A 1,4 glycosidic bonds

Glycogen is a polymer of D-glucose monomers, linked via a 1-4 glycosidic bonds with al-6 links creating branch points (Figure 6.21). 

The process starts in the mouth during mastication where salivary a-amy-lase cleaves some of the a-1,4 glycosidic bonds of starch. 

In the reverse direction, protonation of the phosphate of glucose-1-phosphate destabilizes the glycosidic bond and promotes formation of a glucosyl oxocarbonium ion-phosphate anion pair. In the subsequent step, the phosphate anion becomes essential for promotion of the nucleophilie attack of a terminal glucosyl residue on the carbonium ion. This sequence of reactions brings about a-1,4-glycosidic bond formation and primer elongation. 

Pancreatic (a) amylase is an endoglycosidase, which removes maltose units from amylose or amylopectin. It splits the a-1,4 glycoside bond. In the absence of an a-1,6 glycosidase, limit dextrin is a product in addition to maltose. Sucrose is nonreducing lactose is reducing. 

Amylose, which comprises about 20% of starch molecules, has an unbranched skeleton of glucose molecules with a-1,4 -glycoside bonds. Because of this linkage, an amylose chain adopts a helical arrangement, giving it a very different three-dimensional shape from the linear chains of cellulose. Amylose was first described in Section 5.1. 

Figure 11.10. Maltose, a Disaccharide. Two molecules of glucose are linked by an a -1,4-glycosidic bond to form the disaccharide maltose.

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. 

Starch, the plant storage polysaccharide, which is also a polymer of glucose, differs in its structure in that the glucose monomer units are joined by a(1 4) glycosidic bonds. 

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