How does insulin promote glycogenesis?

Insulin activates signaling through its receptor tyrosine kinase, IRS proteins, PI3K, and Akt. Akt inhibits glycogen synthase kinase 3 (GSK-3), reducing inhibitory phosphorylation of glycogen synthase. Insulin signaling also favors activation of protein phosphatase 1 (PP1), which dephosphorylates glycogen synthase to the more active GSa form. In muscle, insulin stimulates GLUT4 translocation to increase glucose uptake, providing substrate for glycogenesis.

Why do glucagon and epinephrine inhibit glycogenesis?

In liver, glucagon raises cAMP and activates protein kinase A (PKA), which phosphorylates glycogen synthase toward the less active GSb form while simultaneously activating glycogen breakdown. Epinephrine similarly raises cAMP in liver and muscle (beta-adrenergic signaling) and in muscle can also use alpha-adrenergic pathways that increase IP3 and cytosolic Ca2+. The net effect favors glycogenolysis over synthesis during fasting or stress.

What is the role of glucose-6-phosphate in regulating glycogen synthase?

Glucose-6-phosphate is an allosteric activator of glycogen synthase. High cytosolic G6P indicates abundant substrate flux through glycolysis and related pathways; even the phosphorylated GSb form can be partially activated when G6P is elevated, allowing some glycogenesis when substrate is plentiful despite concurrent hormonal phosphorylation.

Chapter 4 of 5 - Glycogenesis Course

Regulation of Glycogenesis

Hormones and small molecules shift glycogen synthase between active and inactive states - aligning storage with the fed state and shutting synthesis down when you need to release glucose or fuel fight-or-flight.

Hormonal Regulation

Insulin is the main physiologic driver of glycogenesis after meals. Glucagon (liver) and epinephrine (liver and muscle) oppose storage when blood glucose must be exported or when muscle needs rapid glycogen mobilization.

Insulin (Fed State)

Activates protein phosphatase 1 (PP1), which dephosphorylates glycogen synthase toward the more active GSa form.
Can lower cAMP in some tissues by activating phosphodiesterases, reducing PKA-driven phosphorylation of synthase and other targets.
In muscle, stimulates GLUT4 translocation to the plasma membrane, increasing glucose uptake and substrate delivery for glycogenesis.

Glucagon (Fasting - Liver)

Binds hepatocyte receptors, activates adenylyl cyclase, raises cAMP, and activates PKA.
PKA phosphorylates glycogen synthase toward the less active GSb form, inhibiting glycogenesis.
Concurrently activates glycogenolysis so the liver can export glucose (via glucose-6-phosphatase).

Epinephrine (Stress - Muscle and Liver)

Similar cAMP-PKA cascade in liver and muscle (especially via beta-adrenergic receptors), phosphorylating glycogen synthase and favoring breakdown over synthesis.
In muscle, alpha-adrenergic signaling can raise IP3 and cytosolic Ca2+, activating phosphorylase kinase and reinforcing glycogenolysis for contraction.

Allosteric Regulation

Glucose-6-phosphate (G6P) is a key allosteric activator of glycogen synthase. When G6P is high, synthase activity increases even for the phosphorylated GSb form, so abundant substrate flux can partially override inhibitory phosphorylation. This couples synthesis to the availability of activated glucose within the cell.

ATP reflects energy charge; high ATP favors anabolic storage. Free glucose in the hepatocyte also contributes to the regulatory picture in liver by modulating phosphatase activity toward glycogen synthase (the “glucose signal” that helps reset synthase toward the active form after a carbohydrate meal).

Quick Check

Which hormone promotes glycogenesis?

Covalent Modification - The Phosphorylation Cascade

Glycogen synthase sits at the intersection of opposing kinase and phosphatase networks. The diagrams below summarize the major signaling routes that push synthase toward the active (dephosphorylated) or inactive (phosphorylated) state.

Insulin - storage favored

Insulin

↓

Receptor tyrosine kinase

autophosphorylation, IRS docking

↓

IRS → PI3K → PIP3

↓

Akt (PKB)

phosphorylates and inhibits GSK-3

↓

Glycogen synthase

less GSK-3-mediated phosphorylation; PP1 favors GSa

Glucagon - synthesis dampened

Glucagon (liver)

↓

cAMP ↑

↓

PKA active

↓

Glycogen synthase phosphorylated

shift toward inactive GSb; glycogenolysis stimulated

Tissue-Specific Differences

Liver

Hepatocytes express glucagon receptors and glucose-6-phosphatase. Typical total hepatic glycogen is on the order of about 100 g in an adult. Liver glycogen buffers blood glucose: it is built when glucose and insulin are high and broken down between meals to maintain euglycemia.

Skeletal Muscle

Muscle stores roughly about 400 g of glycogen in a trained adult, used for local ATP production during contraction. Muscle responds strongly to epinephrine and to insulin (GLUT4). It lacks glucose-6-phosphatase, so glycogen-derived glucose-6-phosphate is committed to glycolysis in the fiber and cannot be released as free glucose into the bloodstream.

Fill in the Blank

Insulin promotes glycogenesis by activating________which dephosphorylates glycogen synthase, converting it from the inactive GSb form to the active GSa form.

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