A
zymogen (), also called a
proenzyme (), is an inactive
precursor of an
enzyme. 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
Golgi bodies, where a specific part of the precursor enzyme is cleaved in order to activate it. The inactivating piece which is cleaved off can be a
peptide unit, or can be independently folding domains comprising more than 100
residues. Although they limit the enzyme's ability, these n-terminal extensions of the enzyme or a “prosegment” often aid in the stabilizing and
folding of the enzyme they inhibit.