Fold recognition study of α3-galactosyltransferase and molecular modeling of the nucleotide sugar-binding domain

The structure and fold of the enzyme responsible for the biosynthesis of the xenotransplantation antigen, namely pig alpha 3 galactosyltransferase, has been studied by means of computational methods. Secondary structure predictions indicated that alpha 3-galactosyltransferase and related protein family members, including blood group A and B transferases and Forssman synthase, are likely to consist of alternating alpha-helices and beta-strands, Fold recognition studies predicted that alpha 3-galactosyltransferase shares the same fold as the T4 phage DNA-modifying enzyme beta-glucosyltransferase. This latter enzyme displays a strong structural resemblance with the core of glycogen phosphorylase b. By using the three-dimensional structure of beta-glucosyltransferase and of several glycogen phosphorylases, the nucleotide binding domain of pig alpha 3-galactosyltransferase was built by knowledge-based methods. Both the UDP-galactose ligand and a divalent cation were included in the model during the refinement procedure. The final three-dimensional model is in agreement with our present knowledge of the biochemistry and mechanism of alpha 3-galactosyltransferases.