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Regulation of Glucose in Blood (Part II)
Our liver and to a much lesser extent our kidney cells are responsible for regulating and maintaining our blood glucose levels. These cells must regulate glycolysis and gluconeogenesis in a reciprocal fashion to ensure that proper glucose levels are maintained. A key element in this regulatory pathway is a bifunctional allosteric enzyme that contains two different regulatory domains. One domain is called phosphofructokinase-2 (PFK-2) while the other regulatory domain is called fructose bisphosphatase-2 (FBPase-2). This bifunctional enzyme exists in two important states. In one state, the PFK-2 domain is unphosphorylated and so it exists in its active state while the other domain is inactive. In this state, the PFK-2 domain will phosphorylate fructose 6-phosphate to form fructose 2,6-bisphosphate. In the other state, a serine residue on the PFK-2 domain becomes phosphorylated and this inactivates it while activating the FBPase-2 domain. The active FBPase-2 domain will go on to dephosphorylate the fructose 2,6-bisphosphate to form fructose 6-phosphate. When blood glucose levels are high (after eating a meal), insulin will be released by the beta-cells of the pancreas, which will in turn stimulates glucose uptake into the cells. Insulin also helps activate the enzyme phosphoprotein phosphatase that is responsible for dephosphorylating the bifunctional enzyme. This actives the PFK-2 domain, which in turn stimulates the formation of fructose 2,6-bisphosphate, an allosteric activator of phosphofructokinase in glycolysis. This in turn stimulates the process of glycolysis and decreases the rate of gluconeogenesis. Under conditions of low blood glucose (during fasting periods), glucagon is activated, which in turn stimulates protein kinase A to turn on FBPase-2. This in turn transforms fructose 2,6-bisphosphate into its fructose 6-phosphate, which stimulates gluconeogenesis and decreases the rate of glycolysis.
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