Synthesis of α-galactosyl epitopes by metabolically engineered Escherichia coli.

The α-Gal epitope is a carbohydrate structure, Galα-3Galβ-4GlcNAc-R, expressed on glycoconjuguates in many mammals, but not in humans. Species that do not express this epitope have present in their serum large amounts of natural anti-Gal antibodies, which contribute to organ hyperacute rejection during xenotransplantation. We first describe the efficient conversion of lactose into isoglobotriaose (Galα-3Galβ-4Glc) using high cell density cultures of a genetically engineered Escherichia coli strain expressing the bovine gene for α-1,3-galactosyltransferase. Attempts to produce the Galili pentasaccharide (Galα-3Galβ-4GlcNAcβ-3Galβ-4Glc) by additionally expressing the Neisseria meningitis lgtA gene for β-1,3-N-acetylglucosaminyltransferase and the Helicobacter pylori gene for β-1,4-galactosyltransferase were unsuccessful and led to the formation of a series of long chain oligosaccharides formed by the repeated addition of the trisaccharide motif [Galβ-4GlcNAcβ-3Galα-3] onto a lacto-N-neotetraose primer. The replacement of LgtA by a more specific β-1,3-N-acetylglucosaminyltransferase from H. pylori, which was unable to glycosylate α-galactosides, prevented the formation of these unwanted compounds and allowed the successful formation of the Galili pentasaccharide and longer α-Gal epitopes.