UDP-Glucose 4-Epimerase and β-1，4-Galactosyltransferase from the Oyster as Valuable Biocatalysts for the Production of Galactosylated Products.

Uridine diphosphate galactose (UDP-galactose) is a valuable building block in the enzymatic synthesis of galactose-containing glycoconjugates. UDP-glucose 4-epimerase (UGE) is an enzyme which catalyzes the reversible conversion of abundantly available UDP-glucose to UDP-galactose. Herein， we described the cloning， expression， purification， and biochemical characterization of an unstudied UGE from the oyster (MgUGE). Activity tests of recombinantly expressed MgUGE， using HPLC (high-performance liquid chromatography)， mass spectrometry， and photometric assays， showed an optimal temperature of 16 °C， and reasonable thermal stability up to 37 °C. No metal ions were required for enzymatic activity. The simple nickel-affinity-purification procedure makes MgUGE a valuable biocatalyst for the synthesis of UDP-galactose from UDP-glucose. The biosynthetic potential of MgUGE was further exemplified in a coupled enzymatic reaction with an oyster-derived β-1，4-galactosyltransferase (MgGalT7)， allowing the galactosylation of the model substrate -nitrophenol xylose (NP-xylose) using UDP-glucose as the starting material.