Butyrate production under aerobic growth conditions by engineered Escherichia coli

Butyrate is an important industrial platform chemical. Although several groups have reported butyrate production under oxygen-limited conditions by a native producer, Clostridium tyrobutylicum, and by a metabolically engineered Escherichia coli, efforts to produce butyrate under aerobic growth conditions have met limited success. Here, we constructed a novel butyrate synthetic pathway that functions under aerobic growth conditions in E. coli, by modifying the 1-butanol synthetic pathway reported previously. The pathway consists of phaA (acetyltransferase) and phaB (NADPH-dependent acetoacetyl-CoA reductase) from Ralstonia eutropha, phaJ ((R)-specific enoyl-CoA hydratase) from Aeromonas caviae, ter (trans-enoyl-CoA reductase) from Treponema denticola, and endogenous thioesterase(s) of E. coli. To evaluate the potential of this pathway for butyrate production, culture conditions, including pH, oxygen supply, and concentration of inorganic nitrogen sources, were optimized in a mini-jar fermentor. Under the optimal conditions, butyrate was produced at a concentration of up to 140 mM (12.3 g/L in terms of butyric acid) after 54 h of fed-batch culture.