Function
Insulin is a protein hormone that aids in carbohydrate metabolism as well as storage. Insulin is manufactured in the beta cells of the pancreas. When glucose is in high enough concentration in the blood, the pancreas releases insulin which signals muscle, fat and liver cells to absorb. Normally the liver releases glucose but when insulin is present in the blood this process stops.
Disease
Type 1 diabetes is an autoimmune disease that kills of beta cells in the pancreas so that insulin is never produced. Without insulin treatment, which is done by injection or with an insulin pump, blood glucose levels will get dangerously high and cells won’t have any energy to fuel themselves.
Type 2 diabetes is quite different from type 1 diabetes in that insulin is present in the body. The cells insulin receptors become resistant to insulin which means that glucose no longer is taken up and blood sugar levels remain high. This causes beta cells in the pancreas to work harder to produce more and more insulin until they become exhausted to the point that very little insulin is being produced.
Structural highlights
Insulin is composed of two polypeptide chains, an A chain and a B chain. The A chain is composed of 21 amino acids while the B chain is 30 amino acids long. They are joined together by 3 , 2 linking the A and B chains together, and one internally linking the A chain. Both chains contain but no beta pleated sheets. Binding of insulin to its insulin receptor is limited to the B chain. The general tertiary structure of insulin is highly conserved among species, and can be used to treat human deficiencies. Pig insulin is a common substitute for human insulin.
The cause the folding of the A and B chains into the protein structure as shown. Hydrogen bonding allows dimers to form between insulin molecules. Insulin molecules naturally form dimers, and do not affect the rate of absorption into a cell. Hexadimers, which are hydrogen bonded complexes between 6 insulin molecules, do slow down the rate of absorption due to its size.
References
https://www.diabetesselfmanagement.com/blog/what-does-insulin-do/
https://www.rcsb.org/structure/3I40
