The Story of Insulin

Insulin Synthesis

The insulin-making cells of the body are called beta cells, and they are found in the pancreas gland. These cells clump together to form the "islets of Langerhans", named for the German medical student who described them.

The synthesis of insulin begins at the translation of the insulin gene, which resides on chromosome 11. During translation, two introns are spliced out of the mRNA product, which encodes a protein of 110 amino acids in length. This primary translation product is called preproinsulin and is inactive. It contains a signal peptide of 24 amino acids in length, which is required for the protein to cross the cell membrane.

Once the preproinsulin reaches the endoplasmic reticulum, a protease cleaves off the signal peptide to create proinsulin. Proinsulin consists of three domains: an amino-terminal B chain, a carboxyl-terminal A chain, and a connecting peptide in the middle known as the C-peptide.

Within the endoplasmic reticulum, proinsulin is exposed to several specific peptidases that remove the C-peptide and generate the mature and active form of insulin. In the Golgi apparatus, insulin and free C-peptide are packaged into secretory granules, which accumulate in the cytoplasm of the beta cells. Exocytosis of the granules is triggered by the entry of glucose into the beta cells. The secretion of insulin has a broad impact on metabolism.

Insulin Structure

In 1958, Frederick Sanger was awarded his first Nobel Prize for determining the sequence of the amino acids that make up insulin. This marked the first time that a protein had had the order of its amino acids (the primary sequence) determined.

Insulin is composed of two chains of amino acids named chain A (21 amino acids) and chain B (30 amino acids) that are linked together by two disulfide bridges. There is a 3rd disulfide bridge within the A chain that links the 6th and 11th residues of the A chain together.

In most species, the length and amino acid compositions of chains A and B are similar, and the positions of the three disulfide bonds are highly conserved. For this reason, pig insulin can be used to replace deficient human insulin levels in diabetes patients. Today, porcine insulin has largely been replaced by the mass production of human proinsulin by bacteria (recombinant insulin).

Insulin molecules have a tendency to form dimers in solution, and in the presence of zinc ions, insulin dimers associate into hexamers. Whereas monomers of insulin readily diffuse through the blood and have a rapid effect, hexamers diffuse slowly and have a delayed onset of action. In the design of recombinant insulin, the structure of insulin can be modified in a way that reduces the tendency of the insulin molecule to form dimers and hexamers but that does not interrupt binding to the insulin receptor. In this way, a range of preparations of insulin is made, varying from short acting to long acting.

Insulin secretion

Rising levels of glucose inside the pancreatic beta cells trigger the release of insulin:

1. Glucose is transported into the beta cell by type 2 glucose transporters (GLUT2). Once inside, the first step in glucose metabolism is the phosphorylation of glucose to produce glucose-6-phosphate. This step is catalyzed by glucokinaseit is the rate-limiting step in glycolysis, and it effectively traps glucose inside the cell.

2. As glucose metabolism proceeds, ATP is produced in the mitochondria.

3. The increase in the ATP:ADP ratio closes ATP-gated potassium channels in the beta cell membrane. Positively charged potassium ions (K+) are now prevented from leaving the beta cell.

4. The rise in positive charge inside the beta cell causes depolarization.

5. Voltage-gated calcium channels open, allowing calcium ions (Ca2+) to flood into the cell.

6. The increase in intracellular calcium concentration triggers the secretion of insulin via exocytosis.

There are two phases of insulin release in response to a rise in glucose. The first is an immediate release of insulin. This is attributable to the release of preformed insulin, which is stored in secretory granules. After a short delay, there is a second, more prolonged release of newly synthesized insulin.

Once released, insulin is active for a only a brief time before it is degraded by enzymes. Insulinase found in the liver and kidneys breaks down insulin circulating in the plasma, and as a result, insulin has a half-life of only about 6 minutes. This short duration of action allows rapid changes in the circulating levels of insulin.

Insulin Receptor

The net effect of insulin binding is to trigger a cascade of phosphorylation and dephosphorylation reactions. These actions are terminated by dephosphorylation of the insulin receptor.

Similar to the receptors for other polypeptide hormones, the receptor for insulin is embedded in the plasma membrane and is composed of a pair of alpha subunits and a pair of beta subunits (Figure 1). The alpha subunits are extracellular and contain the insulin-binding site. The beta subunits span the membrane and contain the enzyme tyrosine kinase. Kinases are a group of enzymes that phosphorylate proteins (the reverse reaction is catalyzed by a group of enzymes called phosphatases).

Insulin binding to the alpha subunits induces a conformational change that is transmitted to the beta subunits and causes them to phosphorylate themselves (autophosphorylation). A specific tyrosine of each beta subunit is phosphorylated along with other target proteins, such as insulin receptor substrate (IRS). As these and other proteins inside the cell are phosphorylated, this in turn alters their activity, bringing about the wide biological effects of insulin.

Insulin Action

The binding of insulin results in a wide range of actions that take place over different periods of time.

Almost immediately, insulin promotes the uptake of glucose into many tissues that express GLUT4 glucose transporters, such as skeletal muscle and fat. Insulin increases the the activity of these transporters and increases their numbers by stimulating their recruitment from an intracellular pool to the cell surface. Not all tissues require insulin for glucose uptake. Tissues such as liver cells, red blood cells, the gut mucosa, the kidneys, and cells of the nervous system use a glucose transporter that is not insulin dependent.

Over minutes to hours, insulin alters the activity of various enzymes as a result of changes in their phosphorylation status.

Over a period of days, insulin increases the amounts of many metabolic enzymes. These reflect an increase in gene transcription, mRNA, and enzyme synthesis.

Source: National Library of Medicine