Branching of the glycogen

In both liver and muscle, glycogen breakdown is initiated by the enzyme phosphorylase which catalyses the sequential removal of terminal glucosyl residues from the branches of the glycogen molecule by phosphorolytic cleavage. The presence of inorganic phosphate is essential and the glucosyl residues are released in the form of glucose 1-phosphate. Phosphorylase is unable to... 

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 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.

Branching enzyme is responsible for the a-1,6-branching of the a-1,4-chain in the synthesis of glycogen. Branching enzyme enhances the rate of polysaccharide (endogenous glycogen) synthesis from glucose-1 -phosphate by phosphorylase. 

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 ... 

We have already considered regulatory cascades initiated respectively by the (3 and a2 adrenergic receptors. The effect of these cascades on glycogen phos-phorylase is outlined on the left side of Fig. 11-4. One branch of the cascade sequence begins with release of... 

Figure 21.1. Glycogen Structure. In this structure of two outer branches of a glycogen molecule, the residues at the nonreducing ends are shown in red and residue that starts a branch is shown in green. The rest of the glycogen molecule is represented by R.

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