Insulin is a hormone that regulates glucose levels in the blood. It binds to insulin receptors on liver cells, activating a series of chemical reactions that increase glucose breakdown, facilitate conversion to glycogen and fat, and inhibit glucose production. Insulin receptors are protein kinases that phosphorylate tyrosine on other proteins, activating enzymes that affect glucose metabolism. When glucose levels drop, insulin production stops, leading to increased glucose production.
In cell biology, a receptor is a region on the cell membrane that binds to a substance. Usually, receptors are proteins found on or within the membrane. Many different types of molecules can bind to receptors on the cell surface, including hormones. An insulin receptor is an example of a receptor that binds to a hormone, especially insulin.
Insulin is a very important hormone as it regulates the level of glucose, a sugar, in the blood. This protein is formed in specialized cells in the pancreas called beta islet cells. In response to blood glucose levels, these cells are activated to produce insulin. In other words, if the glucose level is high, the islet cells produce and secrete insulin. When glucose levels are low, they stop producing the hormone.
Insulin is secreted into the bloodstream so it can be transported throughout the body. When it reaches the liver, insulin interacts with insulin receptors found on the cell membranes of liver cells. Insulin does not enter the cell after it has attached itself to a receptor. Instead, the receptor is activated and causes another substance to be generated or activated within the cell.
When insulin binds to an insulin receptor, several chemical reactions are affected within the liver cell. First, it causes the liver cell to increase the breakdown of glucose. Insulin activation also facilitates the conversion of glucose into glycogen and fat. Glycogen is a polysaccharide made up of many glucose molecules and is the main storage form of carbohydrates in animals.
Finally, the reaction that breaks down glycogen into glucose molecules is inhibited when an insulin receptor is activated. In this way, insulin inhibits the production of glucose by liver cells. By affecting these three processes within the liver cells, insulin effectively reduces the level of glucose in the body.
When glucose levels in the bloodstream drop, the pancreas produces less insulin and eventually stops. With the absence of insulin in the blood, the insulin receptors will not be activated by insulin binding to them. This has the opposite effect on liver cells. The breakdown of glucose is inhibited and additional glucose is formed from storage compounds, leading to increased blood glucose levels.
Insulin receptors are an example of protein kinases. A kinase is an enzyme that catalyzes phosphorylation reactions or reactions that add a phosphate group to a substance using ATP. ATP, or adenosine triphosphate, is an organic compound that has three phosphate groups and serves as an energy store for most organisms. In the case of insulin receptors, an amino acid called tyrosine found on other proteins is phosphorylated, making them tyrosine kinases.
When insulin binds to the receptor on the cell surface, the receptor changes shape so that regions of the kinase within the cell become activated. The activated insulin receptor then activates a number of different targets within the cell. The targets are often enzymes, which lead to the increase or decrease of different chemical reactions involving glucose, as described earlier. Insulin’s effect on liver cells is called the tyrosine kinase second messenger system.
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