Apoptosis

Apoptosis or  programmed   cell  death,  is  executed  through  a  group  of  proteases that is called caspases. In response to particular pro-apoptotic signals the inactive procaspases are proteolytically activated to their active form. That activated caspases then act on other procaspases as well as on another cellular protein to bring about cell death.

Zymogen activation may generate a huge amplification of the initial signal as a single activated enzyme should act on several thousands of substrate molecules to bring about further activation.  Because proteolytic  cleavage does not needs ATP, zymogen  cleavage  is  a  particularly  appropriate  mechanism  for  activation  of proteins outside cells. Moreover, unlike the covalent modification of an enzyme, zymogen activation is not reversible.  Once it was activated, the enzyme stays active.