出典(authority):フリー百科事典『ウィキペディア(Wikipedia)』「2012/12/18 07:08:24」(JST)
Glucose transporters are a wide group of membrane proteins that facilitate the transport of glucose over a plasma membrane. 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.
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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 only produced in hepatocytes, in fasting conditions other tissues such as the intestines, muscles, brain and kidneys are able to produce glucose following activation of gluconeogenesis.
In Saccharomyces cerevisiae glucose transport takes place through facilitated diffusion.[1] The transport proteins are mainly from the Hxt family, but many other transporters have been identified.[2]
Name | Properties | Notes |
Snf3 | low-glucose sensor; repressed by glucose; low expression level; repressor of Hxt6 | |
Rgt2 | high-glucose sensor; low expression level | |
Hxt1 | Km: 100 mM,[3] 129 - 107 mM[1] | low-affinity glucose transporter; induced by high glucose level |
Hxt2 | Km = 1.5[1] - 10 mM[3] | high/intermediate-affinityglucose transporter; induced by low glucose level[3] |
Hxt3 | Vm = 18.5, Kd = 0.078, Km = 28.6/34.2[1] - 60 mM[3] | low-affinity glucose transporter[3] |
Hxt4 | Vm = 12.0, Kd = 0.049, Km = 6.2[1] | intermediate-affinity glucose transporter[3] |
Hxt5 | Km = 10 mM[4] | Moderate glucose affinity. Abundant during stationary phase, sporulation and low glucose conditions. Transcription repressed by glucose.[4] |
Hxt6 | Vm = 11.4, Kd = 0.029, Km = 0.9/14,[1] 1.5 mM[3] | high glucose affinity[3] |
Hxt7 | Vm = 11.7, Kd = 0.039, Km = 1.3, 1.9,[1] 1.5 mM[3] | high glucose affinity[3] |
Hxt8 | low expression level[3] | |
Hxt9 | involved in pleiotropic drug resistance[3] | |
Hxt11 | involved in pleiotropic drug resistance[3] | |
Gal2 | Vm = 17.5, Kd = 0.043, Km = 1.5, 1.6[1] | high galactose affinity[3] |
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,[5][6][7] 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;[8] also, the QLS motif located in the seventh transmembrane segment could be involved in the selection and affinity of transported substrate.[9][10]
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.[11] To date, 13 members of the GLUT/SLC2 have been identified.[12] 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.[13]
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 a bidirectional transporter, allowing glucose to flow in 2 directions. Is expressed by renal tubular cells, small intestinal epithelial cells, liver cells and pancreatic beta cells. Bidirectionality is required in liver cells to uptake glucose for glycolysis, 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-capacity and low-affinity isoform. There is some evidence that GLUT 1 and 3 are actually the functional transporters in beta cells. |
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 | Found in adipose tissues and striated muscle (skeletal muscle and cardiac muscle). | Is the insulin-regulated glucose transporter. Responsible for insulin-regulated glucose storage. |
Class II comprises:
Class III comprises:
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) 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 yet known, but it has clearly been established that insulin does not promote GLUT6 and GLUT8 cell-surface translocation.
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.[16] Crane's discovery of cotransport was the first ever proposal of flux coupling in biology.[17][18]
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リンク元 | 「グルコース輸送体」 |
関連記事 | 「GLUT」 |
GLUT | 発現部位 | 機能 |
GLUT1 | 赤血球、脳、筋肉、脂肪、その他。×肝臓 | |
GLUT2 | 肝臓、脾臓β細胞 | KM≒60mM、hexokinase IV(glucokinase)(50kDa) |
GLUT3 | 脳、神経 | グルコース需要が多い場所で発現 |
GLUT4 | 筋と脂肪組織のみ | インスリン→Vmax↑。KM=2-5mM、hexokinase II(100kDa) |
GLUT5 | 小腸上皮管腔側 | フルクトースの受動輸送 |
GLUT7 | 肝臓小胞体 | G6Pの脱リン酸化に関与(FB. 453) |
-GLUT
-GLUT
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