A proenzyme or zymogen is a chemically inactive protein (a so-called protein precursor), which can be converted into an active enzyme via some reactions. This mechanism allows an enzyme to remain inactive until it has reached its place where it has to do its job. As a result, the operation is more targeted and fewer side effects occur. In addition, an important advantage of pro enzymes is that a stock can be built up and they can be activated quickly when needed. This activation does not require ATP.
Because the proenzyme of a proteolytic (protein cleavage) enzyme is inactive, it is prevented that the enzyme adversely affects its own tissue, which itself is also built up to a large extent by proteins. An example of disease due to activation of the proenzyme in the producing organ itself is pancreatitis.
wiki
