有些酶在刚分泌时时没有催化活性的，这种没有活性的前体物称为酶原。一般生物体会通过肽键的剪切改变酶的构象使酶的活性中心形成或暴露出来而使酶具有活性。酶原的生理意思使为了避免酶消化细胞自身。

A zymogen (/ˈzaɪməjən, -moʊ-/), also called a proenzyme (/ˌproʊˈɛnzaɪm/), 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.