Bovine alpha 1,3-galactosyltransferase catalytic domain structure and its relationship with ABO histo-blood group and glycosphingolipid glycosyltransferases

alpha1,3-galactosyltransferase (alpha 3GaIT, EC 2.4.1.151) is a Golgi-resid ent, type IT transmembrane protein that transfers galactose from UDP-alpha -galactose to the terminal N-acetyllactosamine unit of glycoconjugate glyca ns, producing the Gal alpha1,3Gal beta1,4GlcNAc oligosaccharide structure p resent in most mammalian glycoproteins. Unlike most other mammals, humans a nd Old World primates do not possess alpha 3GalT activity, which is relevan t for the hyperacute rejection observed in pig-to-human xenotransplantation , The crystal structure of the catalytic domain of substrate-free bovine al pha 3GalT, solved and refined to 2.3 Angstrom resolution, has a globular sh ape with an alpha/beta fold containing a narrow cleft on one face, and shar es a UDP-binding domain (UBD) with the recently solved inverting glycosyltr ansferases. The substrate-bound complex, solved and refined to 2.5 Angstrom , allows the description of residues interacting directly with UDP-galactos e. These structural data suggest that the strictly conserved residue E317 i s likely to be the catalytic nucleophile involved in galactose transfer wit h retention of anomeric configuration as accomplished by this enzyme. Moreo ver, the alpha 3GalT structure helps to identify amino acid residues that d etermine the specificities of the highly homologous ABO histo-blood group a nd glycosphingolipid glycosyltransferases.