Sodium/ Glucose Transporter Facilitative

Other cotransporters facilitate the transport of other sugars, osmolytes, and amino acids. In humans, a disorder of intestinal glucose and galactose absorption is due to a defective sodium-glucose transporter. 

Figure 1. Models for the orientation of A.) Members of the facilitative glucose transporter family (CLUT1 to GLUT7), and B.) the sodium-dependent glucose transporter (SGLT1). The branched structure is at the site of glycosylation for both transporters. In A, the open residues represent amino acids which are identical in GLUT1 through GLUT5.

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. 

The mechanism of DHAA uptake by luminal membranes of human jejunum has pharmacological characteristics that clearly differ from those of ascorbate uptake. Sodium-independent carriers take up DHAA by facilitated diffusion, and these are distinct from the sodium-dependent transporters of ascorbate. Glucose inhibits ascorbate uptake but not DHAA uptake, which raises the possibility that glucose derived from food may increase the bioavailability of DHAA relative to ascorbate (Malo and Wilson, 2000). Human enterocytes contain reductases that convert DHAA to ascorbate (Buffinton and Doe, 1995). This conversion keeps the intracellular level of DHAA low, and the resulting concentration gradient favors uptake of oxidized AA across the enterocyte plasma membrane. 

While the characteristics of this type of transport have been studied and identified in a number of cell systems and for a number of substances (Table 6), little is known of the molecular mechanism involved, although kinetic analysis of facilitated diffusion may be found (Stein, 1967 Neame and Richards, 1972). However, because the proposed mechanism involves a reversible reaction of the substrate with a membrane carrier to form a complex which traverses the membrane and releases the substrate at the other side, it may be that facilitated diffusion mechanisms do not differ from those involved in active transport of the same molecule (Csaky, 1965 Wilbrandt, 1972). Specifically, the active transport of D-glucose may simply require the presence and cotransport of sodium ions (Crane, 1962 Stein, 1967). This theory has recently received support from studies of Na+-gradient-dependent uptake of o-glucose by isolated intestinal and renal brush border membranes (Murer and Hopfer, 1974 Kinne et al., 1975). However, the elec-trogenic nature of D-glucose transport is probably more accurately class-... 

The simplest of these functions is that of a permeability barrier that limits free diffusion of solutes between the cytoplasm and external environment. Although such barriers are essential for cellular life to exist, there must also be a mechanism by which selective permeation allows specific solutes to cross the membrane. In contemporary cells, such processes are carried out by transmembrane proteins that act as channels and transporters. Examples include the proteins that facilitate the transport of glucose and amino acids into the cell, channels that allow potassium and sodium ions to permeate the membrane, and active transport of ions by enzymes that use ATP as an energy source. 

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