Changing the Donor Cofactor of Bovine α1,3-Galactosyltransferase by Fusion with UDP-galactose 4-Epimerase

Two fusion enzymes consisting of uridine diphosphogalactose 4-epimerase (UDP-galactose 4-epimerase, EC 5.1.3.2) and α1,3-galactosyltransferase (EC 2.4.1.151) with an N-terminal His6 tag and an intervening three-glycine linker were constructed by in-frame fusion of the Escherichia coli galEgene either to the 3′ terminus (f1) or to the 5′ terminus (f2) of a truncated bovine α1,3-galactosyltransferase gene, respectively. Both fusion proteins were expressed in cell lysate as active, soluble forms as well as in inclusion bodies as improperly folded proteins. Both f1 and f2 were determined to be homodimers, based on a single band observed at about 67 kDa in SDS-polyacrylamide gel electrophoresis and on a single peak with a molecular mass around 140 kDa determined by gel filtration chromatography for each of the enzymes. Without altering the acceptor specificity of the transferase, the fusion with the epimerase changed the donor requirement of α1,3-galactosyltransferase from UDP-galactose to UDP-glucose and decreased the cost for the synthesis of biomedically important Galα1,3Gal-terminated oligosaccharides by more than 40-fold. For enzymatic synthesis of Galα1,3Galβ1,4Glc from UDP-glucose and lactose, the genetically fused enzymes f1 and f2 exhibited kinetic advantages with overall reaction rates that were 300 and 50%, respectively, higher than that of the system containing equal amounts of epimerase and galactosyltransferase. These results indicated that the active sites of the epimerase and the transferase in fusion enzymes were in proximity. The kinetic parameters suggested a random mechanism for the substrate binding of the α1,3-galactosyltransferase. This work demonstrated a general approach that fusion of a glycosyltransferase with an epimerase can change the required but expensive sugar nucleotide to a less expensive one.