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H2 drives metabolic rearrangements in gas-fermenting Clostridium autoethanogenum

Valgepea, Kaspar, de Souza Pinto Lemgruber, Renato, Abdalla, Tanus, Binos, Steve, Takemori, Nobuaki, Takemori, Ayako, Tanaka, Yuki, Tappel, Ryan, Köpke, Michael, Simpson, Séan Dennis, Nielsen, Lars Keld, Marcellin, Esteban
Biotechnology for biofuels 2018 v.11 no.1 pp. 55
Clostridium autoethanogenum, acetates, biomass, carbon, carbon dioxide, carbon monoxide, energy, energy conservation, ethanol, fermentation, formates, hydrogen, metabolomics, models, oxidation, proteome, proteomics, synthesis gas
BACKGROUND: The global demand for affordable carbon has never been stronger, and there is an imperative in many industrial processes to use waste streams to make products. Gas-fermenting acetogens offer a potential solution and several commercial gas fermentation plants are currently under construction. As energy limits acetogen metabolism, supply of H₂ should diminish substrate loss to CO₂ and facilitate production of reduced and energy-intensive products. However, the effects of H₂ supply on CO-grown acetogens have yet to be experimentally quantified under controlled growth conditions. RESULTS: Here, we quantify the effects of H₂ supplementation by comparing growth on CO, syngas, and a high-H₂ CO gas mix using chemostat cultures of Clostridium autoethanogenum. Cultures were characterised at the molecular level using metabolomics, proteomics, gas analysis, and a genome-scale metabolic model. CO-limited chemostats operated at two steady-state biomass concentrations facilitated co-utilisation of CO and H₂. We show that H₂ supply strongly impacts carbon distribution with a fourfold reduction in substrate loss as CO₂ (61% vs. 17%) and a proportional increase of flux to ethanol (15% vs. 61%). Notably, H₂ supplementation lowers the molar acetate/ethanol ratio by fivefold. At the molecular level, quantitative proteome analysis showed no obvious changes leading to these metabolic rearrangements suggesting the involvement of post-translational regulation. Metabolic modelling showed that H₂ availability provided reducing power via H₂ oxidation and saved redox as cells reduced all the CO₂ to formate directly using H₂ in the Wood–Ljungdahl pathway. Modelling further indicated that the methylene-THF reductase reaction was ferredoxin reducing under all conditions. In combination with proteomics, modelling also showed that ethanol was synthesised through the acetaldehyde:ferredoxin oxidoreductase (AOR) activity. CONCLUSIONS: Our quantitative molecular analysis revealed that H₂ drives rearrangements at several layers of metabolism and provides novel links between carbon, energy, and redox metabolism advancing our understanding of energy conservation in acetogens. We conclude that H₂ supply can substantially increase the efficiency of gas fermentation and thus the feed gas composition can be considered an important factor in developing gas fermentation-based bioprocesses.