New strategy for removing acetic acid as a by-product during L-tryptophan production

Abstract

Acetic acid, an important by-product of L-tryptophan production by Escherichia coli, is detrimental to cell growth and tryptophan accumulation. To reduce acetic acid accumulation and further improve the production of L-tryptophan, an acetyl-coenzyme A (acetyl-CoA) synthase (ACS) with high synthetic activity is needed. In this study, E. coli ACS was found to have a superior catalytic capability compared to that of other bacterial strains. To reduce the accumulation of acetic acid by metabolic overflow, a higher expression level of ACS was obtained by promoter engineering and copy number optimisation. Furthermore, reconstruction of the tri-carboxylic acid cycle (TCA) not only enhanced the supply of phosphoenolpyruvate (PEP), which is one of the precursors required for tryptophan synthesis, but also improved bacterial growth. The final engineered strain T16 accumulated 54.6 g/L of L-tryptophan, 11.4% more than the original strain T03, with an accumulation of only 1.4 g/L acetic acid after fermentation, which reduced by 74.5% compared to T03.