Glucose transporters are a wide group of membrane proteins that facilitate the transport of glucose across the plasma membrane, a process known as facilitated diffusion. Because glucose is a vital source of energy for all life, these transporters are present in all phyla. The GLUT or SLC2A family are a protein family that is found in most mammalian cells. 14 GLUTS are encoded by human genome. GLUT is a type of uniporter transporter protein.
Synthesis of free glucose
Most non-autotrophic cells are unable to produce free glucose because they lack expression of glucose-6-phosphatase and, thus, are involved only in glucose uptake and catabolism. Usually produced only in hepatocytes, in fasting conditions, other tissues such as the intestines, muscles, brain, and kidneys are able to produce glucose following activation of gluconeogenesis.
Glucose transport in yeast
|Snf3||low-glucose sensor; repressed by glucose; low expression level; repressor of Hxt6|
|Rgt2||high-glucose sensor; low expression level|
|Hxt1||Km: 100 mM, 129 - 107 mM||low-affinity glucose transporter; induced by high glucose level|
|Hxt2||Km = 1.5 - 10 mM||high/intermediate-affinity glucose transporter; induced by low glucose level|
|Hxt3||Vm = 18.5, Kd = 0.078, Km = 28.6/34.2 - 60 mM||low-affinity glucose transporter|
|Hxt4||Vm = 12.0, Kd = 0.049, Km = 6.2||intermediate-affinity glucose transporter|
|Hxt5||Km = 10 mM||Moderate glucose affinity. Abundant during stationary phase, sporulation and low glucose conditions. Transcription repressed by glucose.|
|Hxt6||Vm = 11.4, Kd = 0.029, Km = 0.9/14, 1.5 mM||high glucose affinity|
|Hxt7||Vm = 11.7, Kd = 0.039, Km = 1.3, 1.9, 1.5 mM||high glucose affinity|
|Hxt8||low expression level|
|Hxt9||involved in pleiotropic drug resistance|
|Hxt11||involved in pleiotropic drug resistance|
|Gal2||Vm = 17.5, Kd = 0.043, Km = 1.5, 1.6||high galactose affinity|
Glucose transport in mammals
GLUTs are integral membrane proteins that 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, it seems, located in transmembrane segments 9, 10, 11; also, the DLS 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, 14 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||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. GLUT1 expression is upregulated in many tumors.|
|GLUT2||Is a bidirectional transporter, allowing glucose to flow in 2 directions. Is expressed by renal tubular cells, liver cells and pancreatic beta cells. It is also present in the basolateral membrane of the small intestine epithelium. Bidirectionality is required in liver cells to uptake glucose for glycolysis and glycogenesis, and release of glucose during gluconeogenesis. In pancreatic beta cells, free flowing glucose is required so that the intracellular environment of these cells can accurately gauge the serum glucose levels. All three monosaccharides (glucose, galactose, and fructose) are transported from the intestinal mucosal cell into the portal circulation by GLUT2.||Is a high-frequency 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, allowing it to transport even in times of low glucose concentrations.|
|GLUT4||Expressed in adipose tissues and striated muscle (skeletal muscle and cardiac muscle).||Is the insulin-regulated glucose transporter. Responsible for insulin-regulated glucose storage.|
|GLUT14||Expressed in testes||similarity to GLUT3 |
Class II comprises:
- GLUT5 (SLC2A5), a fructose transporter in enterocytes
- GLUT7 (SLC2A7), found in the small and large intestine, transporting glucose out of the endoplasmic reticulum
- GLUT9 - (SLC2A9)
- GLUT11 (SLC2A11)
Class III comprises:
- GLUT6 (SLC2A6),
- GLUT8 (SLC2A8),
- GLUT10 (SLC2A10),
- GLUT12 (SLC2A12), and
- GLUT13, also H+/myo-inositol transporter HMIT (SLC2A13), primarily expressed in brain.
The function of these new glucose transporter isoforms is still not clearly defined at present. Several of them (GLUT6, GLUT8) are made of motifs that help retain them intracellularly and therefore prevent glucose transport. Whether mechanisms exist to promote cell-surface translocation of these transporters is not ye known, but it has clearly been established that insulin does not promote GLUT6 and GLUT8 cell-surface translocation.
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. Crane's discovery of cotransport was the first ever proposal of flux coupling in biology. Crane in 1961 was the first to formulate the cotransport concept to explain active transport. Specifically, he proposed that the accumulation of glucose in the intestinal epithelium across the brush border membrane was [is] 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.
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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.