An inactive protein that is activated by the removal of one or more polypeptides. The removal (or cleaving) is done by proprotein convertases, whose function is to convert the proprotein to the protein.

The reason that a proprotein is formed, instead of a protein, is that this mechanism ensures that the biologically active molecules are formed in the appropriate cellular compartments. Thus, the proprotein convertases exists in a compartment, and only there can the protein be rendered active by removal of some of its polypeptides.

See also zymogen.