Oxidation of Methyl-Substituted Naphthalenes: Pathways in a Versatile Sphingomonas paucimobilis Strain

Abstract

Aromatic compounds with alkyl substituents are abundant in fossil fuels. These compounds become important environmental sources of soluble toxic products, developmental inhibitors, etc. principally through biological activities. To assess the effect of methyl substitution on the completeness of mineralization and accumulation of pathway products, an isolate from a phenanthrene enrichment culture,Sphingomonas paucimobilis 2322, was used. Washed cell suspensions containing cells grown on 2,6-dimethylnaphthalene in mineral medium were incubated with various mono-, di-, and trimethylnaphthalene isomers, and the products were identified and quantified by gas chromatography and mass spectrometry. The data revealed enzymes with relaxed substrate specificity that initiate metabolism either by methyl group monoxygenation or by ring dioxygenation. Congeners with a methyl group on each ring initially hydroxylate a methyl, and this is followed by conversion to a carboxyl; when there are two methyl groups on a single ring, the first reaction is aryl dioxygenation of the unsubstituted ring. Intermediates are channeled to primary ring fission via dihydrodiols to form methyl-substituted salicylates. Further evidence that there are multiple pathways comes from the fact that both phthalate and (methyl)salicylate are formed from 2-methylnaphthalene.

A molecular modeling analysis of polycyclic aromatic hydrocarbon biodegradation by naphthalene dioxygenase

“…However, a few variations in the catalyzed reactions have been observed. Transformations of several alkylated PAHs by Sphingomonas paucimobilis 2322 revealed the capability of that organism to perform ring dioxygenation or alkyl group monooxygenation as reported here, but monooxygenation of the methyl group of 2-methylnaphthalene was observed in addition to dioxygenation of the unsubstituted ring [27]. Similarly, an enzyme in Pseudomonas putida CSV86 has been observed to perform ring dioxygenation and methyl group monooxygenation on both 2-methylnaphthalene and 1-methylnaphthalene [28].…”

Bacterial Degradation of Aromatic Compounds

“…In general, methyl-/ethyl-naphthalenes and phenanthrenes are prevalent contaminants in the environment and, however, limited numbers of studies have been done in relation to bacterial degradation [ 28 , 125 , 126 ]. Catabolism of alkyl PAHs in aerobic bacteria suggests a very diversity of enzymes involved [ 125 ].…”

Quantitative structure‐activity relationships for kinetic parameters of polycyclic aromatic hydrocarbon biotransformation

“…Kinetics of the initial catabolic step depend on the kinetics of any of the following: The biotransformation reaction, preceding processes, or any co‐occurring processes. Assuming that in strain EPA505 a common dioxygenase with loose substrate specificity catalyzes the oxygenation of an aromatic ring or alkyl side chain [28,29], preceding processes include electron transport (e.g., by a flavoprotein and a ferredoxin) and membrane binding and transport, whereas co‐occurring processes could involve substrate efflux from the cell interior. Electron‐transport components of a dioxygenase, a multicomponent enzyme system, could become rate‐limiting in diminishing quantities; in our case, however, the variability in the observed kinetic parameters and the fact that the bacterial culture used in the kinetic experiments was induced dismiss this possibility.…”