zymogènes

Zymogens, also called proenzymes, are inactive precursors of enzymes. They are synthesized and stored in cells in an inactive form and require a biochemical change to become active. This regulatory mechanism prevents unintended proteolysis and allows enzymes to be activated at the right time and place.

Zymogens are common in digestion and coagulation, but occur in other pathways as well. Digestive zymogens include pepsinogen in the stomach and pancreatic trypsinogen, chymotrypsinogen, and procarboxypeptidases. Activation typically occurs after secretion: pepsinogen is converted to pepsin by stomach acid; trypsinogen is activated to trypsin by enteropeptidase in the small intestine, and trypsin itself activates additional pancreatic zymogens in a proteolytic cascade.

Other important examples are prothrombin, which is activated to thrombin in the coagulation cascade, and plasminogen, which is activated to plasmin during fibrinolysis. Activation generally involves proteolytic cleavage of a propeptide, often triggered by localization in the correct compartment and by specific activating enzymes. In many systems a zymogen must overcome autolytic risk through inhibitors and tightly regulated conditions.

Clinical relevance: misregulated zymogen activation can cause disease, such as pancreatitis from premature activation of pancreatic zymogens, or bleeding disorders when coagulation zymogens are impaired. Therapeutic approaches sometimes target these activation steps or the active proteases themselves.

Common examples include pepsinogen, trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidases, prothrombin, and plasminogen.