Insulin is a hormone produced by the pancreas, an organ located behind the stomach. This hormone is fundamental for various bodily processes, acting as a messenger that helps regulate how the body uses energy from the food consumed. Without sufficient insulin, or if the body cannot use it properly, significant health issues can arise.
The Insulin Molecule’s Structure
Insulin is a protein composed of two chains, the A-chain and the B-chain. The A-chain consists of 21 amino acids, while the B-chain contains 30 amino acids, making the entire human insulin molecule comprise 51 amino acids with a molecular mass of 5808 Da. These two polypeptide chains are connected by two disulfide bonds, chemical linkages between sulfur atoms. An additional disulfide bond exists within the A-chain itself, stabilizing its shape.
The specific arrangement of these amino acids and disulfide bonds gives insulin a unique three-dimensional shape. This precise folding includes alpha-helical regions in both chains and beta-sheets in the B-chain. This distinct three-dimensional structure is necessary for insulin to interact correctly with specific receptors on cell surfaces throughout the body. While the active form of insulin is a single molecule (monomer), it is often stored in the body as a hexamer, six insulin molecules which are linked together in the presence of zinc atoms.
How Insulin is Made and Released
The creation of insulin begins within the beta cells, which are specialized cells located in clusters called the islets of Langerhans within the pancreas. The process starts with a larger precursor molecule known as preproinsulin. As preproinsulin moves through the endoplasmic reticulum, a section of 24 amino acids, the signal peptide, is removed by enzymes.
This removal transforms preproinsulin into proinsulin, which then folds into its three-dimensional structure. Proinsulin is further processed as it moves into vesicles that bud off from the Golgi body. Inside these vesicles, proinsulin is cleaved in two places, yielding the mature insulin molecule (the A and B chains) and an inactive C-peptide. Both are stored in granules within the beta cells and released into the bloodstream, primarily in response to a rise in blood glucose levels after a meal.
Insulin’s Role in the Body
Once released into the bloodstream, insulin acts as a key to allow glucose, a type of sugar derived from carbohydrates, to enter cells in various tissues such as muscles, fat, and the liver. Insulin binds to specific receptors on the cell surface, which then signals the cell to take up glucose from the blood. By facilitating glucose uptake, insulin helps to lower blood sugar levels and provides cells with energy.
Beyond glucose uptake, insulin promotes the storage of glucose. In liver and muscle cells, it stimulates the conversion of glucose into glycogen. This process, known as glycogenesis, helps to maintain stable blood glucose levels. Insulin also influences fat and protein metabolism, promoting the synthesis and storage of fatty acids and triglycerides in fat cells, while inhibiting their breakdown. It also accelerates protein formation within cells, indicating its anabolic effects on the body’s energy reserves.