Overcoming Energetic Barriers in Acetogenic C1 Conversion

Abstract

Currently one of the biggest challenges for society is to combat global warming. A solution to this global threat is the implementation of a CO2-based bioeconomy and a H2-based bioenergy economy. Anaerobic lithotrophic bacteria such as the acetogenic bacteria are key players in the global carbon and H2 cycle and thus prime candidates as driving forces in a H2- and CO2-bioeconomy. Naturally, they convert two molecules of CO2via the Wood-Ljungdahl pathway (WLP) to one molecule of acetyl-CoA which can be converted to different C2-products (acetate or ethanol) or elongated to C4 (butyrate) or C5-products (caproate). Since there is no net ATP generation from acetate formation, an electron-transport phosphorylation (ETP) module is hooked up to the WLP. ETP provides the cell with additional ATP, but the ATP gain is very low, only a fraction of an ATP per mol of acetate. Since acetogens live at the thermodynamic edge of life, metabolic engineering to obtain high-value products is currently limited by the low energy status of the cells that allows for the production of only a few compounds with rather low specificity. To set the stage for acetogens as production platforms for a wide range of bioproducts from CO2, the energetic barriers have to be overcome. This review summarizes the pathway, the energetics of the pathway and describes ways to overcome energetic barriers in acetogenic C1 conversion.

Genetic evidence reveals the indispensable role of the rseC gene for autotrophy and the importance of a functional electron balance for nitrate reduction in Clostridium ljungdahlii

“…In comparison to C. ljungdahlii, the RNF complex of A. woodii uses sodium ions instead of protons to generate the chemiosmotic gradient, which is then consumed by a sodium-dependent F1FO ATPase to generate ATP (28,29). Overall, this further confirms the meticulous differences in the energy conservation and redox balancing in different acetogens (2), which have to be considered to apply acetogens for biotechnological purposes.…”

“…Thus, the energy balance of the Wood-Ljungdahl pathway alone is net zero (10). All required cellular energy for the anabolism of the microbes during autotrophy is generated via membrane-coupled phosphorylation (2). In C. ljungdahlii, the membrane-bound transhydrogenase Rhodobacter nitrogen fixation (RNF) complex (11,12) utilizes two electrons from the oxidation of reduced ferredoxin to reduce NAD + to NADH, while simultaneously one proton is translocated across the membrane (10,13).…”

“…Acetogenic bacteria (i.e., acetogens), such as Clostridium ljungdahlii, maintain autotrophic growth with mixtures of the gaseous substrates carbon dioxide, carbon monoxide, and hydrogen as carbon and energy sources (1,2). The pathway that allows carbon fixation for autotrophic growth in acetogens is the Wood-Ljungdahl pathway (3,4).…”

A Heterodimeric Reduced-Ferredoxin-Dependent Methylenetetrahydrofolate Reductase from Syngas-Fermenting Clostridium ljungdahlii

“…C. ljungdahlii is one of the most promising strains for fermenting syngas to produce biofuels and biocommodities ( 14 ). Methylenetetrahydrofolate reductase (MTHFR), a key enzyme in the WLP, has attracted considerable attention ( 15 – 17 ), because in the WLP, the methylene-THF/methyl-THF couple has the highest redox potential (E 0 ′ = −200 mV) and is considered to be an important energy-conserving site ( 18 , 19 ). In some acetogens, the MTHFRs have been characterized.…”

“…In recent years, two hypotheses about the MTHFR from C. ljungdahlii / C. autoethanogenum have been proposed. One is that it is an NADH- and Fd-dependent electron-bifurcating enzyme, and the other is that it is NADH-dependent but non-electron-bifurcating ( 16 , 17 , 19 , 27 ). However, the binding site of NAD(P)H was not detected on MetFV in all the multimeric MTHFRs from acetogens, while it was found on the RnfC subunit of the enzyme from A. woodii and the HdrA subunit of M. thermoacetica ( 22 , 23 ).…”