A-l,6-Glycosidic bonds

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.)...

D. a-Amylase hydrolyzes a( 1 4) glycosidic bonds present in starch (amylose and amylopectin) in a random fashion leaving primarily the disaccharide maltose, the trisaccharide maltotriose, and an oligosaccharide known as the a-limit dextrin, which is composed of 6 to 8 glucose residues with one or more a(l—>6) glycosidic bonds. Galactose and fructose are not present in starch. 

Amylopectin is a highly branched amylose in which the branches are attached to the C-6 hydroxyl groups by a(l —> 6) glycosidic bonds (Figure 17.12). The main chains consist of a(l —> 4) glycosidic bonds. Each branch contains 20-25 glucose units, and there are so many branches that the main chain can scarcely be distinguished. 

Starch, the storage polysaccharide of plant cells, is composed of approximately 80% amylose and 20% amy-lopectin. Amylose is a pol5rmer of a-D-glucose units bonded a(l — 4). Amylose forms a helix. Amylopectin has many branches. Its main chain consists of a-D-glucose units bonded a(l — 4). The branches are connected by a(l —> 6) glycosidic bonds. 

Pullulanase Type I (Pullulanase EC3.2.1.41) Hydrolysis of a-l, 6-glycosidic bond in pullulan and straight-chain oligosaccharides to generate maltotriose and linear oligosaccharides. 

Microbial fermentation processes of sucrose yield dextrans, which are mainly linear neutral polymers of a-d-glucose linked by a-(l-> 6) glycosidic bonds (may include variable amounts of a-(1 3) branches). 

Sephadex is a three-dimensional network in which soluble dextran chains are crosslinked by glycerol ether bonds or reaction with epichlorohydrin in alkahne solution. Dextran is a branched-chain polysaccharide composed of o-glucose units which are jointed mainly by means of a-l,6-glycosidic bonds and is branched by 1,2-, 1,3-, and 1,4-glycosidic linkages. Sephadex is stable to chemical attack by, for example, alkali and weak acids and can be heated without any change in properties. 

Figure 3 shows examples of a P-(l,4)glycosidic bond (I), a a-(l,4)-glycosidic bond (II), and a (l,6)-glycosidic bond (ID). This (l,6)-linkage is common to many branched polysac-... 

Amylopectin is a branched polymer of approximately 10,000 D-glucose units joined by a-l,4-glycosidic bonds. Branches consist of 24-30 D-glucose units started by a-l,6-glycosidic bonds. 

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