A nonionic inhibitor with high specificity for the UDP-Gal donor binding site of human blood group B galactosyltransferase: design, synthesis, and characterization.

9-(5-O-α-D-galactopyranosyl)-D-arabinityl-1,3,7-trihydropurine-2,6,8-trione (1) was designed and synthesized as a nonionic inhibitor for the donor binding site of human blood group B galactosyltransferase (GTB). Enzymatic characterization showed 1 to be extremely specific, as the highly homologous human N-acetylgalactosaminyltransferase (GTA) is not inhibited. The binding epitope of 1 demonstrates a high involvement of the arabinityl linker, whereas the galactose residue is only making contact to the protein via its C-2 site, which is very important for the discrimination between galactose and N-acetylgalactosamine, the substrate transferred by GTA. The approach can generate highly specific glycosyltransferase inhibitors.