Expression， characterization， and site-directed mutagenesis of UDP-glycosyltransferase UGT88A1 from Arabidopsis thaliana.

Quercetin-4'-O-glucoside is one of the major quercetin derivatives in the mature red onion bulb. It has an adjuvant effect on allergies， asthma， arthritis， and cancer. The present study aimed to use uridine diphosphate glycosyltransferase 88A1 (UGT88A1) from Arabidopsis thaliana to achieve the enzymatic synthesis of quercetin-4'-O-glucoside from quercetin. The results showed that UGT88A1 was most active at pH 9.0. The optimum temperature of UGT88A1 for synthesizing quercetin-4'-O-glucoside was 45°C， which was a little lower than that for synthesizing quercetin-3-O-glucoside (50°C). One mutant， V18R， of UGT88A1 was obtained by site-directed mutation and showed a greater affinity (K 0.20 mM) and twice the enzyme activity (552.3 mU/mg) towards quercetin compared with the wild-type enzyme (0.36 mM and 227.6 mU/mg， respectively). The possible reason could be attributed to the distance change between the 18th amino-acid residue of UGT88A1 and the substrate quercetin， as deduced by molecular simulation.