Which step is rate-limiting in glycolysis under aerobic conditions?

• A. Phosphofructokinase-1 (PFK-1) step.
• B. Hexokinase step.
• C. Pyruvate kinase step.
• D. Glyceraldehyde-3-phosphate dehydrogenase step.

Answer: (A)

Glycolytic flux is governed by a single regulatory checkpoint that acts as the throttle on the pathway. The phosphofructokinase-1 step, converting fructose-6-phosphate to fructose-1,6-bisphosphate, serves that role under aerobic conditions. This step is effectively irreversible in cells and sits at the committed point of glycolysis, so its activity directly sets how fast glucose is processed through the rest of the pathway. It responds strongly to the cell’s energy status: ATP and citrate inhibit it, while AMP and fructose-2,6-bisphosphate activate it, aligning glycolysis with energy demand. Because of this tight regulation and irreversible chemistry, PFK-1 controls the overall rate of glycolytic flux more than the other steps. The hexokinase step traps glucose but is not the primary throttle; the pyruvate kinase step is downstream and regulated, but does not dictate the initial pace of the pathway. The glyceraldehyde-3-phosphate dehydrogenase step is an early-middle step and does not determine the overall rate.