Chapter 25 - Glycogen Synthesis

Glycogen Synthase

KEY REGULATORY ENZYME

• active when in unphosphorlyated a form

Takes up one glucose unit from UDP glucose and add its to the glycogen chain through a alpha 1,4 glycosidic bond. Creates starter chain

Glycogenin

Priming enzyme, makes alpha 1,4 glycosidic bond remains attached to glycogen via a tyrosine residue.

Glycogen breakdown regulation through hormones

Exercise → epinephrine hormone released in muscle and liver and it breaks down glycogen for immediate energy

Fasting → glucagon hormone released in liver breaking down glycogen to release into blood

What happens to PKA when glucagon/epinephrine signal stops? (meaning body no longer needs to break down glycogen) 

PKA becomes inactive.

What enzyme becomes active after PKA is inactive?

Protein Phosphatase 1 (PP1)

What does PP1 do?

Removes phosphate groups from enzymes reversing what PKA did (dephosphorylates enzymes).

How does PP1 affect glycogen synthase?

Converts glycogen synthase b (inactive) → a (active) → promotes glycogen synthesis. how?: 

• Dephosphorylates glycogen synthase b → active glycogen synthase a
  → 🧱 Starts building glycogen

• Dephosphorylates phosphorylase kinase + glycogen phosphorylase
  → 💥 Stops glycogen breakdown

What results in the phosphorylation and inactivation of glycogen synthase kinase ?

Insulin - increases glucose uptake (via GLUT4), converts glucose to G6P (which activates glycogen synthase), inhibits glycogen synthase kinase, and activates PP1 to dephosphorylate and activate glycogen synthase.

In the liver, what happens if high blood glucose levels

inhibits degradation stimulates glycogen synthesis

What causes Type 1 Diabetes?

An autoimmune attack on pancreatic β-cells, leading to no insulin production.

What hormone is high in Type 1 Diabetes?

Glucagon — because insulin levels are very low or absent.

What happens to glucose in Type 1 Diabetes?

Glucose cannot enter cells, so blood glucose remains high and excess glucose is lost in urine.

Why does ketoacidosis occur in Type 1 Diabetes?

The liver metabolizes fatty acids into ketone bodies for energy, causing acid buildup in the blood.

What causes Type 2 Diabetes?

Cells become resistant to insulin, meaning they don’t respond properly even though insulin is present.

What happens to blood glucose in Type 2 Diabetes?

Glucose uptake is impaired, leading to high blood glucose levels.

Why is fatty acid release from adipose tissue lower in Type 2 Diabetes?

Because insulin still inhibits hormone-sensitive lipase, even though glucose uptake is reduced.