32. The signal transduction pathway involved in glycogen metabolism triggered in the liver by the hormone epinephrine involves the following steps:
(A) Activation of G protein (G_αβγ) by the activated receptor
(B) Protein kinase A activation
(C) Second messenger generation (3′, 5′ cyclic AMP)
(D) Adenylyl cyclase activation
Which one of the following combinations describes these processes in the right order?
(1) A.D-C-B(2) A-C.D-B
(3) D-A-B-C          (4) A-D-B-C

Introduction

Epinephrine, a key hormone regulating glycogen metabolism in the liver, triggers a complex signal transduction pathway that mobilizes glucose. The efficiency and specificity of this pathway depend on the correct sequence of molecular events starting from hormone binding to amplification inside the cell.

The Key Steps in the Pathway

1. Activation of G protein (A): Epinephrine binds to its G protein-coupled receptor (GPCR) on hepatocyte membranes, which activates the heterotrimeric G protein (Gαβγ). Upon activation, the GDP bound to the Gα subunit is exchanged for GTP, releasing the active Gα subunit.

2. Adenylyl Cyclase Activation (D): The activated Gα subunit (specifically Gαs) binds and activates the membrane-bound enzyme adenylyl cyclase.

3. Second Messenger Generation (C): Activated adenylyl cyclase catalyzes the conversion of ATP to cyclic AMP (cAMP), a key second messenger.

4. Protein Kinase A Activation (B): cAMP binds to the regulatory subunits of protein kinase A (PKA), releasing the catalytic subunits that phosphorylate target enzymes involved in glycogen breakdown.

Correct Order of the Processes

The correct chronological sequence is:

A (G protein activation) → D (Adenylyl cyclase activation) → C (cAMP generation) → B (PKA activation)

This corresponds to option (4) A-D-B-C if we consider the question’s labeling.

Insights from Research

Research has demonstrated that epinephrine increases glycogenolysis primarily through this classical GPCR-cAMP-PKA pathway. Additionally, other factors like epidermal growth factor (EGF) may modulate the response by influencing cAMP levels but the fundamental cascade remains as described.

Summary

• Epinephrine binds and activates the GPCR

• GPCR activates G protein (Gαβγ)

• Activated Gα activates adenylyl cyclase

• Adenylyl cyclase produces cAMP

• cAMP activates PKA

• PKA phosphorylates enzymes to mobilize glycogen

This pathway ensures robust and timely glucose release to maintain energy homeostasis in response to stress or fasting.