Zymogen

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A zymogen or a proenzyme, is an inactive enzyme precursor. A zymogen requires a biochemical change (such as a hydrolysis reaction revealing the active site, or changing the configuration to reveal the active site) for it to become an active enzyme. The biochemical change usually occurs in a lysosome where a specific part of the precursor enzyme is cleaved in order to activate it. The amino acid chain that is released upon activation is called the activation peptide.

Very often the suffix "-ogen" can be added to the name of the enzyme to indicate that it is in the precursor form. Examples of zymogens are trypsinogen, chymotrypsinogen, pepsinogen, most of the proteins of the coagulation system, some of the proteins of the complement system, the caspases.

Zymogen is another term for a pro-enzyme. Some enzymes have a pro- prefix to denote their inactive status eg. Proalbumin, which is converted into albumin.