Metabolic engineering of Escherichia coli for producing adipic acid through the reverse adipate-degradation pathway

Adipic acid is an important dicarboxylic acid mainly used for the production of nylon 6-6 fibers and resins. Previous studies focused on the biological production of adipic acid directly from different substrates, resulting in low yields and titers. In this study, a five-step reverse adipate-degradation pathway (RADP) identified in Thermobifida fusca has been reconstructed in Escherichia coli BL21 (DE3). The resulting strain (Mad136) produced 0.3 g L^-1 adipic acid with a 11.1% theoretical yield in shaken flasks, and we confirmed that the step catalyzed by 5-Carboxy-2-pentenoyl-CoA reductase (Tfu_1647) as the rate-limiting step of the RADP. Overexpression of Tfu_1647 by pTrc99A carried by strain Mad146 produced with a 49.5% theoretical yield in shaken flasks. We further eliminated pathways for major metabolites competing for carbon flux by CRISPR/Cas9 and deleted the succinate-CoA ligase gene to promote accumulation of succinyl-CoA, which is the precursor for adipic acid synthesis. The final engineered strain Mad123146, which could achieve 93.1% of the theoretical yield in the shaken flask, was able to produce 68.0 g L^-1 adipic acid by fed-batch fermentation. To the best of our knowledge, these results constitute the highest adipic acid titer reported in E. coli.