Well, for two reasons. First of all, if we have a high concentration of ATP, that means we have plenty of energy inside our cells. And that implies if we have plenty of energy, we can easily use that energy to actually synthesize these nucleuside triphosphates, such as ATP, by using this enzyme right here. Now, the second reason why a high ATP concentration basically stimulates this enzyme to carry out its process is because of the following reasoning. So at a high concentration of ATP, when we have many ATP molecules inside our cells, that means we can use the ATP molecules to actually synthesize nucleocide triphosphates or RNA and DNA molecules. But to actually synthesize these nucleic acids, we not only need the ATP, we also need other nucleotide triphosphates, specifically perimedidine nucleocide triphosphates such as CTP.