A 51-amino-acid peptide hormone consisting of two chains (A: 21aa, B: 30aa) linked by 3 disulfide bonds. The first protein ever sequenced (Sanger, 1951), first genetically engineered drug (1978), and the most important hormone in metabolic medicine.
Insulin is a peptide hormone produced by the beta cells of the pancreatic islets of Langerhans. It is the body's primary anabolic hormone, responsible for facilitating glucose uptake from the blood into cells, promoting glycogen synthesis, stimulating protein synthesis, and inhibiting lipolysis (fat breakdown).
Insulin's discovery in 1921 by Banting and Best (Nobel Prize 1923) transformed Type 1 diabetes from a death sentence into a manageable condition. Frederick Sanger's complete sequencing of insulin's amino acid structure (Nobel Prize 1958) was a landmark in biochemistry — it proved that proteins have defined sequences, not random arrangements.
Insulin holds a singular place in the history of medicine. Its discovery in 1921 by Frederick Banting and Charles Best at the University of Toronto — and its rapid development into a life-saving treatment for Type 1 diabetes — is one of the most celebrated stories in medical science. Before insulin, a diagnosis of Type 1 diabetes was a death sentence. Within months of its first clinical use, patients who had been wasting away were restored to health. Banting and John Macleod received the Nobel Prize in Physiology or Medicine in 1923, just two years after the discovery.
Insulin was the first protein to have its amino acid sequence determined (by Frederick Sanger in 1955, earning him his first Nobel Prize) and the first protein to be chemically synthesized in a laboratory. It was also the first human protein to be produced using recombinant DNA technology — Genentech's biosynthetic human insulin (Humulin), approved in 1982, marked the beginning of the biotechnology era. Today, insulin analogs represent a multi-billion-dollar market, with rapid-acting (lispro, aspart, glulisine), long-acting (glargine, detemir, degludec), and ultra-rapid formulations available.
As a molecule, insulin is a 51-amino-acid peptide consisting of two chains (A chain: 21 residues; B chain: 30 residues) linked by two disulfide bonds, with an additional intrachain disulfide bond within the A chain. This disulfide architecture is essential for biological activity and makes insulin more structurally complex than most peptides on this site.
Insulin binds to the insulin receptor, a receptor tyrosine kinase (RTK) — one of only a few peptide hormones that signal through RTKs rather than GPCRs. Binding to the α-subunit triggers autophosphorylation of the β-subunit, recruiting IRS proteins that activate the PI3K-Akt pathway. Akt phosphorylates multiple targets, most importantly triggering GLUT4 vesicle translocation to the cell membrane, allowing glucose to enter muscle and fat cells.
| Pathway | Effect | Significance |
|---|---|---|
| GLUT4 translocation | Akt → AS160 → GLUT4 vesicle fusion with membrane | Primary mechanism for glucose uptake in muscle/adipose |
| Glycogen synthesis | Akt → GSK3 inhibition → glycogen synthase activation | Stores glucose as glycogen in liver and muscle |
| Protein synthesis | Akt → mTOR activation → ribosomal protein translation | Anabolic effect — promotes muscle protein synthesis |
| Lipogenesis | Activates SREBP-1c → fatty acid synthesis genes | Promotes fat storage; inhibits lipolysis |
| Gene expression | MAPK pathway activation → cell growth and differentiation | Long-term metabolic programming |
| Study | Design | Findings | Level |
|---|---|---|---|
| Type 1 diabetes | 100+ years clinical use | Exogenous insulin is essential for survival in T1D; multiple analogs optimize pharmacokinetics | Level I (standard of care) |
| Type 2 diabetes | Extensive RCTs | Insulin therapy when oral agents insufficient; basal-bolus regimens optimize control | Level I |
| Diabetic ketoacidosis | Standard of care | IV insulin is the definitive treatment for DKA | Level I |
| Insulin analogs | RNCT comparisons | Rapid (lispro, aspart), long-acting (glargine, detemir, degludec) improve glycemic control vs regular insulin | Level I |
Hypoglycemia: The most significant risk of exogenous insulin therapy is hypoglycemia (low blood sugar), which can range from mild symptoms (shakiness, sweating, confusion) to severe and life-threatening events (seizures, loss of consciousness, death). All insulin formulations carry this risk, though the incidence varies by type — rapid-acting analogs cause more meal-related hypoglycemia, while long-acting formulations carry more overnight hypoglycemia risk. Modern analog insulins and continuous glucose monitoring (CGM) have significantly reduced hypoglycemia rates compared to older insulin formulations.
Weight gain: Insulin therapy is associated with weight gain, typically 2-4 kg in the first year of treatment. This is a consequence of improved glucose utilization (less glucosuria) and the anabolic effects of insulin. Weight gain can be a significant barrier to patient adherence, particularly given that many Type 2 diabetes patients already have obesity.
Injection site reactions: Lipohypertrophy (accumulation of fat tissue at injection sites) occurs with repeated injection in the same area. This can impair insulin absorption and lead to variable glycemic control. Rotation of injection sites is essential.
Insulin is one of the safest and most extensively studied drugs in medical history — billions of doses have been administered over more than a century. The risks are well-characterized and manageable with proper dosing, monitoring, and patient education.
| Jurisdiction | Status |
|---|---|
| FDA | Approved: Multiple analogs (Humalog, NovoLog, Lantus, Levemir, Tresiba, Fiasp, etc.) |
| WHO | Essential Medicine |
| History | First genetically engineered drug (recombinant human insulin, Humulin, 1982) |