Synthesis of the O-linked pentasaccharide in glycoproteins of Trypanosoma cruzi and selective sialylation by recombinant trans-sialidase

The mucin-like glycoproteins of Trypanosoma cruzi have novel O-linked oligosaccharides that are acceptors of sialic acid in the trans-sialidase (TcTS) reaction. The transference of sialic acid from host glycoconjugates to the mucins is involved in infection and pathogenesis. The synthesis of the pentasaccharide, β-d-Galp-(1→2)-[β-d-Galp-(1→3)]-β-d-Galp-(1→6)-[β-d-Galf-(1→4)]-d-GlcpNAc and the corresponding alditol, previously isolated by reductive β-elimination of the mucins, is described. The key step was the 6-O-glycosylation of a easily accessible derivative of β-d-Galf-(1→4)-d-GlcpNAc with a β-d-Galp-(1→2)-[β-d-Galp-(1→3)]-d-Galp donor using the trichloroacetimidate method. The β-linkage was diastereoselectively obtained by the nitrile effect. The pentasaccharide is the major oligosaccharide in the mucins of T. cruzi, G strain and presents two terminal β-d-Galp residues for possible sialylation by TcTS. A preparative sialylation reaction was performed with its benzyl glycoside and the sialylated product was isolated and characterized. NMR spectroscopic analysis showed that selective monosialylation occurred at the terminal (1→3) linked galactopyranose.

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