Glycolysis- 5 Activation of glucose; formation of glucose-6-phosphate Glucokinase is specific for glucose and does not catalyze reactions with other sugars. Glucokinase has a high Km (lower affinity) for glucose, so, its activity increases with increases in the concentration of glucose in portal blood. It promotes hepatic uptake of large amounts of glucose after a carbohydrate meal. Glucokinase is found in tissues that require a glucose sensing system” as the β– cells of the pancreas and hypothalamus. Regulation of both insulin secretion and of appetite are functions of glucokinase. Uptake of glucose after a meal can increase the concentration of glucose in portal blood from the normal fasting level of 4-5 mmolar to 20 mmolar or even higher. This activates inward hepatic glucose transport and glucokinase activity. Since glucokinase is not product-inhibited, the liver is able to take up and store large amounts of glucose as glycogen after a meal. This can then be released to the circulation later to stabilize blood glucose levels. Glucokinase is not product-inhibited. At normal systemic blood (4.5 – 5.5 mmol/L) the liver is a net producer of glucose. However, as blood glucose level rises, the output of glucose ceases, and there is a net uptake. Hexokinase is, well-adapted as the initiator of glucose metabolism in tissues utilizing glucose as an energy source, but not as the initiator of energy storage in the liver. Glucokinase enzyme is active in spite of high glucose and G-6-P levels and so, is important for storage of glucose as glycogen in the liver. Pancreatic ß-cells secrete insulin in response to very small increases in blood glucose concentration. The glucose transport protein in these cells (GLUT2) and glucokinase both have high Km values. This appears to be also the case for glucokinase-containing cells in the hypothalamus. The couple GLUT2-glucokinase in these cells acts as a glucose sensor, controlling both insulin secretion and appetite. Glucokinase activity automatically rises and falls with changes in glucose concentration in the ß-cell.
Posted on: Wed, 22 Oct 2014 17:55:22 +0000