Proinsuliinin

Proinsulin is a precursor protein to insulin. It is synthesized in the beta cells of the islets of Langerhans in the pancreas. Proinsulin is a single polypeptide chain that is approximately 86 amino acids long. It contains three distinct domains: an amino-terminal B chain, a connecting peptide (C-peptide), and a carboxy-terminal A chain.

Within the endoplasmic reticulum of the beta cells, proinsulin undergoes folding and disulfide bond formation. It is then transported to the Golgi apparatus for further processing. In the Golgi and subsequent secretory vesicles, proinsulin is cleaved by specific enzymes called prohormone convertases and carboxypeptidase E. This cleavage removes the C-peptide, separating it from the insulin molecule.

Mature insulin consists of the A chain and the B chain, which are held together by two disulfide bonds. The released C-peptide is secreted along with insulin into the bloodstream. The C-peptide's presence in the blood can be used as a marker to assess insulin production by the body. Mutations in the proinsulin gene can lead to defects in insulin processing or secretion, resulting in various forms of diabetes mellitus, often referred to as proinsulin-related diabetes.