Insulin is a hormone that signals cells to capture glucose from the blood and break it down to obtain energy. Abnormalities in insulin synthesis or the tissue’s reaction to it lead to the development of diabetes of the first or second type, respectively. In this scenario, chronic disease tissues of an organism do not receive the desired amount of glucose, whereas its concentration in blood is greatly increased. This condition is called hyperglycemia. According to the World Health Organization diabetes and hyperglycemia is currently causing up to 3.5 million deaths annually (1).
Insulin is a small protein consisting of two polypeptide chains. The A chain (indicated in gray) consists of 21 amino acid residues, and the B chain (orange) — of 30 amino acids. The chains are linked by two disulfide bonds. Another disulfide bond is located inside the A-chain.Insulin is an ancient,conserved molecule. Even nematodes have insulin very similar to humans. This is the first protein in which amino acid sequences were identified in the early 1950s (2).
Insulin molecules can bind zinc ions and form complexes of six subunits. The complexes are not active, but they can gradually break down to release active proteins. This property is used to create insulin medical formulations with longer action.
The name of the protein comes from the Latin word insula — island. It was named after a small group of cells in the pancreatic islets of Langerhans, in which the hormone is synthesized. Pancreatic islets were discovered as a result of inhumane experiments on dogs: the animal had its pancreatic duct ligated while scientists waited until all the cells that secrete digestive enzymes died and were disposed of by immunity. As a result of this experiment only connective tissues and groups of cells that synthesize insulin remained in the pancreas of an animal (3).
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