Human Fucosyltransferases

The X structure (Gal-β-1-4 [Fuc-α-1-3]GlcNAc) occurs in glycoproteins, glycolipids, and free oligosaccharides and may be exposed as a terminal non-reducing end-group or masked by substitution with other sugars. The presence of sialic acid in α-2,3-linkage to the terminal β-galactosyl residue gives the sialyl-X determinant (NeuAc-α-2-3Gal-β-1-4-[Fuc-α-1-3]GlcNAc). Both structures have been identified as human tumour markers and as ligands in cellular adhesion reactions mediated by endogenous lectins (LEC-CAMS). X structures are biosynthesised by a family of GDP-fucose:- N -acetyl-D-glucosaminide α-3-L-fucosyltransferases. The obligatory pathway for biosynthesis of sialyl-X is first the formation of NeuAc α-2-3-Gal-β-1-4GlcNAc, followed by the addition of the fucosyl residue to the N -acetylglucosamine unit. Not all α-3-fucosyltransferase species that synthesise X determinants can add fucose to the GlcNAc residue in NeuAcα-2-3Gal-β-1-4-GlcNAc to form sialyl-X determinants. In myeloid tissues species differing in their activities with sialylated acceptors occur at different stages of maturation; these variants appear to be encoded by different genes from the α-3-fucosyltransferases expressed in other tissues.