Novel Stereoselective Syntheses of (+)-Streptol and (-)-1-epi-Streptol Starting from Naturally Abundant (-)-Shikimic Acid

Multiple Metabolic Engineering Strategies to Improve Shikimate Titer in Escherichia coli

Shikimate is a valuable chiral precursor for synthesizing oseltamivir (Tamiflu^®) and other chemicals. High production of shikimate via microbial fermentation has attracted increasing attention to overcome the unstable and expensive supply of shikimate extracted from plant resources. The current cost of microbial production of shikimate via engineered strains is still unsatisfactory, and thus more metabolic strategies need to be investigated to further increase the production efficiency. In this study, we first constructed a shikimate E. coli producer through the application of the non-phosphoenolpyruvate: carbohydrate phosphotransferase system (non-PTS) glucose uptake pathway, the attenuation of the shikimate degradation metabolism, and the introduction of a mutant of feedback-resistant 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase. Inspired by the natural presence of bifunctional 3-dehydroquinate dehydratase (DHD)-shikimate dehydrogenase (SDH) enzyme in plants, we then designed an artificial fusion protein of DHD-SDH to decrease the accumulation of the byproduct 3-dehydroshikimate (DHS). Subsequently, a repressed shikimate kinase (SK) mutant was selected to promote shikimate accumulation without the supplementation of expensive aromatic substances. Furthermore, EsaR-based quorum sensing (QS) circuits were employed to regulate the metabolic flux distribution between cell growth and product synthesis. The final engineered strain dSA10 produced 60.31 g/L shikimate with a yield of 0.30 g/g glucose in a 5 L bioreactor.

Systems engineering of Escherichia coli for high-level shikimate production

To further increase the production efficiency of microbial shikimate, a valuable compound widely used in the pharmaceutical and chemical industries, ten key target genes contributing to shikimate production were identified by exploiting the enzyme constraint model ec_iML1515, and subsequently used for promoting metabolic flux towards shikimate biosynthesis in the tryptophan-overproducing strain Escherichia coli TRP0. The engineered E. coli SA05 produced 78.4 g/L shikimate via fed-batch fermentation. Deletion of quinate dehydrogenase and introduction of the hydroaromatic equilibration-alleviating shikimate dehydrogenase mutant AroE^T61W/L241I reduced the contents of byproducts quinate (7.5 g/L) and 3-dehydroshikimic acid (21.4 g/L) by 89.1% and 52.1%, respectively. Furthermore, a high concentration shikimate responsive promoter P_rpoS was recruited to dynamically regulate the expression of the tolerance target ProV to enhance shikimate productivity by 23.2% (to 2 g/L/h). Finally, the shikimate titer was increased to 126.4 g/L, with a yield of 0.50 g/g glucose and productivity of 2.63 g/L/h, using a 30-L fermenter and the engineered strain E. coli SA09. This is, to the best of our knowledge, the highest reported shikimate titer and productivity in E. coli.