Question

In response to an external stimulus epinephrine binds to the b-adrenergic receptor activating protein kinase

A.  Explain this process at the molecular level.

b. Explain how the rates of glycogen degradation and synthesis are affected when protein kinase A is activated.

Answer 1

A.

The b-Adrenergic Receptor System Acts through the Second Messenger cAMP.

• Epinephrine binds to its specific receptor.
• The occupied receptor causes replacement of the GDP bound to Gs by GTP, activating Gs.
• Gs ( a-subunit) moves to adenylyl cyclase and activates it.
• Adenylyl cyclase catalyzes the formation of cAMP.
• cAMP activates PKA.( protein kinase A )

B.

Phosphorylation of cellular proteins by PKA causes the cellular response to epinephrine. Each molecule of PKA, catalyzes the phosphorylation of many molecules of the target protein—phosphorylase b kinase. This kinase activates glycogen phosphorylase b, which leads to the rapid mobilization of glucose from glycogen. The net effect of the cascade is amplification of the hormonal signal by several orders of magnitude, which accounts for the very low concentration of epinephrine (or any other hormone) required for hormone activity.