Metabolic Engineering of <i>Escherichia coli</i> for Efficient Production of Redox Cofactors Flavin Mononucleotide and Flavin Adenine Dinucleotide

Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are essential redox cofactors in cellular metabolism. In this study, riboflavin-producing Escherichia coli was engineered for de novo FMN and FAD synthesis. The RibF M-1 mutant with defective FAD synthase activity was overexpressed to produce FMN. NADH supply was enhanced by overexpressing sthA and deleting pntAB, while deletion of ndh encoding NADH dehydrogenase II and appB encoding cytochrome oxidase bd-II improved ATP regeneration. The resulting strain FMN08 produced 3457.3 mg/L FMN in fed-batch fermentation. FMNAT from Candida glabrata was engineered by semirational design to increase catalytic activity 2.14-fold, and coexpression of FMN1 from Saccharomyces cerevisiae enabled FMN-to-FAD conversion. Deletion of ushA encoding 5′-nucleotidase and overexpression of purA further promoted FAD production. The final strain FAD13 produced 2305.2 mg/L FAD in fed-batch fermentation. These results represent the highest reported titers of de novo synthesis of these redox cofactors.