Polysaccharides, besides nucleic acids and proteins, are the third class of macro-molecules, yet they constitute the largest part of the planet's biomass . Unlike the linear nucleic acid and protein molecules, polysaccharides exhibit highly branched structures and a multiplicity of different, although related, types of linkage known as glycosidic bonds. Enzymes that catalyze the formation of these glycosidic bonds are referred to as glycosyltransferases (GTs). These enzymes use activated sugar molecules (donors) to attach saccharide residues to a variety of substrates (acceptors). The donor molecules most often are nucleotide diphospho sugars (e.g., UDP-Glc) but can also be sugar phosphates or disaccharides. The most common acceptor molecules are growing carbohydrate chains; however, a variety of other compounds such as lipids, proteins, or steroids can also serve this function . Generally, glycosyl-transferases are divided into two groups: processive and nonprocessive. Processive group enzymes are usually found in eukaryotic systems and include enzymes such as cellulose synthase and chitin synthase that are capable of sequentially attaching multiple residues to a growing carbohydrate chain. Most microbial transferases are nonprocessive and are responsible for the addition of single sugar residues. They are involved in the synthesis of complex polysaccharide structures with few or no repeating residues. These include various forms of lipopolysaccharides (LPS), the repeat units of succinoglycan and O-antigens on the surfaces of bacterial cells. The biological significance and functions of these complex structures have only recently begun to be unraveled.
Polysaccharide synthesis does not follow a linear template, and the mechanisms that govern this synthesis are difficult to elucidate. Generally, glycosyltransferases display flexibility in their recognition of donor and acceptor substrates, a fact that has been exploited in the synthesis of natural and unnatural oligosaccharides. Gly-cosyltransferase specificities come from the recognition of some critical hydroxyls in their substrates where modifications are not tolerated. This is seen in the case of glucosyltransferases and galactosyltransferases that distinguish their respective substrates solely on the basis of the orientation of the hydroxyl at the fourth carbon. However, the exact in vivo substrate specificities of majority of GTs have not been determined. In the majority of cases, the formation of each type of glycosidic bond requires a different enzyme, and sometimes, different enzymes (encoded by separate genes) can form the same bond type. Bacterial glycosyltransferases, the topic of this chapter, are enzymes responsible for the assembly of bacterial cell walls (e.g., suc-cinoglycan) and lipopolysaccharides or polysaccharide structures attached to the lip-ids of the outer membrane of gram-negative bacterial cells. Referred to as endotoxins because of their physiological and pharmacological properties, lipopolysaccharides are at the forefront of bacterial interactions with the outside world. These structures have been found to be essential in processes ranging from root nodulation to human pathogenicity. Therefore studies on the corresponding glycosyltransferases are of great importance and interest.
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