Bacterial Polysaccharide Structure and Biosynthesis
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Polysaccharides are linear or branched polymers built up exclusively or mainly of monosaccharides. In glycoconjugates, a polysaccharide chain(s) is linked to a protein/peptide (glycoprotein, proteoglycan, and peptidoglycan) or a lipid (lipoglycans, glycolipids). The bacterial glycome consists of a wide repertoire of cell surface polysaccharides and glycoconjugates, including lipopolysaccharides (LPS, endotoxins) of Gram-negative bacteria, cell-wall anionic polysaccharides of Gram-positive bacteria, mycobacterial lipoglycans, capsular and extracellular polysaccharides (EPS), and S-layer glycoproteins (Seltmann and Holst 2002). A rigid protective peptidoglycan layer surrounds the cytoplasmic membrane in all bacterial groups. Specific surface polysaccharides play an important role in bacterial life and, particularly, are implicated in recognition and virulence of pathogens. They determine the immunospecificity of bacteria, and various immunogenic forms of polysaccharides and their fragments are used as vaccine components. Bacteria often produce also nonspecific polysaccharides, such as glycogen, dextran, other glucans, and fructans. The biosynthesis of bacterial polysaccharides is a complex process that involves membrane and soluble enzymes, as well as a mandatory transmembrane translocation step in all cases studied to date.
Extracellular glycans are found in both Gram-negative and Gram-positive bacteria. Some of them, such as K-antigens, are bound to the cell surface (e.g., with the aid of a phosphatidic acid anchor) and form a protective capsule, whereas others are released to the environment as a slime or a component of biofilm, a matrix of polymeric substances encapsulating bacteria and adherent to a living or inert surface (Cescutti 2009). Like cell-wall polysaccharides, exoglycans are built up of oligosaccharide repeating units, which sometimes have the same structure as O-units in Gram-negative bacteria. They usually have anionic character, often including phosphate groups. Some zwitterionic exopolysaccharides exhibit peculiar biological activities and, particularly, induce immunomodulatory T cell response.
Bacterial cells may be surrounded by an S-layer, an envelope of paracrystalline two-dimensional lattices composed of glycoprotein molecules self-aggregated by electrostatic or hydrophobic interactions. S-layer glycan chains are repetitive long polysaccharides analogous to the O-chains or oligosaccharides resembling the LPS core (Messner et al. 2009). Typically, they are O-linked to serine, threonine, or, less common, tyrosine and are much more diverse in composition and structure than glycan chains of eukaryotic glycoproteins. S-layers are characteristic also of archaea, but in contrast to those of bacteria, they have predominantly short-chain glycans that are N-linked to asparagine.
In the Wzy-dependent pathway, the O-unit is assembled from reactions catalyzed by specific glycosyltransferases, which are typically peripheral membrane proteins associated with the plasma membrane by ionic interactions. Individual O-units are translocated across the membrane by an ATP hydrolysis-independent mechanism mediated by the protein Wzx (O-unit flippase). On the periplasmic side of the membrane, the translocated O-unit is polymerized to a certain length, unique to each O-antigen, by the concerted functions of Wzy (O-antigen polymerase) and Wzz (O-antigen chain length regulator). In the ABC-dependent pathway, the O-antigen is elongated in the cytosol by processive glycosyltransferases, and the polymer is translocated across the membrane by an ABC transporter (Wzt/Wzm or Wzk). ABC transporters are also used in the synthesis of capsular polysaccharides (Whitfield 2006) and S-layers (Messner et al. 2009). In some cases, a termination signal that ceases the chain elongation is provided by methylation or other terminal sugar modifications that couple the polymerization with the ABC transporter.
The complete LPS molecule is transported to the outer membrane by the LptABCDEFG protein complex (Fig. 6) (Ruiz et al. 2009). The export of capsules to the outer membrane also requires a distinct complex of proteins, which includes an outer membrane secreting protein (Whitfield 2006).
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