HUMAN FUCOSYLTRANSFERASES - INVOLVED IN THE BIOSYNTHESIS OF X (GAL-BETA-1-4[FUC-ALPHA-1-3]GLCNAC) AND SIALYL-X (NEUAC-ALPHA-2-3GAL-BETA-1-4[FUC-ALPHA-1-3]GLCNAC) ANTIGENIC DETERMINANTS

The X structure (Gal-beta1-4[Fuc-alpha-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 alpha-2,3-linkage to the terminal beta-galactosyl residue gives the sialyl-X determinant (NeuAc-alpha-2-3Gal-beta-1-4-[Fuc-alpha-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 alpha-3-L-fuucosyltransferase. The obligatory pathway for biosynthesis of sialyl-X is first the formation of NeuAc alpha-2-3-Gal-beta-1-4GlcNAc, followed by the addition of the fucosyl residue to the N-acetylglucosamine unit. Not all alpha-3-fucosyltransferase species that synthesise X determinants can add fucose to the GlcNAc residue in NeuAc alpha-2-3Gal-beta-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 alpha-3-fucosyltransferases expressed in other tissues.