Synthetic glycosylation of proteins using N-(beta-saccharide) iodoacetamides: applications in site-specific glycosylation and solid-phase enzymic oligosaccharide synthesis.

A simple and efficient synthetic glycosylation method suitable for use in solid-phase enzymic oligosaccharide synthesis and site-specific glycosylation of recombinant proteins to produce defined glycoforms is described. This strategy utilizes N-(beta-saccharide) haloacetamides for attaching oligosaccharides specifically to cysteine residues of proteins in solution to form neoglycoproteins. The alkylation reaction was tested using N-(beta-chitotriose) bromoacetamide and an unprotected synthetic hexapeptide containing a single cysteine residue. The glycosylated product was confirmed by amino acid and hexosamine analyses as well as laser desorption mass spectrometry. Similarly N-(beta-chitotriose) iodoacetamide was covalently linked to non-reduced BSA to produce a defined glycoform of this protein. The specific attachment of chitotriose at the single cysteine residue in non-reduced serum albumin was suggested by Ellman's assay for free thiols. This was verified by amino acid sequencing of tryptic glycopeptide derived from this neoglycoprotein. Multiple sugar attachment was accomplished using fully reduced serum albumin as demonstrated by the formation of two neoglycoproteins using iodoacetamide derivatives of galactose beta 1-3-N-acetylgalactosamine (Gal beta 1-3GalNAc) and the major xylose/fucose-class plant-type oligosaccharide of horseradish peroxidase. These two neoglycoproteins with an average of 18-21 sugar residues attached were assayed positively for binding to peanut agglutinin and a sugar-specific anti-(horseradish peroxidase) monoclonal antibody YZ1/2.23 respectively. Sialylation of the neoglycoprotein containing Gal beta 1-3GalNAc was accomplished using alpha-2,3-sialyltransferase and radiolabelled CMP-N-acetylneuraminic acid. Significantly, glycan attachment using this conjugation method is reversible as demonstrated by the release of oligosaccharides from these two neoglycoproteins using hydrazinolysis. Therefore this method could provide invaluable reagents for many glycobiological studies.