Notes
Glycogen Phosphorylase
This is a dimer, meaning it has 2 subunits that come together. Like the previous dimers we have seen, it has both R and T states. As a reminder, the R state is relaxed while the T state is tense.
Unlike the previous dimers, it gets covalently modified:
The A form (active) gets phosphorylated by glucagon + epinephrine signaling
The B form (less active) gets dephosphorylated
Listed from “most active” to “least active”
A form, R state
A form, T state
B form, R state
B form, T state
Liver:
In the liver, glycogen phosphorylase is ACTIVE ✔️ by default.
Inhibited by: Glucose
It is inhibited by glucose because when there is glucose around, the liver does not need to make it from glycogen. Its job is essentially complete.
Muscle:
In the muscle cells, glycogen phosphorylase is INACTIVE ❌ by default
Activated by: AMP
Inhibited by: ATP, glucose-6-phosphate
Why would we want glycogen breakdown in muscle cells at high AMP? (activated by AMP)
AMP is indicative of an energy crisis, meaning that the muscle cells are in need of ATP!
Similarly, it will be inactive in muscle cells with ATP because it means that glucose is no longer needed and there is no energy crisis.
Turning it Off…
GTPase activity of G-protein naturally converts GTP to GDP
If you remember from previous units, this is what normally happens to the G-protein coupled receptor
Phosphodiesterase converts cAMP to AMP
This makes it so the signal stops being amplified by our second messenger.
Protein Phosphatase 1 removes the phosphoryl group from Phosphorylate Kinase AND glycogen phosphorylase.