Exploiting the biocatalytic potential of glycosyltransferases through in situ sugar donor recycling

The exceptional regio- and stereoselectivity of glycosyltransferases (GTs) offers synthesis of high value oligosaccharides, glycoproteins or glycosylated natural products through selective single-step glycosylation of complex substrates. However, the dependence of GTs on expensive nucleotide activated sugar donors makes in situ recycling of the sugar donors a prerequisite for cost-efficient synthesis. In contrast to alternative multienzyme strategies, sucrose synthase (SuSy) allows the direct regeneration of the most abundant sugar donor uridine 5'-diphosphate (UDP)-glucose from inexpensive sucrose and UDP, the byproduct of GT reactions. We established one-pot GT-SuSy cascade reactions for regioselective β-D-glucosylation of flavonoids. Glycosylation improves the water solubility of these polyphenolic plant secondary metabolites and tailors biological activities for prospective applications as functional food and pharmaceutical ingredients. By adding the SuSy module UDP-glucose was up to ~100 times recycled. Thereby stoichiometric amounts of costly UDP-glucose were replaced with catalytic quantities of more economic UDP. Furthermore, removal of the GT byproduct UDP improved conversion rates and increased final reaction yields. Poor water solubility of flavonoids and various other hydrophobic acceptors requires the development of solubility enhancement strategies to obtain the high product concentrations requested in industrial processes. We introduced formation of β-cyclodextrin inclusion complex as perfectly biocompatible approach to boost substrate solubility. Thereby ~20 g L^-1 of the C-glucosidic antioxidant nothofagin were produced although the aqueous solubility of the aglycon was only ~90 mg L^-1.