Efficient biosynthesis of retinal from Escherichia coli by metabolic engineering

Retinal, a valuable vitamin A derivative with potent antioxidant and therapeutic properties, exhibits substantial market demand. Due to the low productivity of traditional methods, metabolic engineering was employed in microorganisms for retinal production. In this study, we developed an engineered Escherichia coli BL21 (DE3) strain using systematic and synthetic biology approaches for efficient β-carotene and retinal biosynthesis from biological resources. The β-carotene production was improved via enzyme engineering by directed evolution and translational engineering by RBS optimization of the rate-limiting enzymes. We introduced β-carotene-15, 15'-oxygenase for de novo retinal production, and identified knocking out aldehyde reductase ybbO could prevent retinal loss. Subsequent strategies aimed at enhancing product secretion through membrane engineering and improving NADPH availability via cofactor regeneration were attempted but did not increase retinal yields. Then, systematic metabolic engineering the mevalonate pathway and rewiring the central carbon metabolism successfully increased the metabolic flux to retinal production. Finally, the constructed efficient E. coli cell factory for retinal biosynthesis achieved a retinal titer of 245.73 mg·L^-1 in a 4-L bioreactor. Our work demonstrates the potential for biosynthesis of retinal and confirms the efficacy of our integrated multi-level engineering strategies for the optimized biosynthesis of retinal and other complex terpenoids.