A receptor refers to a region of a cell membrane that attaches to another substance. Receptors are generally proteins and they are found within or on a cell membrane. Insulin receptor refers to a trans-membrane receptor that can be activated by IGF-I, IGF-II and insulin. It belongs to the category of tyrosine kinase receptors.
Insulin receptor is very important when it comes to glucose homeostasis’ regulation. Glucose homeostasis is a functional process that results in different clinical manifestations such as cancer and diabetes while under degenerate circumstances.
Insulin is among the most important hormones in the body because it regulates glucose level. Insulin is formed in the special cells of the pancreas known as beta islet cells. These cells are activated in response to glucose level so that they can secrete insulin. More insulin is produced when glucose level rises and their production stops when glucose level in the body decreases.
After secretion, insulin is released into the blood stream. On reaching the liver, it interacts with insulin receptors that are found on the membranes of the cells of the liver. Once it attaches itself on a receptor, insulin does not enter into the cells. Instead, receptors are activated causing generation of another substance or activation of another substance in the cell.
Insulin receptor has two beta sub-units and two alpha sub-units. Disulfide bonds link these sub-units. Alpha chains of the receptor are extracellular and they harbor the binding domain of insulin while beta chains linked to it penetrate via the plasma membrane.
Being a tyrosine kinase, insulin receptor acts as an enzyme transferring phosphate groups from adonesine tri phosphate or ATP to tyrosine or amino acid residues on the target intracellular proteins. ATP is organic in nature with three phosphate groups. It serves as storage for energy in most organisms.
When insulin binds on alpha sub-units, beta sub-units phosphorylate themselves. This activates a catalytic activity of insulin receptor. Once activated, the receptor phosphorylates several intracellular proteins that in turn change their activity generating a response biologically.
Any time insulin binds on the insulin receptor, several chemical reactions occur in the cells of the liver. For instance, this action causes an increase in the glucose’ breakdown by the cells of the liver. It also causes glucose conversion into fat and glycogen.
Additionally, activation of insulin receptor inhibits the breakdown of glycogen into glucose. Thus, insulin inhibits glucose production by the cells of the liver. By affecting these activities of the cells of the liver, insulin effectively reduces glucose level in the body.
Biochemically, one gene INSR encodes insulin receptor and from this alternate splicing occurs during transcription in either IR-B or IR-A isoforms. The events of downstream post-translation of the isoforms cause the formation of proteolytically cleaved β and α subunits. When combined, the isoforms are capable of hetero or homo-dimerisaton and production of 320 kDa trans-membrane insulin receptors that are linked by disulfide bonds.
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