2. Glycogenolysis
• Glycogenolysis is the metabolic pathway by which glucose 6-
phosphate is produced from glycogen.
• This process is not simply the reverse of glycogen synthesis
(glycogenesis), because it does not require UTP or UDP
molecules.
3.
4. Glycogenolysis
The Pathway Reaction Steps
• Glycogen Degradation consists of three steps:
1. Release of a glucose residue from glycogen,
2. Glycogen remodeling to permit further degradation, and
3. Conversion of glucose 1-phosphate into glucose 6-
phosphate for further metabolism.
5. Step 1
Release of a Glucose Residue from Glycogen
• The enzyme glycogen phosphorylase cleaves a 1-4 glycosidic
bond to release glucose unit from the non-reducing end of the
glycogen molecule as glucose 1-phosphate.
7. Step 2
Glycogen Remodeling to Permit Further
Degradation
• Glycogen phosphorylase sequentially remove glucose units from
the non-reducing ends of all chains, stopping four residues from
a-(1,6) branch point.
• The a-1,6-glycosidic bonds at the branch points are not
susceptible to cleavage by phosphorylase
8. Step 2
Glycogen Remodeling to Permit Further
Degradation
• Two additional enzymes (transferase and a-1,6-Glucosidase)
remodel the glycogen for continued degradation by
phosphorylase.
• The transferase shifts a block of three glycosyl residues from one
outer branch to the non-reducing end of the linear chain.
• This transfer exposes a single glucose residue joined by an a-1,6-
glycosidic linkage.
9. Step 2
Glycogen Remodeling to Permit Further
Degradation
• a-1,6-Glucosidase, also known as the debranching enzyme,
hydrolyzes the a-1,6-glycosidic bond, resulting in the release of a
free glucose molecule.
10.
11. Step 3
Conversion of glucose 1-phosphate into glucose 6-
phosphate for further metabolism
• In the debranching reaction, the enzyme phosphoglucomutase
catalyzes the isomerization of glucose 1-phopsphate to glucose
6-phosphate.
12. Step 3
Conversion of glucose 1-phosphate into glucose 6-
phosphate for further metabolism
13. Fate of G6P derived from Glycogenolysis
• The fate of the glucose 6-phosphate derived from Glycogenolysis
differ in liver and muscle.
1. Liver contains the enzyme glucose 6-phosphatase which
converts the glucose 6-phosphate to free glucose, which
helps to maintain the blood glucose concentration in the
fasting state.
14. Fate of G6P derived from Glycogenolysis
2. During glycogen degradation in muscle, the main aim is to
produce energy quickly and so the glucose 6-phosphate is
metabolized immediately via glycolysis. This tissue does not
contain glucose 6-phosphatase.
