Update on Nucleotide Sugar Synthesis UDP-Sugar Pyrophosphorylase: A New Old Mechanism for Sugar Activation

Simple sugars (e.g. Glc, Gal) are the building blocks of disaccharides (e.g. Suc) and polysaccharides (e.g. cellulose, hemicellulose). To produce disaccharides and polysaccharides, a given simple sugar needs to be “activated.” This activation involves the addition of a nucleoside-diphosphate group to a sugar, resulting in the formation of a nucleotide sugar. Such an activated sugar can then be used by a variety of glucosyltransferases to link the sugar residue with an appropriate receptor molecule. The latter can be a carbohydrate but also a protein, a lipid, or any other molecule that can be glycosylated (Drickamer and Taylor, 2006). Among nucleotide sugars, those with a uridyl group (UDP-sugars) are most prominent, and they serve as precursors to primary metabolites (e.g. Suc in plants), storage compounds (e.g. glycogen in animals and yeast), structural components (e.g. cellulose in plants and bacteria; hemicellulose and pectin in plants), as well as glycoproteins and glycolipids (Feingold and Barber, 1990, Kleczkowski et al., 2010). UDP-sugars are the main precursors for biomass production in plants (Kotake et al., 2010). UDP-sugar pyrophosphorylase (USPase; EC 2.7.7.64) catalyzes a reversible transfer of the uridyl group from UTP to sugar-1-phosphate, producing UDPsugar and pyrophosphate (PPi). The enzyme was unequivocally identified and characterized only a few years ago (Kotake et al., 2004), but the USPase-like activities have been reported for at least 50 years now. In those early studies, USPase-like activities were observed in a wide range of organisms, from bacteria to plants to animals, with reports on “UDP-Ara/UDPXyl pyrophosphorylase(s)” (Ginsburg et al., 1956), “UDP-Gal pyrophosphorylase” (Chacko et al., 1972; Lee et al., 1978; Smart and Pharr, 1981; Studer-Feusi et al., 1999), “UDP-GlcA pyrophosphorylase” (Hondo et al., 1983), and “UDP-Ara pyrophosphorylase” (Feingold and Barber, 1990). The UDP-Gal pyrophosphorylase activities were especially significant, since they implied an alternative to the Leloir pathway believed to be the major, if not the only, mechanism of Gal to Glc conversion (Smart and Pharr, 1981; Frey, 1996). It seems likely now that at least some of those pyrophosphorylase activities correspond to the same protein, namely USPase. The work on USPase has been hampered by its overlapping specificity with some other pyrophosphorylases. The activity with Glc-1-P, which usually represents the most specific substrate for USPase, overlaps with activities of at least three other UTPdependent pyrophosphorylases, namely UDP-Glc pyrophosphorylase (UGPase; EC 2.7.7.9), which exists as distinct UGPase-A and UGPase-B types, and UDP-Nacetyl-glucosamine pyrophosphorylase (UAGPase; EC 2.7.7.23), known in animals as AGX. These proteins share low or very low identity at the amino acid level (below 20%; Kleczkowski et al., 2010). Phylogenetic analyses revealed that USPases, UGPases (either A or B type), and UAGPases should all be categorized into distinct groups (Geisler et al., 2004; Litterer et al., 2006a, 2006b; Okazaki et al., 2009; Kleczkowski et al., 2010). In this review, we focus mainly on recent studies on USPase, with emphasis on its substrate specificity, structure, and role in metabolism. Recent work with purified USPases (Kotake et al., 2004, 2007; Litterer et al., 2006a, 2006b; Dai et al., 2006; Gronwald et al., 2008; Damerow et al., 2010; Yang and Bar-Peled, 2010), studies on transgenic plants with altered USPase contents (Schnurr et al., 2006; Kotake et al., 2007), and resolution of the crystal structure of protozoan USPase (Dickmanns et al., 2011) have reignited the interest in USPase and provided new clues to the possible roles of this multisubstrate-utilizing enzyme.