concept

Glucose transporter

GLUTs 1, 3, and 4 are transporters that have high affinity for glucose ranging in Km of 2–5 mM glucose. Consequently, the functions of these transporters align with the physiological concentration of glucose of about 5 mM. On the other hand, GLUT2 has a low affinity for glucose with its Km of about 15–20 mM glucose. GLUT2, therefore, is able to move glucose into the liver cell and the pancreatic beta cell in proportion to the plasma level of glucose. This low affinity provides a variable rate of transport (proportional to the increasing glucose concentration) so that the high level of glucose following a carbohydrate-rich meal can be accommodated. GLUT3 has a low Km for glucose of 1.6 mM. It transports glucose into brain cells at a rate that is independent of the plasma level of glucose when it exceeds the physiological range of 4–10 mM. GLUT4 functions for the insulin-dependent translocation of glucose. Thus, insulin stimulates the uptake of glucose by GLUT4 in the muscle cell where hexokinase converts it to glucose-6-phosphate so that the cell may utilize it for either glycolysis for energy or for the formation of glycogen when glucose is abundant. In the adipocyte, GLUT4 also responds to insulin for the uptake of glucose where it is again converted to glucose-6-phosphate by hexokinase and then converted to glycerol-3-phosphate for conversion to trigylcerides. Also under the control of insulin, lipoproteins in the plasma form fatty acids within the cell under the action of lipoprotein lipase and these fatty acids also are used for triglyceride synthesis. In addition, insulin inhibits the breakdown of triglycerides to fatty acids in these cells. GLUT5 is located in the plasma membranes of small intestinal epithelial cells where it is responsible for the uptake of fructose. Some cells like the small intestinal epithelial cell contain more than one hexose transporter. This cell absorbs glucose, galactose, and fructose from the intestinal lumen. The apical membrane contains a sodium–glucose symporter (SGLUT1) that functions to transport glucose and galactose into the cell together with sodium ion. GLUT2 is located on the basolateral side of the cell where it moves the three hexoses, glucose, galactose, and fructose, out of the cell into the extracellular space and ultimately move further to access the bloodstream.

Ref:
Gerald Litwack, Chapter 8 - Glycolysis and Gluconeogenesis, Editor(s): Gerald Litwack, Human Biochemistry, Academic Press, 2018, Pages 183-198, ISBN 9780123838643, https://doi.org/10.1016/B978-0-12-383864-3.00008-9. (https://www.sciencedirect.com/science/article/pii/B9780123838643000089)

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Added on Oct 2, 2022 by Barbara Van De Keer
Edited on Oct 2, 2022 by Barbara Van De Keer

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