Isopropanol production via the thermophilic bioconversion of sugars and syngas using metabolically engineered Moorella thermoacetica

Isopropanol (IPA) is a commodity chemical used as a solvent or raw material for polymeric products, such as plastics. Currently, IPA production depends largely on high-CO₂-emission petrochemical methods that are not sustainable. Therefore, alternative low-CO₂ emission methods are required. IPA bioproduction using biomass or waste gas is a promising method.

Moorella thermoacetica, a thermophilic acetogenic microorganism, was genetically engineered to produce IPA. A metabolic pathway related to acetone reduction was selected, and acetone conversion to IPA was achieved via the heterologous expression of secondary alcohol dehydrogenase (sadh) in the thermophilic bacterium. sadh-expressing strains were combined with acetone-producing strains, to obtain an IPA-producing strain. The strain produced IPA as a major product using hexose and pentose sugars as substrates (81% mol-IPA/mol-sugar). Furthermore, IPA was produced from CO, whereas acetate was an abundant byproduct. Fermentation using syngas containing both CO and H₂ resulted in higher IPA production at the specific rate of 0.03 h⁻¹. The supply of reducing power for acetone conversion from the gaseous substrates was examined by supplementing acetone to the culture, and the continuous and rapid conversion of acetone to IPA showed a sufficient supply of NADPH for Sadh.

The successful engineering of M. thermoacetica resulted in high IPA production from sugars. M. thermoacetica metabolism showed a high capacity for acetone conversion to IPA in the gaseous substrates, indicating acetone production as the bottleneck in IPA production for further improving the strain. This study provides a platform for IPA production via the metabolic engineering of thermophilic acetogens.