Mutan biosynthesis

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. 

Figure 10.13. Two-site insertion mechanisms involved in the biosynthesis of (A) L mesenteroides B-512F dextran by dextransucrase, (B) 5. mutans mutan by mutansucrase, and (C) L mesenteroides B-1355 alternan by altemansucrase. X and Y are nucleophiles at the active site of the enzymes X orients glucosyl units to make a-1 -> 6 linkages, and Y orients glucosyl units to make a-1 3 linkages, cxi is sucrose o is glucose and < is...

B Two-site insertion mechanism for the biosynthesis of Streptococcus mutans alternating comb dextran by dextransucrase... 

Figure IQ. 15. Two-site insertion mechanism for the biosynthesis of (A) L. mesenteroides B-742 regular comb dextran and for the biosynthesis of (B) S. mutans alternating comb dextran by the respective dextransucrases.

The complete biosynthesis of xanthan, a water-soluble bacterial polysaccharide with a cellulose backbone (see Chapter 6, Fig. 6.18 for the structure), has been worked out by Dankert et al. [111-114]. The pentasaccharide repeating unit is attached to a polyprenol pyrophosphate lipid carrier [111], and the polymerization takes place by the addition of the repeating pentasaccharide from the polyprenol pyrophosphate pentasaccharide to the reducing end of the growing xanthan chain [114]. The mechanism of xanthan chain elongation is an insertion mechanism similar to those of murein. Salmonella 0-antigen, the dextrans, mutan, and alteman. 

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