Which enzyme is commonly considered the key regulatory enzyme of glycolysis?

• A. Hexokinase
• B. Pyruvate kinase
• C. Phosphofructokinase-1 (PFK-1)
• D. Aldolase

Answer: (C)

The main idea is how glycolysis is controlled and which step acts as the primary on/off switch for the pathway. Phosphofructokinase-1 regulates the flow of glucose into glycolysis by catalyzing the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, a committed and essentially irreversible step under cellular conditions. Because this step commits the glucose molecule to glycolysis and is subject to strong allosteric control, it serves as the primary regulatory point.

ATP binding inhibits PFK-1 when the cell has high energy, while AMP and ADP activate it when energy is low. In the liver, fructose-2,6-bisphosphate is a powerful activator of PFK-1, helping glycolysis proceed when glucose is plentiful. Citrate, reflecting a busy TCA cycle, inhibits PFK-1 and helps coordinate glycolysis with overall energy status. Although other enzymes in glycolysis, like hexokinase and pyruvate kinase, are regulated, they do not serve as the main control point to the same extent as PFK-1.

So, this enzyme is the best answer because it sits at the key regulatory point that determines whether glycolysis proceeds in response to the cell’s energy and metabolic state.