Anaerobic degradation of n -hexane in a denitrifying bacterium: Further degradation of the initial intermediate (1-methylpentyl)succinate via C-skeleton rearrangement

Abstract

The anaerobic degradation pathway of the saturated hydrocarbon n-hexane in a denitrifying strain (HxN1) was examined by gas chromatography-mass spectrometry of derivatized extracts from cultures grown with unlabeled and deuterated substrate; several authentic standard compounds were included for comparison. The study was focused on possible reaction steps that follow the initial formation of (1-methylpentyl)succinate from n-hexane and fumarate. 4-Methyloctanoic, 4-methyloct-2-enoic, 2-methylhexanoic, 2-methylhex-2-enoic and 3-hydroxy-2-methylhexanoic acids (in addition to a few other methyl-branched acids) were detected in n-hexane-grown but not in n-hexanoate-grown cultures. Labeling indicated preservation of the original carbon chain of n-hexane in these acids. Tracing of the deuterium label of 3- d1-(1-methylpentyl)succinate in tentative subsequent products indicated a deuterium/carboxyl carbon exchange in the succinate moiety. This suggests that the metabolism of (1-methylpentyl)succinate employs reactions analogous to those in the established conversion of succinyl-CoA via methylmalonyl-CoA to propionyl-CoA. Accordingly, a pathway is proposed in which (1-methylpentyl)succinate is converted to the CoA-thioester, rearranged to (2-methylhexyl)malonyl-CoA and decarboxylated (perhaps by a transcarboxylase) to 4-methyloctanoyl-CoA. The other identified fatty acids match with a further degradation of 4-methyloctanoyl-CoA via rounds of conventional beta-oxidation. Such a pathway would also allow regeneration of fumarate (for n-hexane activation) from propionyl-CoA formed as intermediate and hence present a cyclic process.

Degradation of o -xylene and m -xylene by a novel sulfate-reducer belonging to the genus Desulfotomaculum

“…The subsequent degradation steps have been proposed on the basis of molecular investigations of the bbs operon (Leuthner and Heider 2000) and it was hypothesize that succinyl-CoA is cleaved from benzoylsuccinyl-CoA to be recycled for further addition reactions and results in the formation of the central intermediate benzoyl-CoA. In contrast, recent investigations on anaerobic degradation of n-hexan proposed a decarboxylation reaction prior to stepwise β-oxidation (Widdel and Rabus 2001;Wilkes et al 2002). In our studies however, the putative 3-o-toluolyl propionic acid detected in supernatants of strain OX39 might be a dead-end metabolite that derives from spontaneous decarboxylation of 2-methylbenzoylsuccinic acid rather than a transient metabolite of o-xylene degradation.…”

Comparative analysis of metagenomes from three methanogenic hydrocarbon-degrading enrichment cultures with 41 environmental samples

“…Following alkane activation (pathway 7; Figure 2), alkylsuccinates are activated to a CoA derivative, followed by carbon-skeleton rearrangement and decarboxylation (Wilkes et al, 2002). In D. alkenivorans AK-01, these reactions are presumably catalyzed by AssK, a putative methylmalonyl-CoA mutase, and a putative methylmalonyl-CoA carboxyl transferase before β-oxidation (Callaghan et al, 2012).…”

Reactivity and fate of secondary alkane sulfonates (SAS) in marine sediments

“…The next stage in the degradation pathway (Fig. 6C) results in the appearance of Me-SC, due to a Cskeleton rearrangement similar to that described for the anaerobic degradation of alkanes (Wilkes et al, 2002). Briefly, it consists on the migration of the carboxyl group from C-3 to C-2 and a subsequent decarboxylation due to the release of the carboxyl group in C-1.…”

“…During this sequence, a C3 unit is released ( Fig. 6D and E), which may be used for the regeneration of fumarate and later activation of other SAS molecules, according to Wilkes et al (2002) and Davidova et al (2005).…”