Glycogen mobilization is controlled hormonally by a metabolic cascade that is activated by cAMP formation and involves successive phosphorylations of enzyme proteins (Figure 13.18)

1. Glucagon or epinephrine interacts with a cellular receptor, which sends a signal via a G protein to the membrane-bound enzyme, adenylate cyclase.

2. Adenylate cyclase, in turn, forms cAMP from ATP.

3. cAMP activates cAMP-dependent protein kinase.

4. cAMP-dependent protein kinase phosphorylates phosphorylase b kinase.

5. Phosphorylase b kinase phosphorylates glycogen phosphorylase b (the inactive form) to convert it to glycogen phosphorylase a (the active form).

6. Glycogen phosphorylase a catalyzes phosphorolysis of glycogen to form glucose-1-phosphate.

Glycogen breakdown is reciprocally regulated with glycogen synthesis as follows:

cAMP exerts two effects in inhibiting glycogen synthesis: (1) phosphorylation of glycogen synthase (the enzyme that makes glycogen), inactivating it in the process, and (2) inhibition of phosphoprotein phosphatase (PP-1), whose activity tends to restore the activity of glycogen synthase. PP-1 and other phosphoprotein phosphatases play converse roles in glycogenolysis, in which dephosphorylation of glycogen phosphorylase b kinase causes its inactivation.