Glucose is an essential substrate for the metabolism of most cells. Because glucose is a polar molecule, transport through biological membranes requires specific transport proteins. Transport of glucose through the apical membrane of intestinal, choroid plexus and kidney epithelial cells depends on the presence of secondary active Na+/glucose symporters, SGLT-1 and SGLT-2, which concentrate glucose inside the cells, using the energy provided by cotransport of Na+ ions down their electrochemical gradient. Facilitated diffusion of glucose through the cellular membrane is otherwise catalyzed by glucose carriers (protein symbol GLUT, gene symbol SLC2 for Solute Carrier Family 2) that belong to a superfamily of transport facilitators (major facilitator superfamily) including organic anion and cation transporters, yeast hexose transporter, plant hexose/proton symporters, and bacterial sugar/proton symporters.
Molecule movement by glucose transporter proteins works by facilitating diffusion.  This makes them energy independent, unlike active transporters which often require the presence of ATP to drive their translocation mechanism and stall if ATP/ADP ratio drops too low.
GLUTs are integral membrane proteins which contain 12 membrane spanning helices with both the amino and carboxyl termini exposed on the cytoplasmic side of the plasma membrane. GLUT proteins transport glucose and related hexoses according to a model of alternate conformation, which predicts that the transporter exposes a single substrate binding site toward either the outside or the inside of the cell. Binding of glucose to one site provokes a conformational change associated with transport, and releases glucose to the other side of the membrane. The inner and outer glucose-binding sites are probably located in transmembrane segments 9, 10, 11; also, the QLS motif located in the seventh transmembrane segment could be involved in the selection and affinity of transported substrate.
Each glucose transporter isoform plays a specific role in glucose metabolism determined by its pattern of tissue expression, substrate specificity, transport kinetics, and regulated expression in different physiological conditions. To date, 13 members of the GLUT/SLC2 have been identified. On the basis of sequence similarities, the GLUT family has been divided into three subclasses.
Class I comprises the well-characterized glucose transporters GLUT1-GLUT4.
|GLUT1||SLC2A1||Is widely distributed in fetal tissues. In the adult, it is expressed at highest levels in erythrocytes and also in the endothelial cells of barrier tissues such as the blood-brain barrier. However, it is responsible for the low-level of basal glucose uptake required to sustain respiration in all cells.||Its levels in cell membranes are increased by reduced glucose levels and decreased by increased glucose levels.|
|GLUT2||SLC2A2||Is expressed by renal tubular cells and small intestinal epithelial cells that transport glucose, liver cells and pancreatic β cells.||-|
|GLUT3||SLC2A3||Expressed mostly in neurons (where it is believed to be the main glucose transporter isoform), and in the placenta.||Is a high-affinity isoform|
|GLUT4||SLC2A4||Found in adipose tissues and striated muscle (skeletal muscle and cardiac muscle).||Is the insulin-regulated glucose transporter. Responsible for insulin-regulated glucose storage.|
Most members of classes II and III have been idenitified recently in homology searches of EST databases and the sequence information provided by the various genome projects.
The function of these new glucose transporter isoforms is still not clearly defined at present. Several of them (GLUT6, GLUT8) comprise motifs that help retain them intracellularly and therefore prevent glucose transport. Whether mechanisms exist to promote cell-surface translocation of these transporters is not yet known, but it has clearly been established that insulin does not promote GLUT6 and GLUT8 cell-surface translocation.
Synthesis of free glucose
Most cells are unable to produce free glucose because they lack expression of glucose-6-phosphatase and thus are only involved in glucose uptake and catabolism. Only hepatocytes and, in more severe fasting conditions, intestine and kidney, are able to produce glucose following activation of gluconeogenesis and glycogenolysis.
- ↑ Hediger M, Rhoads D (1994). "Molecular physiology of sodium-glucose cotransporters". Physiol. Rev. 74 (4): 993-1026. PMID 7938229.
- ↑ Henderson P (1993). "The 12-transmembrane helix transporters". Curr. Opin. Cell Biol. 5 (4): 708-21. PMID 8257611.
- ↑ Oka Y, Asano T, Shibasaki Y, Lin J, Tsukuda K, Katagiri H, Akanuma Y, Takaku F (1990). "C-terminal truncated glucose transporter is locked into an inward-facing form without transport activity". Nature 345 (6275): 550-3. PMID 2348864.
- ↑ Hebert D, Carruthers A (1992). "Glucose transporter oligomeric structure determines transporter function. Reversible redox-dependent interconversions of tetrameric and dimeric GLUT1". J. Biol. Chem. 267 (33): 23829-38. PMID 1429721.
- ↑ Cloherty E, Sultzman L, Zottola R, Carruthers A (1995). "Net sugar transport is a multistep process. Evidence for cytosolic sugar binding sites in erythrocytes". Biochemistry 34 (47): 15395-406. PMID 7492539.
- ↑ Hruz P, Mueckler M (2001). "Structural analysis of the GLUT1 facilitative glucose transporter (review)". Mol. Membr. Biol. 18 (3): 183-93. PMID 11681785.
- ↑ Seatter M, De la Rue S, Porter L, Gould G (1998). "QLS motif in transmembrane helix VII of the glucose transporter family interacts with the C-1 position of D-glucose and is involved in substrate selection at the exofacial binding site". Biochemistry 37 (5): 1322-6. PMID 9477959.
- ↑ Hruz P, Mueckler M (1999). "Cysteine-scanning mutagenesis of transmembrane segment 7 of the GLUT1 glucose transporter". J. Biol. Chem. 274 (51): 36176-80. PMID 10593902.
- ↑ Thorens B (1996). "Glucose transporters in the regulation of intestinal, renal, and liver glucose fluxes". Am. J. Physiol. 270 (4 Pt 1): G541-53. PMID 8928783.
- ↑ Joost H, Thorens B (2001). "The extended GLUT-family of sugar/polyol transport facilitators: nomenclature, sequence characteristics, and potential function of its novel members (review)". Mol. Membr. Biol. 18 (4): 247-56. PMID 11780753.
- ↑ Bell G, Kayano T, Buse J, Burant C, Takeda J, Lin D, Fukumoto H, Seino S (1990). "Molecular biology of mammalian glucose transporters". Diabetes Care 13 (3): 198-208. PMID 2407475.
- ↑ Uldry M, Thorens B (2004). "The SLC2 family of facilitated hexose and polyol transporters". Pflugers Arch. 447 (5): 480-9. PMID 12750891.
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