Dextran dextransucrase

Evidence has been presented that the synthesis of a-(l -> 3)-branch linkages in Leuconostoc mesenteroides B-512 dextran occurs by nucleophilic attack of a hydroxy-group at C-3 of a free dextran molecule on C-1 of the reducing-end of a dextran-dextransucrase complex the dextran is displaced from the complex on forming an a-(l -> 3)-linkage. Thus, the synthesis of branched linkages does not require a separate branching enzyme. 

Homopolysaccharides are synthesised by relatively few spedfic enzymes and are not constructed from subunits. The commerdally important homo polymer dextran is synthesised extracellularly by the enzyme dextransucrase. In Leuconostoc mesenteroides the enzyme is induced by the substrate sucrose. This is deaved to release free fructose and link the glucose to the redudng end of the acceptor dextran chain, which is bound to the enzyme. The product from this bacterium is composed almost exdusively of... 

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. 

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. 

D) regular comb dextran (50% a-1- 6 linkages and 50% o-l- 3 branch linkages) S3mthesized by L. mesenteroides B-742 dextransucrase ... 

Recently, it has been reported that a low molecular weight alternan, with lowered viscosity, can be produced by conducting the s3mthesis of alternan in the presence of D-glucose and endodextranase (23). The latter prevents the formation of dextran by contaminating dextransucrase and the former decreases the molecular size of the alternan by acceptor reactions that terminates alternan polymerization. 

Dextransucrase, which characteristically converts sucrose to dextran plus D-fructose, is capable of acting similarly on certain other oligosaccharides.63 1 However, this range of substrates does not include plante-ose, even though planteose has an unsubstituted a-D-glucopyranosyl moiety. 

The majority of dextrans in nature are produced extracellularly via dextran-sucrase from sucrose by several lactic acid bacteria, principally Leuconostoc and Streptococcus species [13]. Dextran is also synthesised by dextrinase of different Gluconobacter species [56]. Referring to this enzyme, fermentation of maltodextrins leads to a- —4) branched dextrans with comparatively lower Mw. However, dextransucrase from Leuconostoc mesenteroid.es NRRL B-512F has attracted most interest because of commercial use. 

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.

Fig. 8.—Thin-layer chromatographic autoradiogram of the acceptor products formed in the reaction of B-512FM dextransucrase with [14C]sucrose and D-glucose. The first acceptor product of D-glucose is isomaltose, followed in decreasing amounts by isomaltotriose through isomaltooctaose. A small amount of leucrose results from the acceptor reaction of D-fructopyranose. Dextran remains at the origin.

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