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Zymogen

A zymogen, also called a proenzyme, describes a group of proteins that do not show any catalytic activity. They are, however, transformed within an organism into enzymes, primarily enzymes that catalyze reactions that break down proteins. The zymogen requires biochemical change, like hydrolysis, to expose the active site and make it into an active enzyme.

Biochemical change to zymogens generally occur in a lysosome, where the cleaving to expose the active site can be performed in order to activate the enzyme. An amino acid chain released upon activation is called an activation peptide. The suffix "-ogen" is also appended to an enzyme to indicate it is a precursor. Some examples are trypsinogen, chymotripsynogen, and pepsinogen. Other zymogens include proteins involved in blood coagulation (clotting) and the caspases, proteins in the complement system.

Often, zymogens are activated by separate enzymes that catalyze the cleavage of the necessary peptide bonds. There are also specialized zymogenic cells that work only to synthesize and store zymogens in inactive form, ready to send to the rest of the body at need.

Web Resources On Zymogen

Everything2 on Zymogen
Creation of a Zymogen

Book Resources On Zymogen

Endosomes and Lysosomes: A Dynamic Relationship by Storrie & Murphy
Biology of the Lysosome by Lloyd & Mason (eds)

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