Glycogen: an efficient storage form of energy in anaerobic conditions

What is the net energy yield for the oxidation of a glucose unit from glycogen in anaerobic conditions?

In anaerobic conditions, the oxidation of a free glucose to lactate leads to the net production of two molecules of ATP.

Anaerobic Conditions: Glycolysis to Lactate
Fig. 1 – Glycolysis to Lactate

Glucose from the action of glycogen phosphorylase: glucose-1-phosphate release (about 90% of the removed units).

Glycogen synthesis from free glucose costs two ATP units for each molecule; a glucose-1-phosphate is released by the action of glycogen phosphorylase, with recovering/saving of one of the two previous ATP molecules.
Therefore the oxidation of glucose to lactate starting from glucose-6-phosphate and not from free glucose yields three ATP molecules and not two (one ATP is expended in the activation stage instead of two, 4 ATP are produced in the third stage: three ATP gained).
The net rate between cost and yield is 1/3 (an energy conservation of about 66,7%).
The overall reaction is:

glycogen(n glucose residues) + 3 ADP + 3 Pi → glycogen(n-1 glucose residues) + 2 lactate + 3 ATP

If we combine glycogen synthesis, glycogen breakdown and finally glycolysis to lactate we obtain only one ATP molecule per stored glucose unit, that is the overall sum is:

glucose + ADP + Pi → 2 lactate + ATP

Glucose from the action of debranching enzyme: free glucose release (about 10% of the removed units).

The net yield in ATP between glycogen synthesis and breakdown is two ATP molecules expended because of free glucose is released.
In this case the oxidation of glucose starts from the not-prephosphorylated molecule and it yields two ATP molecules.
Therefore the net yield in ATP is zero.
Considering the oxidation of the glucose units from glycogen to lactate we have an energy conservation of:



In anaerobic conditions, there is the conservation of about 60% of energy into the glycogen molecule, a good storage form of energy.


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