Glycogen branch points

The reactions of glycogen, DNA, RNA, and protein synthesis all use activated precursors, UDP-glucose, dNTPs, NTPs, and amino-acyl tRNAs, respectively. However, unlike the others, glycogen synthesis does not use a template in its propagation. And during the formation of glycogen, branch points are introduced into the emerging polymer (Fig. 11-10). 

Glycogenosis type IV (branching enzyme deficiency) results in the formation of a variant of glycogen, characterized by abnormally long inner and outer glucosyl chains and fewer branch points than normal. The abnormal variant is stored in sufficient amounts to cause some vacuolation. The clinical manifestations of this... 

Figure 13-13. The glycogen molecule. A General structure. B Enlargement of structure at a branch point. The molecule is a sphere approximately 21 nm in diameter that can be visualized in electron micrographs. It has a molecular mass of 10 Da and consists of polysaccharide chains each containing about 13 glucose residues. The chains are either branched or unbranched and are arranged in 12 concentric layers (only four are shown in the figure). The branched chains (each has two branches) are found in the inner layers and the unbranched chains in the outer layer. (G, glycogenin, the primer molecule for glycogen synthesis.)...

Branches are created by forming glycosidic linkages with both the 4- and 6-hydroxyl groups of the glucose residue at the branch point. The glycogen polymer is very large and contains multiple branches. 

Figure 9.16 Structures of glucopyranose homopolysaccharides. Cellulose is a linear structure of the glucopyranose units linked /3( 1 —>4). Starch consists of amylose, which has a linear < (1—>4) structure, and amylopectin, which has a(l—>6) branch points on the linear a(l—>4) chains. Glycogen has a similar structure to amylopectin, but with a greater degree of < ( 1—>6) branching.

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). 

Glycogen phosphorylase cannot break a-1,6 bonds and therefore stops when it nears the outermost branch points. 

Figure 6.6 Diagrammatic representation of a small part of the glycogen molecule. Each Line represents a chain of glucose molecules linked between the 1 and 4 positions of the glucose units. The arrowhead represents a branch-point with structure of branch-point indicated in the insert. This link is between the one and six positions.

Figure 6-5. Glycogenolysis. Degradation of glycogen occurs stepwise by hydrolysis of one glucosyl unit at a time from the nonreducing ends by phosphorylase. The limit dextrin occurs as indicated in the second step when there are four glucosyl units remaining to a branch point. Once debranching enzyme has resolved the limit dextrin, degradation by phosphorylase can resume.

A spectrum of the color reaction of glycogen with iodine is recorded. The wavelength of the absorption maximum is positively correlated with the outer chain length of glycogen (i.e., the chain length distal of the branching points) [24]. 

FIGURE 15-3 Removal of a terminal glucose residue from the nonreducing end of a glycogen chain by glycogen phosphorylase. This process is repetitive the enzyme removes successive glucose residues until it reaches the fourth glucose unit from a branch point (see Fig. 15-4). 

FIGURE 15-4 Glycogen breakdown near an (al—>6) branch point. 

Glycogen phosphorylase sequentially cleaves the cx(1 -h>4) glycosidic bonds between the glucosyl residues at the nonreducing ends of the glycogen chains by simple phosphorolysis until four glucosyl units remain on each chain before a branch point (Figure 11.7). [Note ... 

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