Which enzyme is NOT involved in glycogenolysis?

• A. Glycogen phosphorylase
• B. Phosphoglucomutase
• C. Glucose-6-phosphatase
• D. Pyruvate kinase

Answer: (D)

The correct answer identifies an enzyme that does not play a role in glycogenolysis, the process of breaking down glycogen into glucose. Glycogen phosphorylase is the primary enzyme responsible for cleaving glucose units from glycogen, effectively releasing glucose-1-phosphate. Phosphoglucomutase then converts glucose-1-phosphate into glucose-6-phosphate, a crucial step in the pathway of glycogen degradation. Additionally, glucose-6-phosphatase is involved in the dephosphorylation of glucose-6-phosphate to free glucose, allowing its release into the bloodstream, particularly in the liver.

In contrast, pyruvate kinase is primarily involved in glycolysis, not glycogenolysis. It catalyzes the conversion of phosphoenolpyruvate (PEP) to pyruvate, which is a separate metabolic pathway that occurs after glucose has been broken down. As such, pyruvate kinase does not participate in the mobilization of glucose from glycogen stores, making it the enzyme that is not involved in glycogenolysis. This distinction is important for understanding the different roles enzymes play in carbohydrate metabolism.