Question

Which one of the following enzymes does NOT catalyze a physiological irreversible step in glycolysis?

• A. Hexokinase
• B. Phosphofructokinase
• C. Phosphoglycerate kinase
• D. Pyruvate kinase

Hint:

Irreversible enzymes in Glycolysis: Step 1, 3, and 10.
Mnemonic: Harry Potter Pushes (Hexokinase, PFK, Pyruvate kinase).
Any enzyme involving "Kinase" that is not one of these three is typically reversible in metabolism.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
Glycolysis is a series of ten metabolic reactions that convert glucose into pyruvate. While most of these reactions are reversible and exist in equilibrium, a few are highly exergonic and act as "checkpoints" or irreversible steps under physiological conditions. The question asks to identify the enzyme from the list that catalyzes a reversible reaction.

Step 2: Detailed Explanation:

• Irreversible Steps of Glycolysis: There are exactly three irreversible steps in the glycolytic pathway. These steps are highly regulated by the cell.
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Step 1: Catalyzed by Hexokinase (Glucose $\rightarrow$ Glucose-6-phosphate).
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Step 2: Catalyzed by Phosphofructokinase-1 (Fructose-6-phosphate $\rightarrow$ Fructose-1,6-bisphosphate). This is the primary rate-limiting step.
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Step 10: Catalyzed by Pyruvate kinase (Phosphoenolpyruvate $\rightarrow$ Pyruvate).

• Characteristics of Irreversible Steps: These reactions involve a large negative change in Gibbs free energy ($\Delta G << 0$). Because they are one-way streets, the cell must use different enzymes for the reverse process (Gluconeogenesis) to bypass them.

• Role of Phosphoglycerate Kinase: This enzyme catalyzes the 7th step of glycolysis, where 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate. This reaction involves the first "substrate-level phosphorylation" to generate ATP.

• Reversibility of
Step 7: Under standard physiological conditions in the cytoplasm, the actual $\Delta G$ for the phosphoglycerate kinase reaction is near zero. This means the reaction is reversible. In fact, during gluconeogenesis, the exact same enzyme is used to catalyze the reaction in the opposite direction.

• Comparison: Options A, B, and D represent the three "committed" steps that drive the pathway forward. Option C represents a step in the "payoff phase" that functions easily in both directions depending on cellular demand.

Step 3: Final Answer:
Phosphoglycerate kinase catalyzes a reversible reaction in glycolysis and gluconeogenesis, unlike Hexokinase, PFK, and Pyruvate kinase which are irreversible.