Final answer:
Phosphofructokinase is known as the regulatory enzyme of glycolysis because it is a critical control point that is sensitive to the energy needs of the cell, being inhibited by high levels of ATP and activated by lower levels. The correct answer is 2.
Step-by-step explanation:
The regulatory enzyme of glycolysis is phosphofructokinase. This enzyme has a crucial role in one of the early steps of glycolysis, where it converts fructose-6-phosphate into fructose-1,6-bisphosphate and is strongly regulated by various metabolic signals. Other enzymes, such as hexokinase and pyruvate kinase, also play roles in the regulation of glycolysis, but phosphofructokinase is considered the main regulatory enzyme due to its sensitivity to the energy needs of the cell.
It is inhibited by high levels of ATP, indicating that the cell has ample energy, and is stimulated when ATP levels are low, signaling that more energy production is needed.