Towards acetone-uncoupled biofuels production in solventogenic Clostridium through reducing power conservation

Microbial production of butanol by solventogenic Clostridium has long been complicated with the formation of acetone as an unwanted product, which causes poor product yields and creates a most important problem concerning substrate transformation. Intensive attempts concentrate on carbon conversion pathways to eliminate acetone, but have actually achieved little so far. Here, we believe microbial product distribution can largely depend on how the cell plays its energetic cofactors in central metabolism, and demonstrate that by introducing a synthetic 2,3-butanediol synthesis pathway in Clostridium acetobutylicum as an NADH-compensating module to readjust the reducing power at a systems level, the production of acetone can be selectively and efficiently eliminated (< 0.3 g/L). H₂ evolution was reduced by 78%, and the total alcohol yield was strikingly increased by 19% to 0.44 g/g glucose, much higher than those yet reported for butanol fermentation. These findings highlight that it is the loss of reducing power rather than typically manipulated solventogenesis genes that dominates acetone formation. Further study revealed that the NADH-module triggered apparent regulation of pathways involved in electron transfer and reducing power conservation. The study also suggested the key to conservation of intracellular reducing power might essentially lie in the intermediate processes in central metabolism that are related to redox partners, butyrate or C₄ branches, and possibly NADH and NADPH specificity. This study represents the first effective redox-based configuration of C. acetobutylicum and provides valuable understandings for redox engineering of native Clostridium species towards advanced production of biofuels and alcohols.