Enhanced production of l-fuculose by Escherichia coli engineered via genome-scale metabolic modeling.

l-Fuculose is a rare deoxyketohexose sugar and is a structural isomer of l-fucose, which exhibits skin-lightening, moisturizing, and anti-aging effects. Due to their structural similarity, l-fuculose is also expected to provide potential health benefits. However, l-fuculose exists only in trace amounts in nature, making extraction from natural sources virtually impossible; to date, it has been synthesized mainly by enzymatic conversion. This approach, however, suffers from major limitations: both the substrate (l-fucose) and the enzyme involved (l-fucose isomerase) are costly; the enzymatic reaction cannot achieve complete conversion due to the chemical equilibrium; and the methods for purification of l-fuculose from the reaction mixture containing both l-fucose and l-fuculose are inefficient and uneconomical. Microbial cell factories have been explored as an alternative route for l-fuculose biosynthesis, but their production titers remain extremely low, limiting their industrial applicability. In this study, a microbial cell factory was engineered in Escherichia coli by redirecting the pathway of l-fucose metabolism toward l-fuculose production. Overexpression of fucA enabled the aldol condensation of lactaldehyde and dihydroxyacetone phosphate to produce l-fuculose-1-phosphate, which was subsequently dephosphorylated to l-fuculose by a sugar phosphatase. To prevent diversion of substrates and products into competing pathways, the fucI, fucK, tpiA, fucO, and aldA genes were deleted. The final engineered strain produced 50.25 ± 4.30 mg/L of l-fuculose, a 32.4-fold increase compared to that achieved previously by microbial biosynthesis. This study establishes a foundation for the industrial production of l-fuculose, which has potential application as a valuable ingredient in cosmetics, functional foods, and pharmaceuticals.