Which enzyme is a key control point in glycolysis during high-intensity exercise?

• A. Lactate dehydrogenase
• B. Phosphofructokinase
• C. Pyruvate dehydrogenase
• D. Hexokinase

Answer: (B)

The movement of glucose through glycolysis under high-intensity exercise is governed by the enzyme that acts as the gatekeeper of the pathway. Phosphofructokinase catalyzes the committed, rate-limiting step—converting fructose-6-phosphate to fructose-1,6-bisphosphate. Its activity is highly sensitive to the cell’s energy state: high ATP and citrate dampen it, while high AMP/ADP and inorganic phosphate activate it. During intense effort, ATP is rapidly consumed, raising AMP/ADP levels and lowering the energy charge, which strongly stimulates phosphofructokinase. This accelerates glycolysis to produce more pyruvate (or lactate under anaerobic conditions) to meet the urgent need for ATP. The other enzymes either act earlier in the pathway, help recycle NAD+ via lactate formation, or determine what pyruvate becomes (entering the mitochondria as acetyl-CoA), but they don’t regulate the overall rate of glycolysis as directly as phosphofructokinase.