Synthesis of Complex Carbohydrates and Glyconjugates: Enzymatic Synthesis of Globotetraose Using -1,3-N-Acetylgalactosaminyltransferase LgtD From Haem

The lipopolysaccharide of capsule-deficient Haemophilus infuenzae strain Rd contains an N -acetylgalactosamine residue attached to the terminal globotriose moiety in the Hex5 glycoform. Genome analysis identified an open reading frame, HI1578, referred to as LgtD, whose amino acid sequence shows a significant level of similarity to those of a number of bacterial glycosyltransferases involved in lipopolysaccharide biosynthesis. To investigate its function, overexpression and biochemical characterization were performed. Most of the protein was obtained in a highly soluble and active form. Standard glycosyltransferase assay, high-performance liquid chromatography (HPLC), and liquid chromatography (LC)/mass spectrometry (MS) show that LgtD is an N -acetylgalactosaminyltransferase with high donor substrate specificity, and globotriose is a highly preferred acceptor substrate for the enzyme.