Insulin

Overview

Insulin is a 51-amino-acid peptide hormone produced by the beta cells of the pancreatic islets of Langerhans. It is the principal anabolic hormone of the body and the primary regulator of blood glucose concentration. Insulin promotes cellular uptake of glucose from the bloodstream, stimulates glycogen synthesis in liver and muscle, drives lipogenesis in adipose tissue, and facilitates amino acid uptake and protein synthesis across multiple tissues.

The discovery of insulin in 1921 by Frederick Banting and Charles Best at the University of Toronto, with purification contributions from James Collip and supervision by J.J.R. Macleod, represents one of the most transformative breakthroughs in medical history. Prior to insulin's availability, a diagnosis of type 1 diabetes was effectively a death sentence. The first human patient, Leonard Thompson, received insulin injections in January 1922 at Toronto General Hospital. Banting and Macleod were awarded the Nobel Prize in Physiology or Medicine in 1923 — just two years after the initial discovery.

Insulin occupies a singular position in the history of biochemistry and pharmacology. It was the first protein to have its amino acid sequence determined (Frederick Sanger, 1951 — earning Sanger his first Nobel Prize), the first protein to be chemically synthesized (1963-1965, independently by groups in China, the United States, and Germany), and the first human protein produced by recombinant DNA technology (1978, Genentech/Eli Lilly). The evolution from animal-derived pancreatic extracts to recombinant human insulin analogs engineered for specific pharmacokinetic profiles reflects a century of pharmaceutical innovation built around a single peptide hormone.

Structure

Insulin consists of two polypeptide chains linked by disulfide bonds:

A chain: 21 amino acids — GIVEQCCTSICSLYQLENYCN B chain: 30 amino acids — FVNQHLCGSHLVEALYLVCGERGFFYTPKT

• Molecular formula: C₂₅₇H₃₈₃N₆₅O₇₇S₆
• Molecular weight: 5,808 Da
• CAS Number: 11061-68-0 (human insulin)
• Disulfide bonds: Three — A7-B7, A20-B19, and an intrachain bond A6-A11
• Isoelectric point: pH 5.3

Insulin is synthesized as a single-chain precursor, preproinsulin (110 amino acids), which is processed in the endoplasmic reticulum to proinsulin (86 amino acids) by signal peptide cleavage. Proinsulin folds and forms its three disulfide bonds, after which the connecting C-peptide (31 amino acids) is excised by prohormone convertases PC1/3 and PC2 in the Golgi apparatus, yielding mature two-chain insulin. The released C-peptide is co-secreted with insulin in equimolar amounts and serves as a clinical biomarker of endogenous insulin production.

The three-dimensional structure of insulin, first solved by Dorothy Hodgkin using X-ray crystallography in 1969, reveals a compact globular fold. In the presence of zinc ions, insulin assembles into hexamers — the storage form within beta cell secretory granules. Upon secretion into the bloodstream, hexamers dissociate to dimers and then monomers, the biologically active form that binds the insulin receptor.

Mechanism of Action

Insulin Receptor Signaling

Insulin exerts its metabolic effects through binding to the insulin receptor (IR), a transmembrane receptor tyrosine kinase expressed on virtually all mammalian cell types. The receptor is a disulfide-linked heterotetramer consisting of two extracellular alpha subunits and two transmembrane beta subunits.

The signaling cascade proceeds through well-characterized steps:

1. Receptor binding — monomeric insulin binds to the alpha subunit ectodomain, inducing a conformational change
2. Autophosphorylation — the beta subunit tyrosine kinase domains cross-phosphorylate each other on specific tyrosine residues
3. IRS recruitment — insulin receptor substrates (IRS-1 through IRS-4) are phosphorylated, creating docking sites for SH2-domain-containing signaling proteins
4. PI3K/Akt pathway — phosphoinositide 3-kinase activation leads to Akt (protein kinase B) phosphorylation, the central node for metabolic insulin signaling
5. GLUT4 translocation — in muscle and adipose tissue, Akt activation drives translocation of GLUT4 glucose transporters from intracellular vesicles to the cell surface, enabling glucose uptake
6. Metabolic gene regulation — Akt phosphorylates and inhibits GSK-3 (activating glycogen synthase) and FOXO transcription factors (suppressing gluconeogenic gene expression)

Metabolic Effects

Insulin's metabolic effects span carbohydrate, lipid, and protein metabolism:

• Carbohydrate: Stimulates glucose uptake (muscle, adipose), glycogen synthesis (liver, muscle), and glycolysis; inhibits gluconeogenesis and glycogenolysis
• Lipid: Promotes lipogenesis and inhibits lipolysis in adipose tissue; stimulates fatty acid synthesis in liver
• Protein: Facilitates amino acid uptake, stimulates protein synthesis, and inhibits proteolysis — contributing to insulin's net anabolic effect

Two Receptor Isoforms

The insulin receptor exists in two splice variants — IR-A (fetal, mitogenic) and IR-B (metabolic, predominant in adult liver, muscle, and adipose). IR-A has higher affinity for IGF-II and is associated with proliferative signaling, while IR-B mediates the classical metabolic effects of insulin. This distinction has implications for understanding insulin's growth-promoting effects and the biology of insulin analogs.

Research Summary

Applications

Therapeutic Insulin Analogs

Modern insulin therapy employs engineered analogs with tailored pharmacokinetic profiles:

Rapid-acting (onset 10-20 min, peak 1-3 hr):

• Insulin lispro (Humalog) — B28-B29 Pro-Lys inversion
• Insulin aspart (NovoLog) — B28 Pro to Asp substitution
• Insulin glulisine (Apidra) — B3 Asn to Lys, B29 Lys to Glu

Long-acting/basal (onset 1-2 hr, duration 20-42 hr):

• Insulin glargine (Lantus) — A21 Asn to Gly, B chain C-terminal diarginine extension
• Insulin detemir (Levemir) — B29 Lys acylated with C14 fatty acid, B30 Thr deleted
• Insulin degludec (Tresiba) — B29 Lys acylated with C16 fatty diacid via glutamic acid linker

Clinical Indications

• Type 1 diabetes mellitus — absolute requirement for exogenous insulin
• Type 2 diabetes mellitus — when oral agents and GLP-1 receptor agonists (Semaglutide, Tirzepatide) provide insufficient glycemic control
• Gestational diabetes — when diet and exercise are inadequate
• Diabetic ketoacidosis — intravenous insulin as emergency treatment
• Hyperkalemia — insulin-glucose infusion for acute potassium lowering

Research Context in Peptide Science

In the broader peptide research context, insulin is relevant as:

• The prototype for peptide hormone replacement therapy
• A model system for protein engineering and analog design
• The basis for understanding the insulin/IGF-1 signaling axis that intersects with IGF-1 LR3 and growth hormone secretagogue research

Dosing Protocols

The following dosing information reflects FDA-approved clinical guidelines. Insulin is one of the most extensively studied and widely prescribed peptide hormones in medicine. Always consult a qualified healthcare professional for individualized insulin therapy.

Type 1 diabetes: Basal-bolus regimen (total daily dose typically 0.4-1.0 units/kg/day, split approximately 50% basal / 50% bolus) or continuous subcutaneous insulin infusion (insulin pump). Doses are individualized based on carbohydrate intake, blood glucose monitoring, and activity level.

Type 2 diabetes: Often initiated with basal insulin (10 units/day or 0.1-0.2 units/kg/day) when oral agents and GLP-1 agonists are insufficient. Titrated upward by 2-4 units every 3-7 days to achieve fasting glucose targets.

Related Compounds

• Semaglutide — a GLP-1 receptor agonist that enhances endogenous insulin secretion and is increasingly used before or alongside insulin therapy
• Tirzepatide — a dual GIP/GLP-1 agonist with insulin-sensitizing and secretagogue effects
• IGF-1 LR3 — insulin-like growth factor-1 long arginine 3; shares structural homology and partial receptor cross-reactivity with insulin
• Somatostatin — an inhibitory hormone that suppresses insulin secretion from beta cells
• C-peptide — the connecting peptide co-secreted with insulin; used clinically to assess endogenous insulin production
• Proinsulin — the single-chain insulin precursor; elevated levels indicate beta cell dysfunction