Biochemistry of the Anaerobic Degradation of Non-Methane Alkanes

Abstract:

Alkanes (saturated hydrocarbons) are naturally wide-spread compounds that are chemically unreactive. Whereas aerobic alkane-utilizing microorganism have been investigated since the early 20th century, anaerobic alkane-degraders became known relatively recently. Several nitrate- or sulfate-reducing bacteria able to growth with alkanes as sole organic substrates have been described. Furthermore, anaerobic alkane degradation was demonstrated in enriched bacterial communities under conditions of nitrate reduction, sulfate reduction, or methanogenesis. This article presents an overview of the anaerobic metabolism of non-methane alkanes. The anaerobic activation of alkanes in the absence of oxygen presents a “metabolic challenge”. Metabolite analyses suggest that many anaerobic alkane degraders make use of a radical-catalyzed carbon-carbon addition to fumarate yielding alkylsuccinates; usually, the subterminal carbon atom of the alkane is activated, resulting in (1-methylalkyl)-succinates. Isotope labeling studies suggest a further metabolism of (1-methylalkyl)succinyl-CoA via carbon skeleton rearrangement and decarboxylation yielding 4-methyl-branched fatty acid thioesters; the latter can undergo β-oxidation. The pathway involves a delicate stereochemistry. Furthermore, there are hints at an alternative, still unexplored pathway for anaerobic alkane degradation with the introduction of a carbon dioxide-derived carboxyl group at carbon-3.