hydroxy-3-methyl- | 6-oxohepta-2,4-dienoate | benzaldehyde | | dehydrogenase v C | | v v p-Toluate CO2 | | benzoate | 1,2-dioxygenase | v 4-Methylcyclohexa-3,5-diene- 1,2-cis-diol-1-carboxylic acid | | | 4-methylcyclohexa-3,5-diene-1,2-cis-diol- | 1-carboxylic acid dehydrogenase from the | 2-Methyl- v from the naphthalene ------> 4-Methylcatechol <---------------- Toluene-4-sulfonate Pathway | Pathway | +--------------+--------------+ Ralstonia eutropha JMP134 | | catechol | | catechol 2,3-dioxygenase | | 1,2-dioxygenase | | v v 2-Hydroxy-5-methyl- 3-Methyl-cis,cis- cis,cis-muconic hexadienedioate ----------------+ Trichosporon cutaneum ATCC 58094 semialdehyde | | | | | | 2-hydroxymuconic | muconate | | semialdehyde | cycloisomerase | | hydrolase | | | | | v v | 2-Oxohex- 4-methylmucono- | muconate trans-4-enoate lactone | cycloisomerase from the | | | Testosterone | 2-oxopent-4-enoate | 4-methylmuconolactone | Pathway | hydratase | methylisomerase | | | | | | v v | +-> 4-Hydroxy-2-oxohexanoate 3-Methylmucono- <---------------+ | lactone | 4-hydroxy-2- | | oxovalerate | methylmuconolactone | aldolase | isomerase | | v v Pyruvate + Propanal 4-Methyl-3-oxoadipate | | enol-lactone | | | | | | 4-methyl-3-oxoadipate- | | | enol-lactone hydrolase | | | v v v Intermediary Intermediary 4-Methyl- Metabolism Metabolism 3-oxoadipate (KEGG) (KEGG) | | v A | | v CO2
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