Insulin
A 51 amino acid peptide hormone produced by pancreatic beta cells that enables cells to take up glucose from the blood, lowering blood sugar levels. Although insulin is technically a protein, it is the most widely used peptide-based therapeutic globally and its biology is central to understanding GLP-1 receptor agonist therapy.
Technical Context
Insulin consists of two polypeptide chains (A-chain, 21 aa; B-chain, 30 aa) connected by two interchain disulphide bonds plus one intrachain disulphide bond. It is synthesised as preproinsulin → proinsulin (with connecting C-peptide) → mature insulin + C-peptide (released in equimolar amounts). Insulin binds the insulin receptor (a tyrosine kinase receptor), activating PI3K/Akt and Ras/MAPK signalling pathways that promote glucose uptake (via GLUT4 translocation), glycogen synthesis, lipogenesis, and protein synthesis. Insulin biology is fundamental to understanding GLP-1 RA therapy — GLP-1 RAs potentiate glucose-stimulated insulin secretion (they amplify the beta cell response to glucose rather than directly causing insulin release, which is why hypoglycaemia risk is low).