Glucose transporter

Glucose

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

## Contents

* 1 Function
* 2 Mechanism
* 3 Structure
* 4 Types
* 4.1 Class I
* 4.2 Classes II/III
* 5 Synthesis of free glucose
* 6 References
* 7 External links

## Function

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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.[1] 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]

## Mechanism

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Molecule movement by glucose transporter proteins works by facilitating 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 ATP/ADP ratio drops too low.

## Structure

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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

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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.

### Class I

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

Name | Gene | Distribution | Notes

|
|
|

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.

### Classes II/III

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Class II comprises the fructose transporter GLUT5 (SLC2A5) and GLUT7 (SLC2A7), 9 (SLC2A9), 11 (SLC2A11).

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

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

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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.

## References

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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. 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. PMID 2348864.CS1 maint: Multiple names: authors list (link)
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. PMID 7492539.CS1 maint: Multiple names: authors list (link)
6. ↑ Hruz P, Mueckler M (2001). "Structural analysis of the GLUT1 facilitative glucose transporter (review)". Mol. Membr. Biol. 18 (3): 183–93. 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. PMID 9477959.CS1 maint: Multiple names: authors list (link)
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. 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. 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. PMID 2407475.CS1 maint: Multiple names: authors list (link)
12. ↑ Uldry M, Thorens B (2004). "The SLC2 family of facilitated hexose and polyol transporters". Pflugers Arch. 447 (5): 480–9. PMID 12750891.

## External links

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* Glucose+Transport+Proteins,+Facilitative at the US National Library of Medicine Medical Subject Headings (MeSH)

* v
* t
* e

Membrane proteins, carrier proteins: membrane transport proteins solute carrier (TC 2A)

By group

SLC1–10

| (1):|

* high affinity glutamate and neutral amino-acid transporter
* SLC1A1
* 2
* 3
* 4
* 5
* 6
* 7

|

(2):|

* facilitative GLUT transporter
* SLC2A1
* 2
* 3
* 4
* 5
* 6
* 7
* 8
* 9
* 10
* 11
* 12
* 13
* 14

(3):|

* heavy subunits of heterodimeric amino-acid transporters
* SLC3A1
* 2

(4):|

* bicarbonate transporter
* SLC4A1
* 2
* 3
* 4
* 5
* 6
* 7
* 8
* 9
* 10
* 11

(5):|

* sodium glucose cotransporter
* SLC5A1
* 2
* 3
* 4
* 5
* 6
* 7
* 8
* 9
* 10
* 11
* 12

(6):|

* sodium\- and chloride\- dependent sodium:neurotransmitter symporters
* SLC6A1
* SLC6A2
* SLC6A3
* SLC6A4
* SLC6A5
* SLC6A6
* SLC6A7
* SLC6A8
* SLC6A9
* SLC6A10
* SLC6A11
* SLC6A12
* SLC6A13
* SLC6A14
* SLC6A15
* SLC6A16
* SLC6A17
* SLC6A18
* SLC6A19
* SLC6A20

(7):|

* cationic amino-acid transporter/glycoprotein-associated
* SLC7A1
* SLC7A2
* SLC7A3
* SLC7A4
* glycoprotein-associated/light or catalytic subunits of heterodimeric amino-acid transporters
* SLC7A5
* SLC7A6
* SLC7A7
* SLC7A8
* SLC7A9
* SLC7A10
* SLC7A11
* SLC7A13
* SLC7A14

(8):|

* Na+/Ca2+ exchanger
* SLC8A1
* SLC8A2
* SLC8A3

(9):|

* Na+/H+ exchanger
* SLC9A1
* SLC9A2
* SLC9A3
* SLC9A4
* SLC9A5
* SLC9A6
* SLC9A7
* SLC9A8
* SLC9A9
* SLC9A10
* SLC9A11

(10):|

* sodium bile salt cotransport
* SLC10A1
* SLC10A2
* SLC10A3
* SLC10A4
* SLC10A5
* SLC10A6
* SLC10A7
* 10A1
* 10A2
* 10A3
* 10A7

SLC11–20

| (11):|

* proton coupled metal ion transporter
* SLC11A1
* SLC11A211A3

|

(12):|

* electroneutral cation-Cl cotransporter
* SLC12A1
* SLC12A2
* SLC12A3
* SLC12A4
* SLC12A5
* SLC12A6
* SLC12A7
* SLC12A8
* SLC12A9

(13):|

* human Na+-sulfate/carboxylate cotransporter
* SLC13A1
* SLC13A2
* SLC13A3
* SLC13A4
* SLC13A5

(14):|

* urea transporter
* SLC14A1
* SLC14A2

(15):|

* proton oligopeptide cotransporter
* SLC15A1
* SLC15A2
* SLC15A3
* SLC15A4

(16):|

* monocarboxylate transporter
* SLC16A1
* SLC16A2
* SLC16A3
* SLC16A4
* SLC16A5
* SLC16A6
* SLC16A7
* SLC16A8
* SLC16A9
* SLC16A10
* SLC16A11
* SLC16A12
* SLC16A13
* SLC16A14

(17):|

* Vesicular glutamate transporter 1
* SLC17A1
* SLC17A2
* SLC17A3
* SLC17A4
* SLC17A5
* SLC17A6
* SLC17A7
* SLC17A8
* SLC17A9

(18):|

* vesicular monoamine transporter
* SLC18A1
* SLC18A2
* SLC18A3

(19):|

* folate/thiamine transporter
* SLC19A1
* SLC19A2
* SLC19A3

(20):|

* type III Na+-phosphate cotransporter
* SLC20A1
* SLC20A2

SLC21–30

| (21):|

* Organic anion-transporting polypeptide
* SLCO1A2
* SLCO1B1
* SLCO1B3
* SLCO1B4
* SLCO1C1
*     * SLCO2A1
* SLCO2B1
*     * SLCO3A1
*     * SLCO4A1
* SLCO4C1
*     * SLCO5A1(SLCO6A1)

|

(22):|

* organic cation/anion/zwitterion transporter
* SLC22A1
* SLC22A2
* SLC22A3
* SLC22A4
* SLC22A5
* SLC22A6
* SLC22A7
* SLC22A8
* SLC22A9
* SLC22A10
* SLC22A11
* SLC22A12
* SLC22A13
* SLC22A14
* SLC22A15
* SLC22A16
* SLC22A17
* SLC22A18
* SLC22A19
* SLC22A20

(23):|

* Na+-dependent ascorbic acid transporter
* SLC23A1
* SLC23A2
* SLC23A3
* SLC23A4

(24):|

* Na+/(Ca2+-K+) exchanger
* SLC24A1
* SLC24A2
* SLC24A3
* SLC24A4
* SLC24A5
* SLC24A6

(25):|

* mitochondrial carrier
* SLC25A1
* SLC25A2
* SLC25A3
* SLC25A4
* SLC25A5
* SLC25A6
* SLC25A7
* SLC25A8
* SLC25A9
* SLC25A10
* SLC25A11
* SLC25A12
* SLC25A13
* SLC25A14
* SLC25A15
* SLC25A16
* SLC25A17
* SLC25A18
* SLC25A19
* SLC25A20
* SLC25A21
* SLC25A22
* SLC25A23
* SLC25A24
* SLC25A25
* SLC25A26
* SLC25A27
* SLC25A28
* SLC25A29
* SLC25A30
* SLC25A31
* SLC25A32
* SLC25A33
* SLC25A34
* SLC25A35
* SLC25A36
* SLC25A37
* SLC25A38
* SLC25A39
* SLC25A40
* SLC25A41
* SLC25A42
* SLC25A43
* SLC25A44
* SLC25A45
* SLC25A46

(26):|

* multifunctional anion exchanger
* SLC26A1
* SLC26A2
* SLC26A3
* SLC26A4
* SLC26A5
* SLC26A6
* SLC26A7
* SLC26A8
* SLC26A9
* SLC26A10
* SLC26A11

(27):|

* fatty acid transport proteins
* SLC27A1
* SLC27A2
* SLC27A3
* SLC27A4
* SLC27A5
* SLC27A6

(28):|

* Na+-coupled nucleoside transport (SLC28A1
* SLC28A2
* SLC28A3

(29):|

* facilitative nucleoside transporter
* SLC29A1
* SLC29A2
* SLC29A3
* SLC29A4

(30):|

* zinc efflux
* SLC30A1
* SLC30A2
* SLC30A3
* SLC30A4
* SLC30A5
* SLC30A6
* SLC30A7
* SLC30A8
* SLC30A9
* SLC30A10

SLC31–40

| (31):|

* copper transporter
* SLC31A1

|

(32):|

* Vesicular glutamate transporter 1
* SLC32A1

(33):|

* Acetyl-CoA transporter
* SLC33A1

(34):|

* type II Na+-phosphate cotransporter
* SLC34A1
* SLC34A2
* SLC34A3

(35):|

* nucleoside-sugar transporter
* SLC35A1
* SLC35A2
* SLC35A3
* SLC35A4
* SLC35A5
*     * SLC35B1
* SLC35B2
* SLC35B3
* SLC35B4
*     * SLC35C1
* SLC35C2
*     * SLC35D1
* SLC35D2
* SLC35D3
*     * SLC35E1
* SLC35E2
* SLC35E3
* SLC35E4

(36):|

* proton-coupled amino-acid transporter
* SLC36A1
* SLC36A2
* SLC36A3
* SLC36A436A2

(37):|

* sugar-phosphate/phosphate exchanger
* SLC37A1
* SLC37A2
* SLC37A3
* SLC37A4

(38):|

* System A & N, sodium-coupled neutral amino-acid transporter
* SLC38A1
* SLC38A2
* SLC38A3
* SLC38A4
* SLC38A5
* SLC38A6
* SLC38A10

(39):|

* metal ion transporter
* SLC39A1
* SLC39A2
* SLC39A3
* SLC39A4
* SLC39A5
* SLC39A6
* SLC39A7
* SLC39A8
* SLC39A9
* SLC39A10
* SLC39A11
* SLC39A12
* SLC39A13
* SLC39A14

(40):|

* basolateral iron transporter
* SLC40A1

SLC41–48

| (41):|

* Magnesium transporter E
* SLC41A1
* SLC41A2
* SLC41A3

|

(42):|

* Ammonia transporter
* RhAG
* RhBG
* RhCG

(43):|

* Na+-independent, system-L like amino-acid transporter
* SLC43A1
* SLC43A2
* SLC43A3

(44):|

* Choline-like transporter
* SLC44A1
* SLC44A2
* SLC44A3
* SLC44A4
* SLC44A5

(45):|

* Putative sugar transporter
* SLC45A1
* SLC45A2
* SLC54A3
* SLC45A4

(46):|

* Folate transporter
* SLC46A1
* SLC46A2

(47):|

* multidrug and toxin extrusion
* SLC47A1
* SLC47A2

(48):|

* Heme transporter

SLCO1–4

* O1A2
* O1B1
* O1B3
* O2B1
* O431
* O4A1

Ion pumps

| Symporter, Cotransporter|

* Na+/K+,Cl−
* Na+/Pi3
* Na+/Cl−
* Na+/glucose
* Na+/I−
* Cl−/K+
* 4
* 5

|

Antiporter (exchanger)|

* Na+/H+
* Na+/Ca2+
* Na+/(Ca2+-K+) - Cl−/HCO−
3 (Band 3)
* Cl−-formate
* Cl−-oxalate

see also solute carrier disorders

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