Glycogen Metabolism

Glycogenesis

Formation of glycogen when glucose exceeds cellular need for ATP synthesis

Where does glycogenesis occur?

In the cytosol

Step 1 of glycogenesis

Glucose → Glucose 6-phosphate

What enzyme catalyzes step 1 of glycogenesis?

Hexokinase or glucokinase

Step 2 of glycogenesis

Glucose 6-phosphate → Glucose 1-phosphate

What enzyme catalyzes step 2 of glycogenesis?

phosphoglucomutase (PGM)

Step 3 of glycogenesis

Glucose 1-phosphate + Uridine triphosphate → UDP-glucose + inorganic pyrophosphate

What additional reaction occurs at step 3 of glycogenesis?

inorganic phosphate + H2O → 2 inorganic phosphates

What enzyme catalyzes step 3 of glycogenesis?

UDP-glucose pyrophosphorylase (UGP)

Function of UDP-glucose

Tag for glucose that shows its used for glycogenesis

Glycogenin Protein

Primer for glycogen synthesis

Step 4 of glycogenesis

Glycogenin links a string of glycogen residues and recruits glycogen synthase to elongate chain

Step 5 of glycogenesis

Glycogen-branching enzyme branches the glucose chain

Glycogenolysis

Breakdown of glycogen in response to low blood glucose

Where does Glycogenolysis take place?

In the cytosol

Step 1 of glycogenolysis

Glycogen phosphorylase chops the glucose chain and releases glucose-1-phosphate

Step 2 of glycogenolysis

Glycogen debranching enzyme removes glycogen branches → becomes linear

Step 3 of glycogenolysis

Glycogen phosphorylase continues removing glucose-1-phosphate molecules

Step 4 of glycogenolysis

Conversion of glucose-1-phosphate to glucose-6-phosphate

Enzyme in step 4 of glycogenolysis

Phosphoglucomutase (PGM)

High level of ATP/glucose 6-phosphate in liver

Suppresses glycogenolysis

Calcium ions released by muscle cells

Stimulates glycogen release

Insulin

Stimulates glycogenesis 

Glucagon and Epinephrine

Stimulates glycogenolysis

What activates glycogen synthase?

Insulin

What activates phosphorylase kinase?

Glucagon/epinephrine

What inactivates glycogen synthase?

Glucagon/epinephrine

What inactivates phosphorylase kinase?

Insulin

Metabolism of fructose

Enters cytosol → phosphorylated to fructose 6-phosphate → dihydroxyacetone phosphate and glyceraldehyde 3-phosphate → pyruvate

Metabolism of Galactose

Galactose → Galactose 1-phosphate (swaps UDP glucose with galactose) → proceeds to glycolysis