A zymogen (also called a proenzyme) is an inactive precursor form of an enzyme.
It requires a biochemical change (such as cleavage of a specific peptide bond) to become an active enzyme.
Why zymogens exist:
Many enzymes, especially digestive and proteolytic enzymes, could damage the cells that produce them if they were active inside those cells.
So, they are made and stored as inactive zymogens to prevent unwanted reactions.
Activation process:
A zymogen becomes active when a specific part of its structure is removed or altered—usually by:
- Cleavage of a peptide bond by another enzyme, or
- A change in pH or other environmental conditions.
This structural change exposes or reshapes the enzyme’s active site, allowing it to bind to its substrate.
Examples:
- Pepsinogen → Pepsin (activated by stomach acid, low pH)
- Trypsinogen → Trypsin (activated by the enzyme enterokinase in the small intestine)
- Prothrombin → Thrombin (activated during blood clotting)
Summary:
Zymogen: inactive enzyme precursor
Activation: by cleavage or modification that exposes the active site
Purpose: prevents cellular damage and allows controlled enzyme activation