Ten types of glucose/fructose transport carrier proteins designated as Glut 1-10 facilitate glucose/fructose transport across the cell membrane. Molecular cloning of glucose transporters have identified a family of closely related genes that encodes at least 7 proteins exhibiting high degree of amino acid homology (45%-65%) all in the molecular weight range of 40-60 kDa. Individual members of the Glut family have predicted secondary structure characteristic of 12 membrane spanning domains of other transport carriers. Majority of the differences in sequence homology in Glut proteins occur at 4 hydrophilic domains that may play a role in distinct tissue-specific pattern of expression and targeting. All Glut proteins are glycosylated at or near the C-terminus and are present on either cell surface or in intracellular sites. Some transporters exhibit dynamic trafficking between intracellular storage sites and plasma membranes in response to various stimuli. In some tissues Glut proteins are asymmetrically distributed between apical and basolateral membranes as in blood brain barrier and blood testis barriers.
Glut 5 seems to function primarily as a fructose transporter; it is expressed in small intestine, and at much lower levels in kidney, skeletal muscle, and adipose