Function
Insulin is made by the pancreatic islet beta cells in response to elevated blood glucose levels. Insulin signals cells that the body is in the "fed" state, and that it should take up glucose from the blood and make other appropriate response. For example, in the liver glycogen synthesis is turned on, which provides a supply of glucose when the blood glucose levels fall under fasting conditions. Insulin also increases fat synthesis in adipocytes. In type 1 diabetes, the pancreatic cells do not release insulin, resulting in high blood sugar levels and increased fat metabolism. Consequently, there is "spillover" of glucose into the urine, and weight loss due to the loss of body fat stores.
Structure
Insulin is composed of two different types of peptide chains. has 21 amino acids and has 30 amino acids. Both chains contain but no beta strands. There are 3 conserved which help keep the two chains together. Insulin can also form in solution due to the hydrogen bonding between the B chains (shown as white lines). The dimers can further interact to form due to interaction between hydrophobic surfaces. This the hydrophobic (gray) and polar (purple) parts of an insulin monomer at a pH of 7.
A number of insulin variants have been made to favor either the monomeric or hexameric form. Deletion of the creates a monomer only form. This portion of the B chain is involved in between the B chain of one monomer and the A (marked C) and B (marked D) chain of another monomer; without this region, the hexamers do not form. Lispro is another fast acting insulin; it has the lysine and proline at the end of the B subunit swapped. This allows the lysine to make a with tyrosine 26, which also favors formation of the active monomer state.
