Insulin: Structure, Secretion, and Function

Insulin is a polypeptide hormone secreted by the pancreatic beta cells. It is synthesized as proinsulin, then cleaved into insulin and C-peptide. Within the beta cells, insulin is stored in secretory granules as a zinc-stabilized hexamer (six insulin molecules bound to two zinc ions). Upon stimulation by elevated blood glucose, insulin is released via exocytosis through the Golgi apparatus into the bloodstream.

Once in circulation, insulin dissociates into monomers and binds to its receptor on target cells (primarily liver, muscle, and adipose tissue), triggering a cascade that promotes glucose uptake, glycogen synthesis, lipogenesis, and protein synthesis—ultimately lowering blood glucose levels.