During glycolysis, phosphofructokinase catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate using ATP as one of its other substrates. However, high levels of ATP inhibit phosphofructokinase. In this scenario, ATP is acting as a ________.
1) Homotropic inhibitor
2) Homotropic activator
3) Heterotropic inhibitor
4) Heterotropic activator