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How does the hormone insulin regulate glucose levels in the body? Explain the molecular mechanism of insulin signaling pathway and its effects on glucose uptake and metabolism.

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Insulin is a peptide hormone produced by the beta cells of the pancreatic islets. It plays a crucial role in regulating glucose levels in the body by promoting the uptake and storage of glucose in cells, particularly in the liver, muscle, and adipose tissues. The molecular mechanism of the insulin signaling pathway and its effects on glucose uptake and metabolism can be explained as follows:1. Insulin binding: Insulin exerts its effects by binding to the insulin receptor  IR , a transmembrane protein found on the surface of target cells. The insulin receptor is a tyrosine kinase receptor, which consists of two extracellular alpha subunits and two transmembrane beta subunits. When insulin binds to the alpha subunits, it induces a conformational change in the receptor, activating the intrinsic tyrosine kinase activity of the beta subunits.2. Receptor autophosphorylation: The activated insulin receptor undergoes autophosphorylation, where it adds phosphate groups to specific tyrosine residues within its own beta subunits. This autophosphorylation event further enhances the kinase activity of the receptor and creates docking sites for downstream signaling molecules.3. Recruitment of insulin receptor substrates  IRS : The phosphorylated tyrosine residues on the insulin receptor serve as docking sites for adaptor proteins, such as insulin receptor substrates  IRS . These proteins are then phosphorylated by the insulin receptor on their tyrosine residues, creating additional docking sites for downstream signaling molecules.4. Activation of PI3K/Akt pathway: One of the key downstream signaling molecules that bind to phosphorylated IRS proteins is phosphoinositide 3-kinase  PI3K . PI3K phosphorylates a membrane lipid called phosphatidylinositol 4,5-bisphosphate  PIP2  to generate phosphatidylinositol 3,4,5-trisphosphate  PIP3 . PIP3 serves as a docking site for the serine/threonine kinase Akt  also known as protein kinase B or PKB . Akt is then activated by phosphorylation by phosphoinositide-dependent kinase 1  PDK1  and mTOR complex 2  mTORC2 .5. Glucose uptake and metabolism: Activated Akt has several downstream effects that promote glucose uptake and metabolism:   a. Glucose transporter  GLUT  translocation: Akt stimulates the translocation of glucose transporter proteins, particularly GLUT4, from intracellular vesicles to the plasma membrane in muscle and adipose tissues. This increases the cell's capacity to take up glucose from the bloodstream.   b. Glycogen synthesis: Akt activates glycogen synthase kinase 3  GSK3 , which in turn activates glycogen synthase, the enzyme responsible for converting glucose into glycogen for storage in the liver and muscle cells.   c. Protein synthesis: Akt also activates the mammalian target of rapamycin  mTOR  pathway, which promotes protein synthesis and cell growth.   d. Inhibition of gluconeogenesis: Akt inhibits the expression of gluconeogenic enzymes, such as phosphoenolpyruvate carboxykinase  PEPCK  and glucose-6-phosphatase  G6Pase , in the liver. This reduces the production of glucose from non-carbohydrate sources, such as amino acids and glycerol.In summary, the insulin signaling pathway regulates glucose levels in the body by promoting glucose uptake and storage in target cells, as well as inhibiting gluconeogenesis in the liver. This ensures that glucose is efficiently utilized and prevents excessive glucose accumulation in the bloodstream.
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