Epinephrine activates glycogenolysis in liver via adenylyl cyclase. In this signaling cascade, which step is specifically amplified by the protein kinase A (PKA)-mediated phosphorylation of phosphorylase kinase?

• A Conversion of inactive Gs-alpha to active Gs-alpha by GTP exchange
• B Degradation of cAMP by phosphodiesterase to terminate the signal
• C Inhibition of glycogen synthase by direct phosphorylation by PKA
• D Activation of phosphorylase b to phosphorylase a, massively amplifying glucose-1-phosphate production ✓

Explanation

PKA phosphorylates (activates) phosphorylase kinase, which in turn phosphorylates glycogen phosphorylase b (inactive) to glycogen phosphorylase a (active). This sequential kinase cascade amplifies the hormonal signal enormously, releasing glucose-1-phosphate from glycogen. Gs-alpha activation by GTP occurs upstream of PKA. PKA does directly phosphorylate glycogen synthase (inactivating it), but the amplification step described is the phosphorylase b-to-a conversion. Phosphodiesterase terminates signaling, not amplifies it.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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