Nutrition Essay

Nutrition Essay - Insulin and Sulphonylureas Insulin is the...

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Insulin and Sulphonylureas Insulin is the peptide hormone that is released when the body is in the fed state, causing liver and muscle cells to take up glucose and convert it to glycogen, and causing fat cells to take up blood lipids, converting them to triglycerides. The raw form of Insulin starts with a precursory mRNA transcript, which is translated into the inactive protein preproinsulin, consisting of two polypeptide chains; A and B. Chains A and B are covalently joined by two disulphide bridges. Preproinsulin migrates to the endoplasmic reticulum (ER) and is then post-translationally modified by having its signal peptide cleaved off, forming proinsulin. Proinsulin is stored in immature granules coated with clathrin and undergoes proteolysis (catalysed by the enzyme PC1) where the C-peptide is cleaved off as a result of hydrolysis of the two peptide bonds. Insulin secretion occurs mainly as a response to glucose uptake, which seems appropriate because insulin is described to be ‘in control’ of facilitating glucose entry into cells. However it can also be secreted in response to sulphonyureas which increase insulin secretion via the beta cells of the islets of Langerhans in the pancreas. Insulin acts upon many sites in different tissues, it primarily works by circulating throughout the blood stream until it binds to insulin receptors. The insulin receptors promote the uptake of glucose into various tissues that contain type 4 glucose transporters (GLUT4). Such tissues include skeletal muscles, which burn glucose for energy, and adipose tissues, which convert glucose to triglycerides. The initial binding of insulin to its receptor initiates a signal transduction cascade that communicates to signal receptors to remove glucose from the blood stream and store it as glycogen. In a healthy person, insulin is secreted in direct proportion to the uptake of glucose in the beta cells of the pancreas. Glucose is taken up into beta cells by the type 2 glucose transporter (GLUT2), which has a high K m for glucose, therefore its competence in transporting glucose across the plasma membrane also increases proportionally with the concentration of glucose. These specialised glucose transporters are known as glucose sensors because of this specialised function. As glucose is the raw fuel for the energy producing pathway glycolysis, beta cells contain the rate-
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This note was uploaded on 02/25/2012 for the course BIO 2400 taught by Professor Mclean during the Fall '09 term at Texas State.

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Nutrition Essay - Insulin and Sulphonylureas Insulin is the...

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