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SUMMARY

Glycogen, a readily mobilized fuel store, is a branched polymer of glucose residues. Most of the glucose units in glycogen are linked by α-1,4-glycosidic bonds. At about every 10th residue, a branch is created by an α-1,6-glycosidic bond. Glycogen is present in large amounts in muscle cells and in liver cells, where it is stored in the cytoplasm in the form of hydrated granules.

24.1 Glycogen Breakdown Requires Several Enzymes

Most of the glycogen molecule is degraded to glucose 1-phosphate by the action of glycogen phosphorylase, the key enzyme in glycogen breakdown. The glycosidic linkage between C-1 of a terminal residue and C-4 of the adjacent one is split by orthophosphate to give glucose 1-phosphate, which can be reversibly converted into glucose 6-phosphate by phosphoglucomutase. Branch points are degraded by the concerted action of a transferase and an α-1,6-glucosidase.
24.2 Phosphorylase Is Regulated by Allosteric Interactions and Reversible Phosphorylation

Phosphorylase b, which is usually inactive, is converted into active phosphorylase a by the phosphorylation of a single serine residue in each subunit. This reaction is catalyzed by phosphorylase kinase. The b form in muscle can also be activated by the binding of AMP, an effect counteracted by ATP and glucose 6-phosphate. The a form in the liver is inhibited by glucose. Liver phosphorylase is activated to liberate glucose for export to other organs, such as skeletal muscle and the brain. In contrast, muscle phosphorylase is activated to generate glucose for use inside the cell as a fuel for contractile activity.
24.3 Epinephrine and Glucagon Signal the Need for Glycogen Breakdown

Epinephrine and glucagon stimulate glycogen breakdown through specific 7TM receptors. Muscle is the primary target of epinephrine, whereas the liver is responsive to glucagon. Both signal molecules initiate a kinase cascade that leads to the activation of glycogen phosphorylase.