Proinsulin

Proinsulin is the precursor to insulin. It is synthesised by the pancreas within the cells which make up the the Islets of Langerhans. Proinsulin is formed from pre-proinsulin when the signal peptide is cleaved off, and successively mature insulin is formed when the C peptide is cleaved via endoproteases from proinsulin.

Without proinsulin, insulin would not be able to be generated which would cause a wide range of disorders and illnesses within humans.