Glycogenolysis

Animals store glucose as glycogen, while plants store it as starch.

It is primarily stored in muscle (1–2%) and the liver (6–8%). Because of more muscular mass, the amount of stored glycogen in muscle (250 g) is almost three times greater than that in the liver (75 g).

Glycogen is highly branched polysaccharide polymer composed of many monomers of alpha-glucose containing both α-1,4 and α-1,6 glycosidic linkages.

• The prime function of liver glycogen is to maintain the blood glucose levels, particularly between meals.
• Muscle glycogen serves as a fuel reserve for the supply of ATP during muscle contraction.
• Brain depends on continuous glucose supply.
• Glycogen can generate energy in the absence of oxygen (anaerobic condition) because it is broken down into glucose and that glucose is used in anaerobic glycolysis to produce ATP (energy) without requiring oxygen.

The process of degradation of glycogen (which is stored in liver and muscle) is called glycogenolysis.

The set of enzymes which are participating in glycogenolysis are present in the cytosol.

Glycogen is degraded and release its monomer by breaking of α-1,4- and α -1,6-glycosidic bonds.

The first step in glycogenolysis is the cleavage of glycogen by the enzyme glycogen phosphorylase. This enzyme breaks sequentially the α(1→4) glycosidic bonds present in glycogen by adding inorganic phosphate (Pi) to yield glucose 1-phosphate.

This process—called phosphorolysis— continues until four glucose residues remain on either side of branching point (α -1,6-glycosidic link).

The glycogen so formed is known as limit dextrin which cannot be further degraded by enzyme Glycogen Phosphorylase.

The branches of limit dextrin are then cleaved by is a bifunctional enzyme which contains two enzyme called debranching enzyme.

• Glycosyl 4 : 4 transferase (oligo α -1,4 → α 1,4 glucan transferase): this enzyme transfer 3 glucose residues from shorter branch to another long branch, now these 3 shifted glucose molecules attached to another branch, having only α-1-4-glycosidic bond.

• Amylo α -1,6-glucosidase: breaks the α -1,6 bond at the branch here only a single glucose residue remains and releases a free glucose.

Glycogen phosphorylase again start to break the α -1,4-glycosidic bonds and single-single unit of glucose 1-phosphate gets released from the linear branch of glycogen.

The action of this enzyme starts again (as in the step 1) because the linear chain has only α -1,4-glycosidic bonds.

Glucose 1-phosphate is converted to glucose 6-phosphate by the enzyme phosphoglucomutase.

The liver, kidney and intestine contain the enzyme glucose 6-phosphatase that convert glucose 6-phosphate to glucose.Therefore, liver is the major glycogen storage organ to provide glucose into the circulation when needed.

This enzyme is absent in muscle and brain, hence glucose 6-phosphate is not converted into free glucose in the muscle and brain.

Regulator	Effect on Glycogenolysis	Mechanism
Glucagon	Stimulates	Activates cAMP pathway
Epinephrine	Stimulates	Activates cAMP pathway
Insulin	Inhibits	Causes dephosphorylation
AMP	Stimulates	Allosteric activation
ATP	Inhibits	Allosteric inhibition
Ca2+	Stimulates	Activates phosphorylase kinase