The most important enzyme in glycolysis is called phosphofructokinase (PFK)and catalyzes the third reaction in the sequence. Since this reaction is so favorable under physiologic conditions, it is known as the "committed step" in glycolysis. In other words, glucose will be completely degraded to pyruvate after this reaction has taken place. With this in mind, PFK seems as if it would be an excellent site of control for glucose metabolism. In fact, this is exactly the case. When ATP or energy is plentiful in the cell, PFK is inhibited and the breakdown of glucose for energy slows down. Therefore, PFK can regulate the degradation of glucose to match the energy needs of the cell. This type of regulation is a recurring theme in biochemistry.