Enzymes dextransucrase

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... 

Strain NRRL B-512(F) produces large proportions of the extracellular enzyme dextransucrase,339 which is responsible for the synthesis of linear sequences ofa-D-(l —> 6)-linked D-glucosyl residues. The enzyme transfers the D-glucosyl group from a sucrose molecule to an enlarging dextran chain and liberates the D-fructose portion. As dextransucrase is an extracellular enzyme, production of dextran by cell-free, culture filtrates can result in enhanced yield and quality, and ease of purification of the product. By suitable adjustment of the conditions, products in a chosen molecular-weight range can be obtained. Formation of branches is not yet well understood, but the enzymes responsible will certainly be found. 

The dextrans are obtained microbiologically from sucrose by fermentation with microbes from the genera Leuconostoc, Acetobacter and Streptococcus, which contain the enzyme dextransucrase. 

K. D. Reh, M. Noll-Borchers, and K. Buchholz, Productivity of immobilized dextransucrase for leucrose formation, Enzyme Microb. Technol., 19 (1996) 518-524. 

V. Monchois, M. Remaud-Simeon, R. R. Russell, P. Monsan, and R. M. Willemot, Characterization of Leuconostoc mesenteroides NRRL B-512F dextransucrase (DSRS) and identification of amino-acid residues playing a key role in enzyme activity, Appl. Microbiol. Biotechnol., 48 (1997) 465 -72. 

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. 

By treating the glucose-enzyme complexes with pepsin, Mooser et al.63 later isolated and sequenced the active-site peptides from dextransucrase (GTF-S) and mutansucrase (GTF-I), each of which has a covalently linked glucose. The two peptides had nine amino acids and the following similar, but not identical, sequences ... 

Fig. 4.—Mechanism for the cleavage of sucrose and the formation of an a-(l — 6)-glycosidic bond by dextransucrase. Reaction 1 Nucleophilic displacement and protonation of the fructose moiety to form a glucosyl-enzyme intermediate. Reaction 2 Formation of an a-( 1 — 6)-glycosidic bond by attack of a 6-hydroxyl group at C-l of a glucosyl-enzyme complex the attack is facilitated by abstraction of a proton from the hydroxyl group by the imidazole group. (From Ref. 92, reprinted by permission of Elsevier Science Press B.V.)...

---