Glucose facilitator protein

Microsomal glucose transport protein (GLUT ), which is a member of the family of facilitative glucose transport proteins (Chapter 13) and which transports glucose into the cytosol. 

Insulin stimulates translocation of the glucose transporter protein from cytosol to the cell surface. Glucose transport protein carries out the facilitated transport of glucose. 

Another kind of pore is gramicidin A, which is a simple 15-residue polypeptide that allows potassium and sodium ions to pass through it (Figure 10,22). Still another pore-facilitated system is that of the glucose transport protein of erythrocytes which strongly favors transport of D-glucose over other sugars. 

Glucose, galactose, and fructose formed by the digestive enzymes are transported into the absorptive epithelial cells of the small intestine by protein-mediated Na -dependent active transport and facilitative diffusion. Monosaccharides are transported from these cells into the blood and circulate to the liver and peripheral tissues, where they are taken up by facilitative transporters. Facilitative transport of glucose across epithelial cells and other cell membranes is mediated by a family of tissue-specific glucose transport proteins (GLUT I-V). The type of transporter found in each cell reflects the role of glucose metabolism in that cell. 

The entry rate of glucose into red blood cells is far greater than would be calculated for simple diffusion. Rather, it is an example of facilitated diffiision (Chapter 41). The specific protein involved in this process is called the glucose transporter or glucose permease. Some of its properties are summarized in Table 52-3-The process of entry of glucose into red blood cells is of major importance because it is the major fuel supply for these cells. About seven different but related glucose transporters have been isolated from various tissues unlike the red cell transporter, some of these are insidin-dependent (eg, in muscle and adipose tissue). There is considerable interest in the latter types of transporter because defects in their recruitment from intracellular sites to the surface of skeletal muscle cells may help explain the insulin resistance displayed by patients with type 2 diabetes mellitus. 

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