The rate-limiting step of glycolysis is catalyzed by which enzyme?

• A. Hexokinase
• B. Lactate dehydrogenase
• C. Phosphofructokinase-1 (PFK-1)
• D. Pyruvate dehydrogenase

Answer: (C)

The pace of glycolysis is controlled by a single gatekeeper enzyme that sits at the committed step of the pathway. This step is the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, and it is carried out by phosphofructokinase-1. Because this step effectively commits glucose to being broken down through glycolysis, its activity sets the overall flux through the pathway.

PFK-1 is highly regulated by the cell’s energy state. It is activated when energy is low (high AMP/ADP, low ATP) to accelerate glycolysis and generate ATP, and inhibited when energy is abundant (high ATP) or when citrate levels rise, helping to prevent unnecessary glucose breakdown. Fructose-2,6-bisphosphate is another powerful activator of PFK-1, produced in response to insulin and opposing glucagon signals, further tuning glycolytic flow.

In contrast, hexokinase starts glycolysis by phosphorylating glucose, but it does not serve as the main control point for the pathway’s rate. Lactate dehydrogenase and pyruvate dehydrogenase act at later stages or in related branches, not at the rate-limiting gate of glycolysis.

So, the enzyme that best fits as the rate-limiting step is phosphofructokinase-1.