Scientific Correspondence Three-Dimensional Structures of UDP-Sugar Glycosyltransferases Illuminate the Biosynthesis of Plant Polysaccharides

Cell-free preparations of the bacterium Acetobacter xylinum were first reported to synthesize cellulose from UDP-Glc over 40 years ago (Glaser, 1958). Despite the elucidation of the primary sequence of the cellulose synthase from this organism in 1990 (Wong et al., 1990), a consistent mechanism to account for the biosynthesis of this and related polysaccharides has remained elusive. Opposing views of the number of catalytic centers and the molecular directionality of the synthesis have been presented (for review, see Delmer, 1999). A comprehensive classification of glycosyltransferases harnessed to the recent structural determinations of UDP-sugar dependent b-glycosyltransferases, including a cellulose synthase homolog, permits a preliminary illumination of this controversial area. The number of glycosyl transfer centers in the catalytic domain of cellulose synthase remains controversial: a two-center model has been proposed (Saxena et al., 1995), but we find it hard to reconcile with the wealth of experimental data on the three-dimensional structure of glycosyltransferases. The sequence family classification system, originally developed for the glycoside hydrolases, has recently been extended to include the activated-sugar dependent glycosyltransferases (Campbell et al., 1997). Forty-eight families are known at the present date and may be found in a continuously updated database at http://afmb.cnrs-mrs.fr/;pedro/CAZY/db. html (for review, see Henrissat and Davies, 2000). Activated-sugar dependent transferases account for the vast majority of glycosyl transfer on earth. The activating group may be a phosphate, a lipid phosphate, or a nucleotide, and the reaction mechanism proceeds with either retention or inversion of the anomeric configuration of the donor sugar. One of the features of the sequence-family classification is that the reaction mechanism is conserved within each family. Cellulose synthase, in family “GT-2,” is an “inverting” glycosyltransferase, i.e. it uses a-linked UDP-sugars to generate a b-linked product. Inverting transferases are assumed to use a single displacement mechanism with nucleophilic attack by the acceptor species at the C-1 (anomeric) carbon of the donor sugar. Such a mechanism is generally believed to demand a base to activate the sugar acceptor for nucleophilic attack by deprotonation because sugar hydroxyls are in themselves quite poor nucleophiles. For most enzymes the reaction also involves an additional carboxylate or carboxylates to coordinate a divalent metal ion on the phosphate group(s) of the nucleotide (Fig. 1).