The specificities of glycosyltransferases make them useful for the synthesis of biologically active oligosaccharides, but also restrict their range of products. in substrate engineering, Substrate promiscuity IS enhanced by attaching removable interactive groups to weak Substrates. Thus, the attachment of beta p-nitrophenyl converts galactose from a poor into a good Substrate of alpha-1,3-galactosyltransferase. The crystallographic structure of a complex of alpha 3GT containing p-nitrophenyl-beta-galactoside shows that the p-nitrophenyl binds similarly to the N-acetylglucosamine of the substrate, N-acetyllactosamine, interacting with the indole of Trp249. p-Nitrophenyl, unlike N-acetylglucosamine, makes no H-bonds but has more non-polar interactions, making it an effective monosaccharide mimetic. (c) 2009 Elsevier Inc. All rights reserved.