Insulin stimulates glycogen synthesis in hepatocytes within minutes of its release. Which immediate post-receptor event accounts for activation of glycogen synthase during this period?
- A Insulin receptor phosphorylates glycogen synthase directly on serine residues, activating it
- B Insulin increases intracellular calcium, which activates calmodulin-dependent glycogen synthase kinase
- C Insulin signaling activates protein phosphatase-1, which dephosphorylates glycogen synthase to its active a form ✓
- D Insulin triggers proteasomal degradation of phosphorylase kinase, removing tonic inhibition
Correct answer: C. Insulin signaling activates protein phosphatase-1, which dephosphorylates glycogen synthase to its active a form
Explanation
The phosphorylated (b) form of glycogen synthase is inactive, and the dephosphorylated (a) form is active. Insulin signaling leads to activation of protein phosphatase-1, which removes phosphate groups from glycogen synthase and also from phosphorylase kinase and glycogen phosphorylase, simultaneously promoting synthesis and blocking degradation. Direct receptor phosphorylation of the synthase does not occur, and calcium signaling opposes glycogen synthesis in liver.
Reference: Lippincott Illustrated Reviews: Biochemistry, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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