Shicheng Mu

Cucurbitacins, a group of diverse tetracyclic triterpenes, display a variety of biological effects. Glycosylation mediated by glycosyltransferases (UGTs) plays a vital role in structural and functional diversity of natural products and influences their biological activities. In this study, GT-SM, a mutant of UGT74AC1 from Siraitia grosvenorii, was chosen as a potential catalyst in glycosylation of cucurbitacins, and its optimal pH, temperature, and divalent metal ions were detected. This enzyme showed high activity (k_cat/K_m, 120 s⁻¹ µM⁻¹) toward cucurbitacin F 25-O-acetate (CA-F₂₅) and only produced CA-F₂₅ 2-O-β-d-glucose which was isolated and confirmed by ¹D and ²D nuclear magnetic resonance. A pathway for uridine diphosphate glucose (UDP-Glc) regeneration and cucurbitacin glycoside synthesis was constructed by combing GT-SM and sucrose synthase to cut down the costly UDP-Glc. The molar conversion of CA-F₂₅ was 80.4% in cascade reaction. Molecular docking and dynamics simulations showed that CA-F₂₅ was stabilized by hydrophobic interactions, and the C2-OH of CA-F₂₅ showed more favorable catalytic conformation than that of C3-OH, explaining the high regioselectivity toward the C2-OH rather than the ortho-C3-OH of CA-F₂₅. This work proved the important potential application of UGT74AC1 in cucurbitacins and provided an understanding of glycosylation of cucurbitacins.

Alginase lyase is an important enzyme for the preparation of alginate oligosaccharides (AOS), that possess special biological activities and is widely used in various fields, such as medicine, food, and chemical industry. In this study, a novel bifunctional alginate lyase (AlgH) belonging to the PL7 family was screened and characterized. The AlgH exhibited the highest activity at 45 °C and pH 10.0, and was an alkaline enzyme that was stable at pH 6.0–10.0. The enzyme showed no significant dependence on metal ions, and exhibited unchanged activity at high concentration of NaCl. To determine the function of non-catalytic domains in the multi-domain enzyme, the recombinant AlgH-I containing only the catalysis domain and AlgH-II containing the catalysis domain and the carbohydrate binding module (CBM) domain were constructed and characterized. The results showed that the activity and thermostability of the reconstructed enzymes were significantly improved by deletion of the F5/8 type C domain. On the other hand, the substrate specificity and the mode of action of the reconstructed enzymes showed no change. Alginate could be completely degraded by the full-length and modified enzymes, and the main end-products were alginate disaccharide, trisaccharide, and tetrasaccharide. Due to the thermo and pH-stability, salt-tolerance, and bifunctionality, the modified alginate lyase was a robust enzyme which could be applied in industrial production of AOS.