Stereochemical Investigations Reveal the Mechanism of the Bacterial Activation of n‐Alkanes without Oxygen

Abstract

Anaerobic growth of the bacterium strain HxN1 with n‐hexane gives nearly equal amounts of (2R,1′R)‐ and (2S,1′R)‐(1‐methylpentyl)succinate, which are formed by the radical addition of the hydrocarbon to fumarate (see scheme). The highly selective attack on the pro‐S hydrogen atom at C2 of n‐hexane is associated with inversion of the configuration at C2 during binding to fumarate and exhibits isotopic discrimination against a C2H bond.

Re-analysis of omics data indicates Smithella may degrade alkanes by addition to fumarate under methanogenic conditions

“…The very low expression of AWGX01000974 under caprylate conditions (Supplementary Figure S1) correlates with the low abundance of Smithella ME-1 in the caprylate culture versus overwhelming dominance in the hexadecane culture (Figure 4b in Embree et al, 2013), supporting our contention that assA expression correlates with growth of Smithella ME-1 on hexadecane in a methanogenic mixed culture. Further supporting our proposal, Embree et al (2013) detected genes encoding a-methylacylcoA racemase and methyl-malonyl-coA that are proposed to be involved in epimerization and carbon skeleton rearrangement of metabolic intermediates, respectively, in the proposed fumarate activation pathway used under nitrate-and sulfate-reducing conditions (Callaghan et al, 2012;Jarling et al, 2012). Genes for b-oxidation of fatty acids were also highly transcribed during growth on hexadecane (Embree et al, 2013), consistent with utilization of n-hexadecane via a fumarate activation pathway.…”

Anaerobic Activation of p -Cymene in Denitrifying Betaproteobacteria: Methyl Group Hydroxylation versus Addition to Fumarate

“…Identification of metabolites was based on comparison of GC retention times and mass spectra with those of authentic standards or from interpretation of mass spectra. For stereochemical assignment, arylsuccinates were derivatized with (R)-1-phenylethanamine and analyzed as recently described by Jarling et al (39). 4-Ethylbenzyl alcohol, 4-ethylbenzaldehyde, 4-ethylbenzoic acid, 4-isopropylbenzyl alcohol, 4-isopropylbenzaldehyde, 4-isopropylbenzoic acid, N-methyl-N-nitroso-p-toluenesulfonamide (Diazald), and sodium sulfate were purchased from Sigma-Aldrich, Germany.…”

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

“…However, because CoAsynthetases, along with methylmalonyl-CoA mutase and carboxylase (involved in the methylmalonylCoA pathway) are widespread in microbes, their presence cannot be solely associated with n-alkane degradation. It has been proposed that, before carbon skeleton rearrangement, the diastereomers produced from fumarate addition undergo epimerization catalyzed by alpha-methylacyl-CoA racemase to produce one intermediate for carbon skeleton rearrangement (Jarling et al, 2012). In a partial ass operon found in the genome of Peptococcaceae from SCADC (Tan et al, 2014B), putative genes involved in fumarate addition and downstream pathways clustered together, including alpha-methylacyl-CoA racemase, methyl malonyl-CoA mutase and β-oxidation, suggesting that alpha-methylacyl-CoA racemase may indeed be involved in n-alkane degradation.…”