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Glucose transporters (GLUT or SLC2A family) are a family of membrane proteins found in most mammalian cells.

Contents

Function

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.

Active Transport - cotransporters

Transport of glucose through the apical membrane of intestinal 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.[1]

Passive transport - GLUTs

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.[2] Molecule movement by such transporter proteins occurs by facilitated diffusion. [1] This makes them energy independent, unlike active transporters which often require the presence of ATP to drive their translocation mechanism, and stall if the ATP/ADP ratio drops too low.

Structure

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,[3][4][5] 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;[6] also, the QLS motif located in the seventh transmembrane segment could be involved in the selection and affinity of transported substrate.[7][8]

Types

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.[9] To date, 13 members of the GLUT/SLC2 have been identified.[10] On the basis of sequence similarities, the GLUT family has been divided into three subclasses.

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Class I

Class I comprises the well-characterized glucose transporters GLUT1-GLUT4.[11]

Name Distribution Notes
GLUT1 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. Levels in cell membranes are increased by reduced glucose levels and decreased by increased glucose levels.
GLUT2 Is expressed by renal tubular cells and small intestinal epithelial cells that transport glucose, liver cells and pancreatic β cells. All three monosaccharides are transported from the intestinal mucosal cell into the portal circulation by GLUT2 Is a high capacity and low affinity isoform
GLUT3 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 Found in adipose tissues and striated muscle (skeletal muscle and cardiac muscle). Is the insulin-regulated glucose transporter. Responsible for insulin-regulated glucose storage.

Classes II/III

Class II comprises:

Class III comprises GLUT6 (SLC2A6), 8 (SLC2A8), 10 (SLC2A10), and 12 (SLC2A12) and the H+/myoinositol transporter HMIT (SLC2A13).[13]

Most members of classes II and III have been identified 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.

Discovery of sodium-glucose cotransport

In August 1960, in Prague, Robert K. Crane presented for the first time his discovery of the sodium-glucose cotransport as the mechanism for intestinal glucose absorption.[14] Crane's discovery of cotransport was the first ever proposal of flux coupling in biology. [15][16]

See also

References

  1. ^ Hediger M, Rhoads D (1994). "Molecular physiology of sodium-glucose cotransporters". Physiol. Rev. 74 (4): 993–1026. PMID 7938229.  
  2. ^ Henderson P (1993). "The 12-transmembrane helix transporters". Curr. Opin. Cell Biol. 5 (4): 708–21. doi:10.1016/0955-0674(93)90144-F. PMID 8257611.  
  3. ^ 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. doi:10.1038/345550a0. PMID 2348864.  
  4. ^ 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.  
  5. ^ 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. doi:10.1021/bi00047a002. PMID 7492539.  
  6. ^ Hruz P, Mueckler M (2001). "Structural analysis of the GLUT1 facilitative glucose transporter (review)". Mol. Membr. Biol. 18 (3): 183–93. doi:10.1080/09687680110072140. PMID 11681785.  
  7. ^ 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. doi:10.1021/bi972322u. PMID 9477959.  
  8. ^ Hruz P, Mueckler M (1999). "Cysteine-scanning mutagenesis of transmembrane segment 7 of the GLUT1 glucose transporter". J. Biol. Chem. 274 (51): 36176–80. doi:10.1074/jbc.274.51.36176. PMID 10593902.  
  9. ^ 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.  
  10. ^ 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. doi:10.1080/09687680110090456. PMID 11780753.  
  11. ^ 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. doi:10.2337/diacare.13.3.198. PMID 2407475.  
  12. ^ Page 995 in: Walter F., PhD. Boron (2003). Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. pp. 1300. ISBN 1-4160-2328-3.  
  13. ^ Uldry M, Thorens B (2004). "The SLC2 family of facilitated hexose and polyol transporters". Pflugers Arch. 447 (5): 480–9. doi:10.1007/s00424-003-1085-0. PMID 12750891.  
  14. ^ Robert K. Crane, D. Miller and I. Bihler. “The restrictions on possible mechanisms of intestinal transport of sugars”. In: Membrane Transport and Metabolism. Proceedings of a Symposium held in Prague, August 22–27, 1960. Edited by A. Kleinzeller and A. Kotyk. Czech Academy of Sciences, Prague, 1961, pp. 439-449.
  15. ^ Ernest M. Wright and Eric Turk. “The sodium glucose cotransport family SLC5”. Pflügers Arch 447, 2004, p. 510. “Crane in 1961 was the first to formulate the cotransport concept to explain active transport [7]. Specifically, he proposed that the accumulation of glucose in the intestinal epithelium across the brush border membrane was coupled to downhill Na+ transport cross the brush border. This hypothesis was rapidly tested, refined and extended [to] encompass the active transport of a diverse range of molecules and ions into virtually every cell type.”
  16. ^ Boyd, C A R. “Facts, fantasies and fun in epithelial physiology”. Experimental Physiology, Vol. 93, Issue 3, 2008, p. 304. “the insight from this time that remains in all current text books is the notion of Robert Crane published originally as an appendix to a symposium paper published in 1960 (Crane et al. 1960). The key point here was 'flux coupling', the cotransport of sodium and glucose in the apical membrane of the small intestinal epithelial cell. Half a century later this idea has turned into one of the most studied of all transporter proteins (SGLT1), the sodium–glucose cotransporter.”

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