Improved soluble bacterial expression and properties of the recombinant flavonoid glucosyltransferase UGT73G1 from Allium cepa

Glycosylation of quercetin using flavonol-specific glycosyltransferases offers an alternate method for isoquercitrin production. Obtaining sufficient quantities of bioactive enzymes is an important prerequisite for highly effective biocatalysis and biotransformation. In this study, a codon-optimized gene for the flavonoid glucosyltransferase UGT73G1 from Allium cepa was heterologously expressed in the preferred prokaryotic expression host Escherichia coli. By combining expression as a fusion protein with 6-histidine tags with coexpression with molecular chaperones, increased soluble expression of UGT73G1 was achieved in E. coli. Two-terminal 6-histidine tags contributed more to the expression than molecular chaperones, as demonstrated by comparison of specific activities in crude extracts obtained from the recombinant E. coli strains. Studies of the catalytic properties of purified UGT73G1 indicated that its activity was significantly promoted by Mn²⁺ and Mg²⁺, while it was strongly inhibited by Cu²⁺. These expression strategies enhanced the solubility and activity of the overexpressed protein and enabled characterization of this plant-derived glucosyltransferase expressed in a prokaryotic host.