And that's because phosphorylation actually requires the presence of ATP. And ATP molecules are found abundantly only inside our cells. So that means phosphorylation can control only those enzymes and proteins which exist inside our cells. On the other hand, unlike phosphorylation, proteolytic activation does not require ATP. And what that means those enzymes and proteins which are found outside our cells in the extracellular environment can readily undergo the process of proteolytic activation. Now unlike allosteric regulation and covalent modifications such as phosphorylation, which can take place many times on any given enzyme, when a zymogen undergoes proteolytic activation it will only undergo this process once in the lifetime of that enzyme.