Problem 16 - Biochemistry

Question

The product of the glycogen phosphorylase reaction is glucose-1-P. Is there a difference in glycolytic ATP yield comparing the yield from the metabolism of glucose-1-P derived from glycogen degradation with the yield from the metabolism of dietary glucose? Explain.

Step-by-Step Solution

Answer

Question: Compare the glycolytic ATP yield between glucose-1-phosphate derived from glycogen degradation and dietary glucose. Answer: The glycolytic ATP yield from glycogen-derived glucose-1-phosphate is 4 ATP molecules per glucose molecule, while the yield from dietary glucose is 3 ATP molecules per glucose molecule. The higher yield from glycogen-derived glucose is due to the fact that it does not require an ATP molecule to enter glycolysis, unlike dietary glucose.

Step 1: Glycolytic ATP yield from dietary glucose

Dietary glucose enters glycolysis by first being converted to glucose-6-phosphate (G6P) through the action of the enzyme hexokinase. This step requires the consumption of 1 ATP molecule. The glycolytic pathway later generates 4 ATP molecules through substrate-level phosphorylations, specifically at the steps catalyzed by phosphoglycerate kinase and pyruvate kinase. Therefore, the net ATP yield from dietary glucose is 4 ATP - 1 ATP = 3 ATP molecules per glucose molecule.

Step 2: Glycolytic ATP yield from glycogen-derived glucose-1-phosphate

Glucose-1-phosphate, derived from glycogen degradation, is converted to glucose-6-phosphate by the enzyme phosphoglucomutase without the consumption of ATP. From there, G6P enters glycolysis and follows the same steps as dietary glucose, generating 4 ATP molecules. Since no ATP is consumed to form G6P from glucose-1-phosphate, the net ATP yield from glycogen-derived glucose is 4 ATP molecules per glucose molecule.

Step 3: Comparing ATP yields

Comparing the net ATP yields calculated in Steps 1 and 2, we can conclude that the glycolytic ATP yield from glycogen-derived glucose-1-phosphate (4 ATP) is higher than that from dietary glucose (3 ATP). The difference is because glucose-1-phosphate does not require an ATP molecule to enter glycolysis, while dietary glucose does.