a) With low levels of ATP, there is a lower level of one reactant (ATP), but, more importantly, the inhibitory site is also going to be empty. This will lead to moderate to fast reaction rate when the cell needs more ATP, while b) high levels of ATP will see the inhibitory site occupied much of the time, c) shutting the reaction down mostly or completely.
Work Step by Step
Glycolysis produces some ATP, feeding materials into cellular respiration which produces a lot of ATP. If a lot of ATP is already around, the cell will slow this reaction, and thus all of glycolysis and cellular respiration. The carbon in fructose can then be shunted to other cellular needs. When ATP levels are high, the inhibitory site will be occupied most of the time, shutting down the reaction even though ATP will also be in the active site.