Structural basis for glucosylsucrose synthesis by a member of the α-1,2-glucosyltransferase family

Glucosylsucroses are potentially useful as additives in cosmetic and pharmaceutical formulations. Although en-zymatic synthesis of glucosylsucroses is the most efficient method for their production, the key enzyme that produces themhas remained unknown. Here, we report that glucosylsucrose synthase fromThermosynechococcus elongatus (TeGSS) catalyzes the synthesis of glucosylsucrose using sucrose and UDP-glucose as substrates. These saccharides are homologous to glucosylsucroses produced by Nostoc sp. PCC 7120 (referred to as protein alr1000). When the ratio of UDP-glucose to sucrose is relatively high, TeGSS from cyanobacteria can hydrolyze excess UDP-glucose to UDP and glucose, indicating that sucrose provides a feedback mechanism for the control of gluco-sylsucrose synthesis. In the present study, we solved the crystal structure of TeGSS bound to UDP and sucrose. Our structure shows that the catalytic site contains a circular region that may allow glucosylsucroses with a right-hand helical structure to enter the catalytic site. Because active site residues Tyr18 and Arg179 are proximal to UDP and sucrose, we mutate these residues (i.e., Y18F and R179A) and show that they exhibit very low activity, supporting their role as catalytic groups. Overall, our study provides insight into the catalytic mechanism of TeGSS.