Which second messenger system is activated by glucagon in the liver to stimulate glycogenolysis?
- A cAMP via adenylyl cyclase leading to PKA activation ✓
- B IP3/DAG via phospholipase C
- C cGMP via guanylyl cyclase
- D Tyrosine kinase phosphorylation cascade
Correct answer: A. cAMP via adenylyl cyclase leading to PKA activation
Explanation
Glucagon binds its GPCR (Gs-linked) on hepatocytes, activating adenylyl cyclase to generate cAMP from ATP. Elevated cAMP activates PKA (protein kinase A), which phosphorylates phosphorylase kinase (activating it), which then phosphorylates glycogen phosphorylase b to its active 'a' form, initiating glycogenolysis. PKA simultaneously phosphorylates and inactivates glycogen synthase, preventing glycogen synthesis.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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