Dextransucrase dextran synthesis

A. Tanriseven and J. F. Robyt, Inhibition of dextran synthesis by acceptor reactions of dextransucrase and the demonstration of a separate acceptor binding-site, Carbohydr. Res., 225 (1992) 321-329. 

McCabe and Smith" have observed that the cell-associated dextransucrase elaborated by S. mutans strain Kl-R is not bound to the cell-wall structure, but is derived from a water-soluble form of the enzyme that becomes incorporated into the (dextran) cell-capsule during dextran synthesis. As synthesis of dextran proceeds, the initially soluble enzyme is converted into two dextran-bound enzyme fractions, one bound reversibly and the other bound irreversibly to the water-insoluble polysaccharide. The soluble dextransucrase is progressively changed to the irreversibly bound enzyme and is inactivated. In terms of the reaction... 

Neely proposed that three activities characterize the acceptor (or receptor) site of dextransucrase, namely, binding of an acceptor molecule, transfer of an a-n-glucopyranosyl group to the acceptor molecule, and the extrusion of the n-glucopyranosyl acceptor to an adjacent site on the enzyme (dextran synthesis) or into solution (oligosaccharide synthesis). 

The uncertainty that exists in oiu- understanding of the precise nature of die role played by dextransucrases in branched-dextran synthesis (see p. 418) is well reflected in the numerous mechanisms of branch-formation that have thus far been formulated. 

L. mesenteroides also contains a sucrose phosphorylase. However, since it has been demonstrated that sucrose is converted to dextran by dex-transucrase in the absence of a detectable quantity of inorganic phosphate and that the enzyme does not form polysaccharide from glucose-l-phos-phate, it is clear that phosphorylase is not involved in dextran synthesis. The evidence indicates that the dextransucrase acts by a direct transfer of glucose units. The reaction appears to involve the substitution of a 1,6-glucosidic linkage for a glucose-fructose bond and can be represented by. the following equation ... 

J. F. Robyt and S. H. Eklund, Stereochemistry involved in the mechanism of action of dextransucrase in the synthesis of dextran and the formation of acceptor products, Bioorg. Chem., 11 (1982) 115-132. 

F. Paul, E. Oriol, D. Auriol, and P. Monsan, Acceptor reaction of a highly purified dextransucrase with maltose and oligosaccharides. Application to the synthesis of controlled molecular weight dextrans, Carbohydr. Res., 149 (1986) 433-441. 

Fig. 7 Mechanism proposed for the synthesis of the a-(l 6) glycosidic linked dextran backbone (A) and a-(l -> 3) branches (B) by B-512F dextransucrase (adapted from [12])...

FIG. 3.—Two-site insertion mechanism for the synthesis of Leuc. mesenteroides B-512F dextran by dextransucrase. The symbols are the same as in Fig. 2. X orients the glucosyl units so that their 6-hydroxyl groups can attack at C-l of the apposed glucosyl unit. (Adapted from ref. 58 and reprinted by permission of Academic Press.)... 

Fig. 7.—Two-site insertion mechanism for the synthesis of Leuc. mesenteroides B-742 regular comb dextran by dextransucrase. The symbols are the same as in Figs. 3 and 5.

Isomaltooligosaccharides (as mixtures of different a-glucooligosaccharides) are produced in part by dextransucrases using sucrose as the substrate and maltose as an acceptor. They have been used as sweeteners in Europe and Japan for years, mainly in the field of prebiotics, and dermocosmetics [11-13]. Several different methods can be used to produce them by using such carbohydrates as sucrose, maltose, starch, and dextran. For the industrial production of a-glucooligosaccharides with sucrose as the substrate and maltose as the acceptor by one specific dextransucrase (of L. mesenteroides NRRL B-1299), the kinetic behavior of the enzyme has been characterized in order to optimize the synthesis [14]. 

In principle, several routes exist for enzymatic isomaltose synthesis. With respect to cost-effectiveness it is obvious that one should use substrates like sucrose or starch that can be exploited by dextransucrase (EC 2.4.1.5) or glucoamylase (EC 3.2.1.3), respectively. However, in both cases, isomaltose represents a side product which is released only in small proportions next to dextran or glucose as main products. Realization of higher yields requires extensive time and effort with respect to engineering of reaction and catalyst design. Based on kinetic investigations dextransucrase has been chosen for the production of isomaltose with sucrose as the substrate, and glucose as an acceptor (see Sect. 2.1). 

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