Zymogens or Proenzymes


Original Question


 Answer to E-02: (e) zymogens.






                                            Representation of a zymogen: Chymotrypsinogen


Zymogens or proenzymes are inactive precursors of enzymes. Observe that it is different than saying that they are “inactive enzymes”. An inactive enzyme is an enzyme that has lost its activity because of different factors, like physical factors, chemical factors or even metabolic factors. A zymogen is a molecule that needs to be activated in order to become an active enzyme, so it is more accurate to say that they are inactive precursors of enzymes, than to say that they are inactive enzymes.  Digestive enzymes, some coagulation factors and other proteins are synthesized as zymogens.


The synthesis of digestive enzymes in an inactivated form is a “safe” mechanism for the cells synthesizing some enzymes, since the proteolytic enzymes synthesized in this way are not activated until they abandon the cell.


Pepsin is synthesized as pepsinogen, trypsin is synthesized as trypsinogen, chymotripsin as chymotrypsinogen,  carboxypeptidase as procarboxypeptidase, and these zymogens are activated (usually when an external factor release an inhibitor peptide from their structure), only when they have been secreted in the gastrointestinal tract.


A good example of what occurs when some zymogens become active enzymes inside the cells is seen in acute pancreatitis , in which the premature activation of some of the pancreatic enzymes  like trypsine, phospholipase A2 and elastase, produce the autodigestion of pancreatic tissue.