1 INTRODUCTION
Sucrose synthase (SuSy, EC 2.4.1.13) belongs to the glycosyltransferase-4 subfamily (GT-4), which can catalyze the reversible reaction of sucrose synthesis and cleavage.[1] A large number of studies have shown that the activity of SuSy depended on pH value.[2, 3] At pH 7.5–9.5, it displays optimal activity in the direction of sucrose synthesis, while acidic pH promotes the reverse reaction and decomposes sucrose at pH 5.5–7.5 to produce nucleoside diphosphate (NDP) glucose and fructose. Recently, using sucrose to recover the “donor” uridine 5’-diphosphate (UDP) glucose (UDPG) by combining SuSy with Leloir GT (SuSy-GT) has aroused considerable interest in the development of biocatalytic glycosylation process, because the glycosylation of most known conjugates by Leloir GT requires the participation of UDPG.[4-6] In the SuSy-GT cascade reaction (Scheme 1), a UDP cycle is created using sucrose and SuSy which makes UDPG continuously regenerated as an expedient donor for glucoside production, which is the shortest, and probably the most appealing one among the three routes involving synthase, phosphorylase, and kinase for UDPG synthesis.[7]