Question

4. What signals lead to the breakdown/synthesis of glycogen?

5. When is phosphorylase active/inactive? When is glycogen synthase active/inactive? What role do phosphorylase kinase, PP1c, GSK-3, and the GL or GM proteins play in regulation of glycogen synthesis/breakdown?

6. What role does Ca2+ and AMP play in the breakdown of glycogen in the skeletal muscle?

Answer 1

4. Signals like PKA begins the cascade of phosphorylation reaction that shuts down glycogen synthesis and activates glycogen breakdown . Harmones like glucagon and epinephrine trigger the breakdown of glycogen .

5.During most physiological conditions phosphorylase b is active because of inhibitory effects of ATP and glucose-6 phosphate .Where as phosphorylase a is active regarless of levels of ATP, AMP and glucose-6 phosphate.

Glycogen synthase plays an important biological role in regulating glycogen or glucose levels and is activated by dephosphorylation. In this form-a is physiologically active form of tge enzyme and form-b, phosphorylated is an inactive form.

Phosphorylase kinase is a regulatory protein and helps in contractions. PP1c helps in contraction and activation of glycogen synthase . GSK-3 activity decreases with muscle contraction. GM is essential for regulation of glycogen synthase during exercise .

6. In the muscle, glycogen is energy source for muscular activity and hence in skeletal muscle cells glycogenolysis occurs and is stimulated by adrenaline and regulated by positive and negative allosteric effects, AMP and Ca+2, ATP and glucose-6 phosphate respectively .

The delta subunit of phosphorylase kinase is a a regulatory protein that binds to calcium and is called calmodulin.It acts as calcium sensor and response to changes in intracellular calcium concentration and influences the activity of proteins with which it interacts.A rise in intracellular calcium concentration is a signal for muscle contraction and once released from sarcoplasmic reticulum, calcium binds to calmodulin of the enzyme and activates it.

AMP accumulates in the muscle during contraction due to consumption of ATP and binds to and activates the enzyme.