Phosphofructokinase is best described as...

• A. A transporter protein
• B. A rate-limiting enzyme of glycolysis
• C. An enzyme in the TCA cycle
• D. A ribosome component

Answer: (B)

Phosphofructokinase is best described as a rate-limiting enzyme of glycolysis. It catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, a committed step that largely governs how quickly glucose is processed through glycolysis. Its activity serves as a major control point because it’s highly regulated by the cell’s energy status: high ATP or citrate levels tell the cell there’s plenty of energy and slow glycolysis, while AMP/ADP levels indicate energy demand and stimulate glycolysis. In liver, fructose-2,6-bisphosphate can strongly activate PFK-1, further adjusting flux in response to hormonal signals. This regulatory role makes it the key driver of glycolytic throughput.

It isn’t a transporter protein, so it doesn’t move substances across membranes. It isn’t part of the TCA cycle, whose enzymes include citrate synthase and isocitrate dehydrogenase. And it isn’t a ribosome component, which would relate to protein synthesis.