Insulin: Definition

Detailed Explanation

Insulin is a 51-amino-acid peptide hormone consisting of an A-chain (21 amino acids) and a B-chain (30 amino acids) connected by two inter-chain disulfide bonds (A7-B7, A20-B19), with an additional intra-chain disulfide bond within the A-chain (A6-A11). It is produced by pancreatic β-cells in the islets of Langerhans as a single-chain precursor (proinsulin, 86 amino acids), which is cleaved to yield mature insulin and C-peptide.

Insulin is the master regulator of blood glucose metabolism. When blood glucose rises after a meal, β-cells secrete insulin, which binds to the insulin receptor (a receptor tyrosine kinase) on target cells — primarily liver, muscle, and adipose tissue. This triggers glucose transporter (GLUT4) translocation to the cell surface, enabling glucose uptake. Insulin also stimulates glycogen synthesis, lipogenesis, and protein synthesis while suppressing gluconeogenesis, glycogenolysis, and lipolysis. Deficiency of insulin (type 1 diabetes) or resistance to its actions (type 2 diabetes) results in hyperglycemia and, if untreated, life-threatening ketoacidosis.

The discovery of insulin in 1921 by Frederick Banting, Charles Best, James Collip, and J.J.R. Macleod at the University of Toronto is one of the most consequential events in medical history — type 1 diabetes changed from a death sentence to a manageable condition. Insulin holds multiple 'firsts' in protein science: the first protein sequenced (Frederick Sanger, 1955, Nobel Prize 1958), the first protein chemically synthesized (several groups, 1963-1965), and the first human protein produced by recombinant DNA technology (Genentech/Eli Lilly, 1982, marketed as Humulin). Modern insulin analogs include rapid-acting (lispro, aspart, glulisine), long-acting (glargine, detemir, degludec), and ultra-long-acting formulations.

Key Facts

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