title: Metabolism of Hydrocarbons in n-Alkane-Utilizing Anaerobic Bacteria.
abstract: The glycyl radical enzyme-catalyzed addition of n-alkanes to fumarate creates a C-C-bond between two concomitantly formed stereogenic carbon centers. The configurations of the two diastereoisomers of the product resulting from n-hexane activation by the n-alkane-utilizing denitrifying bacterium strain HxN1, i.e. (1-methylpentyl)succinate, were assigned as (2S,1'R) and (2R,1'R). Experiments with stereospecifically deuterated n-(2,5-2H2)hexanes revealed that exclusively the pro-S hydrogen atom is abstracted from C2 of the n-alkane by the enzyme and later transferred back to C3 of the alkylsuccinate formed. These results indicate that the alkylsuccinate-forming reaction proceeds with an inversion of configuration at the carbon atom (C2) of the n-alkane forming the new C-C-bond, and thus stereochemically resembles a SN2-type reaction. Therefore, the reaction may occur in a concerted manner, which may avoid the highly energetic hex-2-yl radical as an intermediate. The reaction is associated with a significant primary kinetic isotope effect (kH/kD ≥3) for hydrogen, indicating that the homolytic C-H-bond cleavage is involved in the first irreversible step of the reaction mechanism. The (1-methylalkyl)succinate synthases of n-alkane-utilizing anaerobic bacteria apparently have very broad substrate ranges enabling them to activate not only aliphatic but also alkyl-aromatic hydrocarbons. Thus, two denitrifiers and one sulfate reducer were shown to convert the nongrowth substrate toluene to benzylsuccinate and further to the dead-end product benzoyl-CoA. For this purpose, however, the modified β-oxidation pathway known from alkylbenzene-utilizing bacteria was not employed, but rather the pathway used for n-alkane degradation involving CoA ligation, carbon skeleton rearrangement and decarboxylation. Furthermore, various n-alkane- and alkylbenzene-utilizing denitrifiers and sulfate reducers were found to be capable of forming benzyl alcohols from diverse alkylbenzenes, putatively via dehydrogenases. The thermophilic sulfate reducer strain TD3 forms n-alkylsuccinates during growth with n-alkanes or crude oil, which, based on the observed patterns of homologs, do not derive from a terminal activation of n-alkanes.
authors.affiliation: Institute for Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University Oldenburg, Oldenburg, Germany.
authors.forename: Heinz
authors.initials: H
authors.lastname: Wilkes
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Buckel
authors.forename: Bernard T
authors.initials: BT
authors.lastname: Golding
authors.forename: Ralf
authors.initials: R
authors.lastname: Rabus
mesh_terms: D000214:Acyl Coenzyme A / Q000378:metabolism; D000473:Alkanes / Q000737:chemistry* / Q000378:metabolism*; D000693:Anaerobiosis; D001421:Bacteria, Anaerobic / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism*; D001673:Biodegradation, Environmental; D053858:Metabolic Networks and Pathways; D010084:Oxidation-Reduction
chemical_list: D000214:Acyl Coenzyme A; D000473:Alkanes; C037817:benzoyl-coenzyme A