Biosynthesis of dextrans

Several types of enzymatic reactions were studied during the last decade for the biosynthesis of dextran. Reports on the application of chromatographic reactors for an enzyme catalyzed reaction were first demonstrated in the late 1980s [171, 172]. 

Several types of reaction may be carried out in a chromatographic reactor. The reaction can be chemical or biochemical, taking place on the stationary phase, in the mobile phase, or both. The stationary phase must be chosen to have a good retention (affinity) for at least one component of the reaction system, and in some cases it has to act as a catalyst or catalyst support. Chromatographic reactors are particularly suited to enzyme-catalysed reactions such as the inversion of sucrose and biosynthesis of dextran, to various... 

Biosynthesis of dextran polymer 81 Chromatographic pulse reactor... 

Dextran is a unique polysaccharide because of its structure (only glucose units), purity, defined branching pattern depending on the microbial sources and defined molecular weight. Today, it is produced on a commercial scale resulting from optimised biotechnological processes for the biosynthesis of dextran using preferably Leuconostoc mesenteroides. 

The mechanism of biosynthesis of dextran has been studied by incubation of Leuconostoc mesenteroides dextransucrase, either attached to the cells or in-solubilized on polyacrylamide beads, with [ C]sucrose, It is proposed that the enzyme has two catalytic sites, which both attack sucrose to give D-glucosyl complexes (see Scheme 1). The hydroxy-group at C-6 of one of the o-glucosyl... 

Flavonoids, biosynthesis of, 268 Flow birefringence, studies on dextran, 349... 

Vanillin, biosynthesis of, 268 Vi Antigen, 335 Viscose process, 95 Viscosity, of dextran solutions, 350 effects of structure on, 350... 

Most microbial exopolysaccharides are apparently synthesized in-tracellularly. However, with various Leuconostoc, Streptococcus, and Bacillus species, such exopolysaccharides as dextran and levan can be formed by adding proper substrates that do not penetrate the cell membrane.216,217 Surprisingly little information is available about the biosynthesis of biopolymers of commercial value. However, as most of them are probably formed intracellularly, the process by which substrates enter microbial cells, where they are modified by various enzymic reactions and finally excreted in polymerized form into the medium, bears attention. Even with a lack of complete biosynthetic information, the results of research on related micro-organisms may be extrapolated to form a reasonable hypothesis for the biosynthesis of polysaccharides. 

Neely, W. Brock, Dextran Structure and Synthesis, 15, 341 — 369 Neely, W. Brock, Infrared Spectra of Carbohydrates, 12, 13 — 33 Neuberg, Carl, Biochemical Reductions at the Expense of Sugars, 4, 75—117 Neufeld, Elizabeth F., and Hassid, W. Z., Biosynthesis of Saccharides from Glycopyranosyl Esters of Nucleotides ( Sugar Nucleotides ), 18, 309 - 356... 

The number of the branch PMMA chain per one molecule of of dextran was 0.05-0.3 for M 6,000 of the dextran (d1), 0.35 0.55 for P 61,000 (d2), and 0.8-1.6 for M 196,000 (D3), respectively. If some information are given from matrix molecule (backbone polymer) to the resultant pol3nner (the branch polymer), it may be very important to solve the template reaction of nucleic acid (DNA, RNA) in biosynthesis. These graft pol3nnerization of vinyl compounds onto water-soluble polysaccharides must be consequence from this point as same as matrix polymerization by Kargin et al.. As seen in Fig. 10, with the sample of D1 and D2, the plot of In(lOO-i) ) versus time (t) gave the straight line in accordance with the equation ... 

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...

A Two-site insertion mechanism for the biosynthesis of Leuconostoc mesenteroides B-742 regular 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. 

---