Linked Life Data

48887BiochemicalReaction450

Source:http://www.reactome.org/biopax/48887BiochemicalReaction450

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GLUT (glucose transporter) homotetramers associated with the plasma membrane mediate the facilitated diffusion of glucose between the extracellular space and the cytosol. Four human GLUT isoforms have been identified, members of a larger family of transporter proteins (Joost et al. 2002), encoded by the SCLC2A1, 2, 3, and 4 genes. Conserved sequence motifs in the GLUT proteins suggest the existence of shared structural features confirmed by in situ labeling and mutagenesis studies. Each GLUT protein has twelve membrane spanning domains organized to form an aqueous channel. While monomeric protein can form such a channel and transport glucose, kinetic studies suggest that the functional form of the protein is a homotetramer.<p>Different GLUT proteins are expressed in different tissues. GLUT1 is expressed by many cell types, notably endothelial cells, red blood cells and cells of the brain. Its low Km for glucose (~1 mM) relative to normal blood glucose concentration (~5 mM) allows these cells to take up glucose independent of changes in blood glucose levels. Its abundance in red blood cells has allowed it to be purified and biochemically characterized (Hruz and Mueckler 2001; Liu et al. 2001). Cytosolic ATP associates with GLUT1 and inhibits its glucose transporter activity. GLUT2 is expressed on hepatocytes and pancreatic beta cells. Because of its high Km for glucose (~15-20 mM), GLUT2-mediated glucose uptake normally occurs only in the fed state, when its rate is proportional to blood glucose concentration. This feature of the transporter is thought to enable efficient uptake of large amounts of glucose by the liver in the fed state, and to allow it to function as part of a glucose sensor coupled to insulin release in pancreatic beta cells (Thorens 2001). GLUT2 also mediates glucose export from liver cells when gluconeogenesis is underway (Colville et al. 1993; Santer et al. 1997; Wu et al. 1998). GLUT3 is expressed by cells of the brain and possibly other cell types with a high constitutive requirement for glucose. Its low Km for glucose (~1.5 mM) allows these cells to import glucose independent of fluctuations in blood glucose levels (Colville et al. 1993). GLUT4, like GLUT1 and 3, has a high affinity for glucose. GLUT4 is abundant in cells of skeletal muscle and adipose tissue. GLUT4 molecules are translocated from an intracellular store to the cell surface in response to increased insulin levels (Bryant et al. 2002; Fukumoto et al. 1989).<br>
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alpha-D-glucose (extracellular) <=> alpha-D-glucose (cytosol)
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Glucose is taken up across the plasma membrane by a glucose transport protein (GLUT)
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