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2b. Certain mutations of the β-subunit of the insulin receptor result in the loss of enzymatic activity of the subunit leading to diabetes. Describe the specific role of the β-subunit and explain why loss of activity of these subunits would result in diabetes.The tyrosine kinase domain of each β chain catalyzes autophosphorylation of tyrosine residues in the adjacent kinase domain. Due to the mutation, the subunits lack tyrosine-kinase activity and cannot catalyze autophosphorylation or other phosphorylation reactions. Autophosphorylation of the beta subunits of the receptor activates a local tyrosine kinase, which in turn causes phosphorylation of multiple other intracellular enzymes including a group called insulin-receptor substrates (IRS). Different types of IRS (e.g., IRS-1, IRS-2, IRS-3) are expressed in different tissues. The net effect is to activate some of these enzymes while inactivating others.
The hormone insulin stimulates high rates of glucose uptake into skeletal and heart muscle cells and adipocytes via the transporter GLUT4.When insulin binds to receptors on the cell surface, intracellular vesicles that have GLUT4 embedded in their membranes fuse with the cell surface by exocytosis thereby increasing the capacity of the cells to transport glucose. When there is loss of β-subunit activity, insulin can`t stimulate glucose transport into the cell so glucose concentration increase outside the cell....
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