Abstract
The thermophilic acetogen Thermacetogenium phaeum uses the Wood-Ljungdahl pathway (WLP) in both directions, either for the production of acetate from various compounds or for the oxidation of acetate in syntrophic cooperation with methanogens. In this study, energy–conserving enzyme systems in T. phaeum were investigated in both metabolic directions. A gene cluster containing a membrane-bound periplasmically oriented formate dehydrogenase directly adjacent to putative menaquinone synthesis genes was identified in the genome. The protein products of these genes were identified by total proteome analysis, and menaquinone MK-7 had been found earlier as the dominant quinone in the membrane. Enzyme assays with membrane preparations and anthraquinone-2,6-disulfonate as electron acceptor verified the presence of a quinone-dependent formate dehydrogenase. A quinone-dependent methylene–THF reductase is active in the soluble fraction and in the membrane fraction. From these results we conclude a reversed electron transport system from methyl-THF oxidation to CO2 reduction yielding formate as reduced product which is transferred to the methanogenic partner. The redox potential difference between methyl-THF (Eo’ = −200 mV) and formate (Eo’ = −432 mV) does not allow electron transfer through syntrophic formate removal alone. We postulate that part of the ATP conserved by substrate-level phosphorylation has to be invested into the generation of a transmembrane proton gradient by ATPase. This proton gradient could drive the endergonic oxidation of methyl-THF in an enzyme reaction similar to the membrane-bound reversed electron transport system previously observed in the syntrophically butyrate-oxidizing bacterium Syntrophomonas wolfei. To balance the overall ATP budget in acetate oxidation, we postulate that acetate is activated through an ATP-independent path via aldehyde:ferredoxin oxidoreductase (AOR) and subsequent oxidation of acetaldehyde to acetyl-CoA.
Highlights
The Wood-Ljungdahl pathway (WLP) or reductive acetylCoA pathway is the central pathway in acetogens and most strictly anaerobic acetate-oxidizing bacteria (AOB)
The WLP was investigated in depth the mechanism of energy conservation of most acetogens and AOB remained unclear since no net ATP is gained in this pathway by substrate level phosphorylation (Schuchmann and Müller, 2014)
As described earlier (Keller et al, 2019), syntrophic cultures of T. phaeum with M. thermautotrophicus grown with 40 mM acetate needed 21 days to reach early stationary phase with a doubling time of 42 h as described before (Keller et al, 2019)
Summary
The Wood-Ljungdahl pathway (WLP) or reductive acetylCoA pathway is the central pathway in acetogens and most strictly anaerobic acetate-oxidizing bacteria (AOB). The thermophile M. thermoacetica was shown to have a heterohexameric methylene-THF reductase (MTHFR) (Mock et al, 2014) which does not catalyze the reduction of methyleneTHF with NADH. It was proposed that the MTHFR could be coupled via formate dehydrogenase to the Ech hydrogenase, similar to a membrane-bound formate hydrogenlyase complex found in Escherichia coli. This system could be used to create a proton gradient across the membrane and conserve energy during acetogenesis (Mock et al, 2014). A. woodii lacks this putatively proton translocating system and instead has a heterotrimeric NADHoxidizing methylene-tetrahydrofolate (THF) reductase which is not coupled to energy conservation (Bertsch et al, 2015)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
