This enzyme facilitates the rate-limiting step of glycolysis

• A. Hexokinase
• B. Phosphofructokinase
• C. G3P dehydrogenase
• D. Pyruvate kinase

Answer: (B)

The pace of glycolysis is controlled at a single key checkpoint: the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate by phosphofructokinase-1. This step is effectively irreversible in cells and acts as the main gatekeeper for the pathway, so its activity largely determines how fast glycolysis proceeds. Its regulation mirrors the cell’s energy needs: high ATP or citrate inhibits it, slowing glycolysis, while high AMP/ADP (and, in many tissues, fructose-2,6-bisphosphate) activates it, speeding things up.

The other enzymes play important roles, but they don’t set the overall rate in the same way. Hexokinase traps glucose by forming glucose-6-phosphate, kickstarting glycolysis, but this step isn’t the primary bottleneck. Glyceraldehyde-3-phosphate dehydrogenase is part of the energy-harvesting phase, yet its rate isn’t the limiting one. Pyruvate kinase finishes glycolysis and is regulated, but the rate-limiting gate is the phosphofructokinase-1 step.