Exocellular biosynthesis

Formation of L-guluronic acid, a component of the alginic acid-like polysaccharide produced by P. aeruginosa and Azotobacter vinelandii, requires special comment. In this case, a polymer built from /3-(l- 4)-linked D-mannosyluronic acid residues serves as an intermediate in the biosynthesis.204,205 Part of the D-mannosyluronic acid residues in the polymer is subjected to an epimerization at C-5 catalyzed by an exocellular enzyme of the micro-organism,205-207 producing a polysaccharide composed of structural blocks that contain only D-mannosyluronic acid or only l-gulosyluronic acid residues, as well %s some having both. The mechanism of the epimerization remains unclear. 

Other monosaccharide components (of bacterial polysaccharides) that are structurally related to D-ribose include D-riburonic acid,232 identified in the exocellular polysaccharide produced by a strain of Rhizobium meliloti, and D-arabinose, frequently present as the furanose, in polysaccharides of mycobacterial cell-wall.233,234 L-Xylose235,236 should probably be included in the group, as it may be derived from D-arabinose through epimerization at C-4. Biosynthesis of these monosaccharides was not investigated. 

The term monomeric mechanism will be used for the mechanism depicted in the left-hand part of Scheme 2 (sequence a). In this case, the monosaccharide residues are transferred consecutively from the corresponding glycosyl donors (Z-A or Z -B) onto a membrane-bound glycosyl acceptor. The acceptor is generally a monosaccharide residue, which may be a fragment of an oligosaccharide chain linked to a hydrophobic molecule embedded in a cell membrane. In many instances, the acceptor that is used for assembly of the polymeric chain (Y) is not identical to the final acceptor (X) of the chain, and further transfer of the chain from Y to X, or liberation of the polysaccharide molecule in the case of exocellular polysaccharides, is a necessary step in the biosynthesis. 

A series of poly prenyl diphosphates was formed when EDTA-treated cells of Acetobacter xylinum were incubated with glycosyl nucleotides. The most complicated of them is the heptasaccharide derivative344 (32), which is considered to be an intermediate in the biosynthesis of the exocellular... 

Other examples of exocellular homopolysaccharides whose biosynthetic process has been investigated include D-mannuronan,204,205 an intermediate in the biosynthesis of bacterial alginic acid (mentioned in Section III,l,c), and bacterial cellulose. 

The material presented in previous sub-sections clearly shows that both of the possible mechanisms of polysaccharide chain-assembly may operate in the biosynthesis of bacterial polysaccharides. There is no clearcut, mechanistic difference in the biosynthesis of O-specific chains of lipopoly-saccharides, exocellular polysaccharides, and carbohydrate chains of Grampositive, cell-wall polymers for every class of polymer, the existence of both mechanisms of chain assembly was demonstrated. 

The synthesis of exocellular polysaccharides by lactic acid bacteria is a very widespread character. L. mesenteroides and Streptococcus mutans produce glucose homopolymers such as dextran and glucan fructose homopolymers (levans) and heteropolymers are also synthesized. Dextran of L. mesenteroides is the best known, as much for its different structures and its biosynthesis as for its various applications. 

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