This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Synonyms: 4-nitrotoluene catabolism I
|Superclasses:||Degradation/Utilization/Assimilation → Aromatic Compounds Degradation → Nitroaromatic Compounds Degradation → Nitrotoluene Degradation → 4-Nitrotoluene Degradation|
Expected Taxonomic Range: Proteobacteria
Nitroaromatic compounds such as nitrophenols, nitrotoluenes and nitrobenzoates, are used in the synthesis of pesticides, plasticizers, dyes, pharmaceuticals, and explosives. These compounds are contaminants of waste waters, rivers and groundwater, and of the atmosphere [Leuenberger88]. In most cases nitroaromatic compounds are highly toxic to living organisms, and several of them have been listed as priority pollutants by the U.S. Environmental Protection Agency [Callahan79]. 4-nitrotoluene is a precursor to the explosive 2,4,6-trinitrotoluene (TNT), which is a major product of the explosives industry. Many sites are contaminated by TNT and by mono- and dinitrotoluenes [RhysWilliams93].
Despite their toxicity, these compounds can be degraded by several microorganisms. These degradation pathways almost always start with the removal or replacement of the nitro group [MarvinSikkema94]. The mechanisms in which bacteria are able to remove the nitro group can be divided into two broad categories - oxidative and reductive. In oxidative pathways the nitro group is released in the form of nitrite, usually catalyzed by a mono- or di-oxygenase enzymes. For examples, see 2-nitrotoluene degradation and 4-Nitrotoluene Degradation. In reductive pathways the nitro group is reduced via a nitroso intermediate to a hydroxylamino group, and sometime further to an amino group, and is released in the form of ammonia. For examples see 4-nitrotoluene degradation II and 4-nitrobenzoate degradation.
About This Pathway
Pseudomonas putida TW3 can metabolize 4-nitrotoluene via 4-nitrobenzoate to protocatechuate, which is subsequently broken down to central metabolites. The first part of this process (up to 4-nitrobenzoate) is very similar to the process with which P. putida catabolizes toluene via a set of enzymes encoded by the xyl genes, which are present on the TOL plasmid (see the pathway toluene degradation to benzoate). In fact, toluene is an alternative substrate for this pathway.
Three of the genes encoding enzymes in this pathway ( ntnA, ntnM and ntnC) are very similar to the xylAMC genes [James98]. However, a fourth gene in this operon, encoding 4-nitrobenzyl alcohol oxidase (ntnB), which was originally similar to the xylB gene, has been disrupted by foreign DNA and lost its functionallity. This function has been replaced in strain TW3 by a novel NAD(P)+-independent alcohol dehydrogenase, which is encoded by ntnD, a gene that bears no resemblence to the xylB gene [James00].
An almost identical pathway is found Pseudomonas sp. NT4 [Haigler93]. Strains TW3 and 4NT differ only in that the novel 4-nitrobenzyl alcohol dehydrogenase in strain TW3 is not dependent on the cofactors NAD+ or NADP+, whereas the enzyme from strain 4NT (which has no insertion, and thus is almost identical to the benzyl alcohol dehydrogenase XylB from the TOL pathway) is NAD+ dependent .
In this pathway, P. putida retains the nitro group during a sequential oxidation of the methyl group to form 4-nitrobenzoate. The nitro group is subsequently released as ammonia during protocatechuate formation (see the pathway "4-nitrobenzoate degradation").
Variants: 4-nitrotoluene degradation II
Callahan79: Callahan, M. A., Slimak, M. W., Gabel, N. W., May, J. P., Fowler, C. F., Freed, J. R., Jennings, P., Durfee, R. L., Whitmore, F. C., Maestri, B., Mabey, W. R., Holt, B. R., Gould, C. (1979). "Water-related environmental fate of 129 priority pollutants." EPA report-440/4-79-029b. U.S. Environmental Protection Agency, Washington, D.C.
James00: James KD, Hughes MA, Williams PA (2000). "Cloning and expression of ntnD, encoding a novel NAD(P)(+)-independent 4-nitrobenzyl alcohol dehydrogenase from Pseudomonas sp. Strain TW3." J Bacteriol 2000;182(11);3136-41. PMID: 10809692
James98: James KD, Williams PA (1998). "ntn genes determining the early steps in the divergent catabolism of 4-nitrotoluene and toluene in Pseudomonas sp. strain TW3." J Bacteriol 1998;180(8);2043-9. PMID: 9555884
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