Sucrose polysaccharide formation

Figure 34.5 Extracellular polysaccharide formation from dietary sucrose by bacteria within dental plaque...

Tarr and Hibbert27 determined the optimal conditions for the preparation of dextran. Sucrose was found to be the only suitable carbohydrate substrate, although some strains produced a small amount of dextran from D-glucose. The formation of dextran from D-glucose had been observed occasionally by earlier workers, but such formation is of somewhat transient character, occurring only when the cultures are very active with respect to the formation of polysaccharides from sucrose.27 The nutrient solution (pH 8.0) used by Tarr and Hibbert27 for dextran... 

The number of enzymes responsible for the formation of polysaccharides from sucrose is also restricted. The two best known enzymes that form polysaccharides from sucrose are the dextran, synthesized by the microorganism Leuconostoc mesenteroides (8) and related organisms, and the levan, produced from the same substrate by Acetobacter levanicum (9) and other species. 

In 1861, Louis Pasteur1 reported a polysaccharide that was produced from sucrose. In 1874, Scheubler2 determined its empirical formula and named it dextran. The formation of polysaccharide (dextran) was observed as the result of bacterial transformation of sucrose solutions into viscous solutions, gels, and/or flocculent precipitates.3 The synthesis of dextran from sucrose by a cell-free... 

The genera of bacteria recognized to produce enzymes capable of synthesizing polysaccharides from sucrose are principally Leuconostoc and Streptococcus. These two genera are gram-positive, facultatively anerobic cocci that are closely related to each other. One notable difference between them is that, until recently (see Section n.l), Leuconostoc species required sucrose in the culture medium to induce the formation of the enzyme(s), whereas the Streptococcus species did not require sucrose in the culture medium for the formation of the enzymes. Thus, the Leuconostoc species were INDUCIBLE for the formation of glucansucrases, and the Streptococcus species were constitutive for their formation. 

The kinetics of the xanthation of sucrose were studied in the same year by Cherkasskaya, Pakshver, and Kargin, who determined potentiometric-ally the concentrations of the dithiocarbonate derivative and also of inorganic sulfide and trithiocarbonate. The rate of formation of 0-(sodium thiol-thiocarbonyl)sucrose was found to pass through a maximum with increasing alkali concentration, presumably due to a shift of the equilibrium in favor of side reactions in strongly alkaline solution. This result appears to parallel the qualitative findings of Lieser and Hackl for polysaccharides. 

Disaccharides and polysaccharides are formed from sugars during condensation reactions, in which water is a byproduct. Though there are many more steps that are not shown here, the net equation below describes the formation of the disaccharide sucrose. 

With few exceptions, enzymatic processes in carbohydrates cause degradation. Enzymes are used in the form of pure or semipure preparations or together with their producers, i.e., microorganisms. Currently, semisynthetic enzymes are also in use. Alcoholic fermentation is the most common method of utilization of monosaccharides, sucrose, and some polysaccharides, e.g., starch. Lactic acid fermentation is another important enzymatic process. Lactic acid bacteria metabolize mono- and disaccharides into lactic acid. This acid has a chiral center thus either D(-), L(+), or racemic products can be formed. In the human organism, only the L(+) enantiomer is metabolized, whereas the D(-) enantiomer is concentrated in blood and excreted with urine. Among lactic acid bacteria, only Streptococcus shows specificity in the formation of particular enantiomers, and only the L(+) enantiomer is produced. 

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