The X-ray crystal structures of two glycosyltransferases (GTs)-beta1,3-glucuronyltransferase I (GlcAT1) and alpha1,4-N-acetyl-hexosaminyltransferase (EXTL2)-have now been determined in the presence of both donor and acceptor substrates. These enzymes are involved in glucosaminylglycan (GAG) synthesis where they catalyze inverting and retaining transfer reactions, respectively. As members of a large family of enzymes that transfer sugar groups from donor nucleotide-sugars to acceptor substrates, GlcAT1 and EXTL2 retain conserved GT folds. Comparative analysis of these structures reveals signature features for selecting specific donor sugars. Adaptive binding of the disaccharide moiety of the acceptor sugars enables the enzymes to catalyze either an inverting S(N)2-type displacement reaction or a retaining S(N)i-like transfer reaction. (C) 2003 Elsevier Science (USA). All rights reserved.