QS Study

Biosynthesis of the Insulin

Insulin is synthesized in the rough endoplasmic reticulum of the β cells. It is then transported to the Golgi apparatus, where it is packaged in membrane-bound granules. These granules move to the plasma membrane by a process involving microtubules, and their contents are expelled by exocytosis. The insulin then crosses the basal laminas of the β cell and a neighboring capillary and the fenestrated endothelium of the capillary to reach the bloodstream. When the beta cell is appropriately stimulated, insulin is secreted from the cell by exocytosis and diffuses into islet capillary blood. C peptide is also secreted into the blood but has no known biological activity.

Pre-proinsulin (RER) → Proinsulin → Insulin (CA)

The A and B chains in proinsulin are connected by a peptide segment called Connecting peptide (C peptide).

C peptide has no physiological activity. Insulin and free C peptide are packaged in the Golgi into secretory granules which accumulate in the cytoplasm.

B cells secrete insulin and an equimolar amount of C peptide. The beta-cell ER appears to be specially adapted to supporting these processes in the face of varying demands for the hormone.

Fig: Biosynthesis of the Insulin

A general characteristic in the biosynthesis of many proteins, but in exacting for proteins exported from cells, is that the protein is produced in a precursor form then modified to produce the final form during storage and before discharge. Insulin is synthesized by a group of cells in the pancreas called Islets of Langerhans, stored in granules then released into the blood when required.