Branching and elongation with lactosaminoglycan chains of N‐linked oligosaccharides result in a shift toward termination with α2→3‐linked rather than with α2→6‐linked sialic acid residues

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Abstract
The activity of bovine colostrum CMP-NeuAc: Galβ1→4GlcNAcβ-R α2→6-sialyltransferase (α6-NeuAcT) toward oligosaccharides that form part of complex-type, N-linked glycans appears significantly reduced when a bisecting GlcNAc residue or additional branches are present, or when core GlcNAc residues are absent. By contrast human placenta CMP-NeuAc:Galβ1→4GlcNAcβ-Rα2→3-sialyltransferase (α3-NeuAcT) is much less sensitive to structural variations in these acceptors. Furthermore the α3-NeuAcT shows a much higher activity than the α6-NeuAcT with oligosaccharides that form part of linear and branched lactosaminoglycan extensions. These results indicate that, in tissues that express both enzymes, branching and lactosaminoglycan formation of N-linked glycans will cause a shift from termination with α2→6-linked sialic acid to termination with α2→3-linked sialic acid residues. These findings provide an enzymatic basis for the sialic acid linkage-type patterns found on the oligosaccharide chains of N-glycoproteins

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