Re: How is insulin de-activated once it helps transport glucose?

	Insulin is a hormone which is produced by the Beta cells within the 
islets of Langerhans of the pancreas.  As with most hormones, they are 
active at very low concentrations, meaning, they bind with very high 
affinity to their receptors.  Insulin functions in reducing the level of 
blood glucose by increasing the uptake of glucose from the blood into 
muscle cells and adipocytes, and modulating the activities of enzymes 
involved in glycogen synthesis.  
	When insulin binds to its appropriate receptor, the receptor is 
autophosphorylated which, in turn, activates its receptor-bound, 
intracellular tyrosine kinase.  This Receptor-Tyrosine Kinase or RTK, in 
turn, phosoporylates and activates phosphatase which dephosphorylates and 
activates glycogen synthase.  The signal transduction pathway mediated by 
insulin binding to its receptor not only induces glycogen synthesis but 
also stimulates GLUT4-containing intracellular vesicles to fuse with the 
plasma membrane, thereby increasing the number of glucose transporters on 
the cell surface. 
	The insulin response is terminated by the insulin receptor's ability to 
self-internalize when insulin is bound.  Because insulin binds with high 
affinity to its receptors, almost all the insulin in the body is bound to 
its receptor, and compared to the insulin which is circulating.  By 
internalizing the insulin-bound receptors, therefore, the response is 
deactivated. 
	Furthermore, the same Beta cells of the pancreas also produce another 
hormone, glucogon which functions to increase blood-glucose levels, and, 
therefore, works antagonistically to insulin.  Glucagon inhibits gylcogen 
synthesis by phosporylating and inactivating glycogen synthase.  Therefore, 
the Beta cells play a crucial role within the body to detect blood-glucose 
levels and secrete the appropriate hormone, glucagon or insulin.