Insulin Structure & Function

Insulin is a natural peptide hormone that lowers blood sugar levels by moving glucose from the blood to cells all over the body. In the pancreas, there are clusters of cells called the islet of Langerhans shown below.  The islet of Langerhans is made of are alpha cells (dark blue), beta cells (aqua blue), delta cells (purple), ducts, red blood cells, and pancreatic acini.  Insulin is secreted from the beta cells. 

The insulin protein is made up of 51 amino acids.  An A and B chain polypeptide chain are covalently linked by two inter-chain disulfide bridges with an additional intrachain disulfide bridge. Insulin's secondary structure composed of two alpha helices in the A chain and an alpha helix region and a beta sheet region in the B chain. 

PDB: 3E7Y - Human Insulin 

Insulin Secretion

An increase in glucose level in the blood stimulates insulin release. Facilitated diffusion brings glucose into the beta cells through GLUT 2 channels. As the glucose level increases inside the beta cell, membrane depolarization of ATP-sensitive K+ channels occurs opening Ca²⁺ channels. The influx of Ca²⁺ triggers insulin release by moving the insulin vesicles to the cell surface (marginalization) and fusion them with the cell membrane followed by the release of insulin (exocytosis).