Biosynthesis of amino deoxy-sugar-conjugated flavonol glycosides by engineered Escherichia coli

• E. coli BL21(DE3) mutants were generated to divert carbon flux from glucose-1-phosphate to activated NDP-sugars.
• The pool of three different rare NDP-amino deoxy-sugars was generated in E. coli cytosol.
• Engineered strains were used for the in vivo glycosylation of flavonols.
• Four unnatural quercetin and kaempferol amino deoxy-sugars conjugated glycosides were produced.

Escherichia coli BL21(DE3) was engineered to divert the flow of carbon flux from glucose-1-phosphate to thymidine diphosphate 4-keto 4,6-dideoxy-D-glucose (dTKDG), an intermediate of various dTDP-sugars. Glucose phosphate isomerase (pgi), glucose-6-phosphate dehydrogenase (zwf) and uridylyltransferase (galU) genes were deleted while two additional genes, dTDP-D-glucose synthase (tgs) and dTDP-D-glucose 4,6-dehydratase (dh), were overexpressed to produce a pool of dTKDG in the cell cytosol. The flow of dTKDG was further diverted to dTDP-D-viosamine, dTDP 4-amino 4,6-dideoxy-D-galactose, and dTDP 3-amino 3,6-dideoxy-D-galactose sugars using sugar aminotransferases (gerB, wecE, and fdtB, respectively) from different sources. These sugar moieties were transferred to the 3-hydroxyl position of quercetin and kaempferol with the help of Arabidopsis thaliana glycosyltransferase (ArGT3). As a result, 4-amino 4,6-dideoxy-D-galactose and 3-amino 3,6-dideoxy-D-galactose sugars conjugated to quercetin and kaempferol were biosynthesized successfully from exogenously supplemented quercetin and kaempferol. However, no D-viosamine conjugated kaempferol or quercetin derivatives were produced during the biotransformation. All the synthesized glycosides are novel unnatural compounds, which could have potent biological activities.

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