Epinephrine Signaling and Glycogen Breakdown Pathway

Vocabulary flashcards covering the components, steps, regulation, and perturbations of the epinephrine-stimulated glycogen breakdown pathway.

Epinephrine pathway

A signaling cascade utilized during fasting when the body needs glucose, resulting in the conversion of glycogen to glucose catalyzed by glycogen phosphorylase.

Target tissues of epinephrine

Muscle and liver, which are the two tissues where epinephrine triggers glycogen breakdown.

G protein-coupled receptor (GPCR)

The cell-surface receptor on muscle or liver cells to which epinephrine binds to initiate the signaling cascade.

Heterotrimeric G protein subunits

The three subunits—$\alpha$, $\beta$, and $\gamma$—that make up the G protein in the epinephrine pathway.

G protein $\alpha$ subunit

The subunit of the G protein that binds GDP in its inactive ('off') state or GTP in its active ('on') state and activates adenylyl cyclase.

GDP (Guanosine diphosphate)

The nucleotide bound to the G protein's $\alpha$ subunit when it is in the inactive ('off') conformation.

GTP (Guanosine triphosphate)

The nucleotide bound to the G protein's $\alpha$ subunit when it is active ('on'), enabling it to activate adenylyl cyclase.

Adenylyl cyclase

The enzyme activated by the GTP-bound $\alpha$ subunit that converts ATP into cAMP and PPi ($\text{ATP} \rightarrow \text{cAMP} + \text{PPi}$).

Pyrophosphate ($\text{PPi}$)

The molecular byproduct formed alongside cyclic AMP when adenylyl cyclase converts ATP into cAMP.

Cyclic AMP (cAMP)

The intracellular second messenger generated from ATP by adenylyl cyclase that activates protein kinase A (PKA).

Protein kinase A (PKA)

The kinase activated by cAMP ('A' for AMP) that activates phosphorylase kinase and inactivates glycogen synthase.

Glycogen synthase

The glycogen-synthesizing enzyme that is inactivated by PKA so the cell stops making glycogen while breaking it down.

Dual regulation by PKA

The simultaneous activation of glycogen breakdown and inhibition of glycogen synthesis, ensuring both effects push unidirectionally toward releasing glucose.

Phosphorylase kinase

The kinase activated by PKA that subsequently activates glycogen phosphorylase.

Glycogen phosphorylase

The terminal enzyme activated by phosphorylase kinase that breaks down glycogen to release glucose 1-phosphate.

Glucose 1-phosphate

The primary end product released from glycogen breakdown by glycogen phosphorylase in the epinephrine pathway.

Phosphodiesterase

The enzyme that terminates the epinephrine signal by destroying the second messenger through converting cAMP into AMP.

AMP (Adenosine monophosphate)

The inactive product generated when phosphodiesterase breaks down cAMP to shut down intracellular signaling.

Signal amplification

A mechanism occurring at several steps—where one adenylyl cyclase produces many cAMP molecules and kinases activate many targets—allowing one epinephrine molecule to release a huge amount of glucose.

Epinephrine receptor antagonist

A drug that blocks the epinephrine GPCR, preventing receptor activation so the G protein remains GDP-bound, producing no cAMP, no PKA activation, and no glycogen breakdown.

Adenylyl cyclase inhibition

A condition where epinephrine binding and G protein activation still occur, but no cAMP is formed, leaving PKA, phosphorylase kinase, and glycogen phosphorylase off.

Phosphodiesterase inhibition (e.g., caffeine)

The blockage of cAMP degradation, causing cAMP levels to stay elevated and PKA to remain active, resulting in stronger and longer-lasting glycogen breakdown.

Loss of $\alpha$ subunit GTPase activity

A defect causing the G protein's $\alpha$ subunit to stay GTP-bound, leading adenylyl cyclase to continually produce cAMP even after epinephrine departs so glycogen breakdown cannot be shut off.

PKA cAMP-binding mutation

A mutation preventing PKA from binding cAMP; upstream signaling through cAMP occurs, but downstream kinases remain off while glycogen synthase stays on.

Phosphorylase kinase deficiency

A situation where cAMP levels rise, PKA activates, and glycogen synthase is shut off, but glycogen phosphorylase is never activated, preventing glycogen breakdown.

Why does the epinephrine pathway occur?

he body is fasting and needs glucose from glycogen.

What is the first step?

Epinephrine binds a G protein-coupled receptor (GPCR) on muscle or liver cells.

Epinephrine pathway, step 2 of 8. Step 1: Epinephrine binds its GPCR. What happens next?

The G protein's α subunit releases GDP and binds GTP, so it becomes active.

Epinephrine pathway, step 3 of 8. Step 2: The G protein's α subunit swaps GDP for GTP. What happens next?

The GTP-bound α subunit activates adenylyl cyclase.

step 4 of 8. Step 3: The α subunit activates adenylyl cyclase. What happens next?

Adenylyl cyclase converts ATP into cAMP + PPi.

step 5 of 8. Step 4: Adenylyl cyclase converts ATP into cAMP. What happens next?

cAMP activates protein kinase A (PKA).

step 6 of 8. Step 5: cAMP activates PKA. What happens next?

PKA activates phosphorylase kinase. It also inactivates glycogen synthase, so the cell stops making glycogen.

tep 7 of 8. Step 6: PKA activates phosphorylase kinase. What happens next?

Phosphorylase kinase activates glycogen phosphorylase

step 8 of 8. Step 7: Phosphorylase kinase activates glycogen phosphorylase. What happens next?

Glycogen phosphorylase breaks down glycogen, releasing glucose 1-phosphate.