Answer
Examining the graph reveals that the curve with higher phosphofructokinase activity occurs when ATP concentrations are low.
Work Step by Step
Recall that during the typical glycolytic pathway, glucose is being broken down (this is also the function of phosphofructokinase—to break down glucose). However, during glycolysis, 2 ATP molecules are usually used to break down the glucose.
When ATP is present in high concentrations, it breaks down the glucose instead of the phosphofructokinase breaking down the glucose, which explains the curve in which ATP concentration is high and phosphofructokinase activity is low.
When ATP concentrations are low, not as much glucose can be broken down, so phosphofructokinase activity levels rise to make up for the lack of ATP to do its job.
In fact, ATP binds allosterically to phosphofructokinase and inhibits phosphofructokinase activity when ATP is high in concentration. When ATP is low in concentration and AMP (adenine monophosphate) is high in concentration, phosphofructokinase enzymes are allosterically activated.
