|Topic:||Comparison of the kinetic activities of hexokinases (HK1, HK2, and HK3) and glucokinase (GCK, HK4). .|
|Details:||Comparison of the kinetic activities of hexokinases (HK1, HK2, and HK3) and glucokinase (GCK, HK4). The Km, for glucose, of the hexokinases is significantly lower than that of glucokinase. HK1 exhibits a glucose Km around 0.03 mM, HK2 around 0.3 mM, and HK3 around 0.003 mM. The Km for glucose of glucokinase is around 6 mM. This difference ensures that non-hepatic tissues (which contain hexokinase) rapidly and efficiently trap blood glucose within their cells by converting it to glucose-6-phosphate. One major function of the liver is to deliver glucose to the blood and this is ensured by having a glucose phosphorylating enzyme (glucokinase) whose Km for glucose is sufficiently higher that the normal circulating concentration of glucose (normal fasting plasma glucose: 5 mM). In addition to the Km differences between the hexokinases (HK1, HK2, HK3) and glucokinase, glucokinase also exhibits sigmoidal (as opposed to hyperbolic) kinetics characteristic of multisubunit and/or allosteric enzymes despite the enzyme functioning in the monomeric state.|
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