Facilitative glucose transporters structure

Olson, A. L. and J. E. Pessin. Structure, function, and regulation of the mammalian facilitative glucose transporter gene family. Annu. Rev. Nutr. 1996, 36, 235-256. 

Hruz, P. W. and M. M. Mueckler. Structural analysis of the GLUT1 facilitative glucose transporter. Mol. Membr. Biol. 2001, 38, 183-193. 

Pessin JE, Bell GI (1992), Mammalian facilitative glucose transporter family. Structure and molecular regulation, Annu. Rev. Physiol. 54 911-930. 

Structural Requirements of Glucose Binding to Facilitative Glucose Transporters 74... 

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.

The primary, secondary, and presumably tertiary, and higher, structure of the family of facilitative glucose transporters are similar. It is beginning to be understood which domains of these transporters are important for substrate and inhibitor binding and for subcellular localization. Future work will undoubtably uncover the molecular mechanism by which the facilitative transporters catalyze the translocation of sugars across the membrane. 

Amino acid transporters, oligopeptide transporters, glucose transporters, lactic acid transporters, monocarboxylic acid transporters, phosphate transporters, bile acid transporters and other transporters present on the apical membrane of the epithelial cells serve as carriers to facilitate nutrient absorption by the intestine. On the basolateral membrane, amino acid and oligopeptide transporters also exist. Drag moieties possessing similar structures to nutrients that are absorbed by such carriers may also be absorbed in this manner. 

Passive facilitated diffusion involves movement down a concentration gradient without an input of energy. This mechanism, which may be highly selective for specific conformational structures, is necessary for transport of endogenous compounds whose rate of transport by simple diffusion would otherwise be too slow. The classical example of facilitated diffusion is transport of glucose into red blood cells. 

Facilitated diffusion is very similar to passive diffusion with the difference that transfer across membranes is assisted by the participation of carrier proteins embedded in the membrane bilayer. Again, the direction of passage will be from the side of the membrane with high concentration of a chemical to the side with low concentration this also occurs without energy expenditure by the cell. Such a process is somewhat specific in the sense that it applies to molecules that are able to bind to a carrier protein. Absorption of nutrients such as glucose and amino acids across the epithelial membrane of the gastrointestinal tract occurs by facilitated diffusion. Since a finite number of carriers are available for transport, the process is saturable at high concentrations of the transported molecules and competition for transport may occur between molecules of similar structure. 

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