Glycogen degradation Part II

Main Ideas

Glycogen breakdown is regulated through both:

• Covalent modification: adding or removing phosphate groups.
• Allosteric regulation: molecules bind to an enzyme and change its shape and activity.

Regulation differs between the liver and muscle because they have different roles: the liver helps maintain blood glucose, while muscle breaks down glycogen mainly to meet its own energy needs.

Hormonal Activation of Glycogen Breakdown

During fasting, low blood glucose prompts the pancreas to release glucagon. During exercise, the adrenal glands release epinephrine. These signals activate a cascade:

1. Glucagon or epinephrine binds to a G-protein-coupled receptor.
2. The G protein exchanges GDP for GTP and activates adenylate cyclase.
3. Adenylate cyclase produces cyclic AMP (cAMP).
4. cAMP activates protein kinase A (PKA).
5. PKA phosphorylates and activates phosphorylase kinase.
6. Phosphorylase kinase phosphorylates glycogen phosphorylase, increasing its activity and promoting glycogen breakdown.

Calcium ions released during muscle contraction can also bind to phosphorylase kinase and partially activate it. PKA phosphorylation further activates the kinase.

Glycogen Phosphorylase States

Glycogen phosphorylase has two phosphorylation forms:

• Phosphorylase A is phosphorylated and generally more active.
• Phosphorylase B is unphosphorylated and generally less active.

The enzyme can also shift between two conformations:

• R state: More active, with a more accessible catalytic site.
• T state: Less active, with regulatory regions more obstructive to the catalytic site.

Together, phosphorylation and conformation produce four possible states. The most active is phosphorylated A in the R state; the least active is unphosphorylated B in the T state. Hormonal signaling controls phosphorylation, while allosteric regulators influence the balance between the R and T states.

Differences Between Liver and Muscle Regulation

Liver

The liver helps maintain blood glucose, so its default glycogen phosphorylase state is described as A, with the R state favored—ready to promote glycogen breakdown and glucose production.

• Glucose acts as a negative allosteric regulator. High glucose shifts the enzyme toward the less-active T state.
• When glucose is very high, insulin promotes dephosphorylation, shifting the enzyme toward the less-active B form.

Muscle

Muscle glycogen is reserved for the muscle’s own energy needs, so the default state is phosphorylase B, with the T state favored.

• AMP signals low energy and shifts the enzyme toward the active R state, promoting glycogen breakdown.
• ATP and glucose 6-phosphate signal that energy is sufficient and shift the enzyme toward the less-active T state.

How Glycogen Breakdown Is Turned Off

The signaling cascade can be stopped or reduced in several ways:

• The G protein’s built-in GTPase activity converts GTP back to GDP, reducing activation of adenylate cyclase.
• Phosphodiesterase breaks down cAMP to AMP, so cAMP no longer stimulates PKA.
• Protein phosphatase 1 removes phosphate groups from phosphorylase kinase and glycogen phosphorylase, reducing their activity.

Central Takeaway

The same basic phosphorylation cascade can promote glycogen breakdown, but the liver and muscle regulate the enzyme differently. The liver prioritizes maintaining blood glucose, while muscle responds primarily to its immediate energy needs.

Speakers or Sources Featured

One unnamed narrator or presenter explains the regulation of glycogen breakdown. No other speakers or sources are identified in the subtitles.