Glucose transporters are a wide group of membrane proteins that facilitate the transport of glucose across the

plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...

, a process known as facilitated diffusion. Because glucose is a vital source of energy for all life, these transporters are present in all

phyla Phyla, the plural of ''phylum'', may refer to: * Phylum, a biological taxon between Kingdom and Class * by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another Phyl ...

. The GLUT or SLC2A family are a

protein family A protein family is a group of evolutionarily related proteins. In many cases, a protein family has a corresponding gene family, in which each gene encodes a corresponding protein with a 1:1 relationship. The term "protein family" should not be c ...

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 Catabolism () is the set of metabolic pathways that breaks down molecules into smaller units that are either oxidized to release energy or used in other anabolic reactions. Catabolism breaks down large molecules (such as polysaccharides, lipids, ...

. 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 Gluconeogenesis (GNG) is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrat ...

Glucose transport in yeast

In '' Saccharomyces cerevisiae'' glucose transport takes place through facilitated diffusion. The transport proteins are mainly from the Hxt family, but many other transporters have been identified.

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 In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. Th ...

side of the

. GLUT proteins transport glucose and related hexoses according to a model of alternate conformation, which predicts that the transporter exposes a single

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (locomotion), the surface over which an organism lo ...

binding site toward either the outside or the inside of the cell. Binding of glucose to one site provokes a

conformational change In biochemistry, a conformational change is a change in the shape of a macromolecule, often induced by environmental factors. A macromolecule is usually flexible and dynamic. Its shape can change in response to changes in its environment or oth ...

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.

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. 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

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

Classes II/III

Class II comprises: *

GLUT5 GLUT5 is a fructose transporter expressed on the apical border of enterocytes in the small intestine. GLUT5 allows for fructose to be transported from the intestinal lumen into the enterocyte by facilitated diffusion due to fructose's high concen ...

(), a

fructose Fructose, or fruit sugar, is a Ketose, ketonic monosaccharide, simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galacto ...

transporter in enterocytes *

GLUT7 Solute carrier family 2, facilitated glucose transporter member 7 also known as glucose transporter 7 (GLUT7) is a protein that in humans is encoded by the ''SLC2A7'' gene. SLC2A7 belongs to a family of transporters that catalyze the uptake of ...

(), found in the small and large intestine, transporting glucose out of the

endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...

* GLUT9 - () *

GLUT11 Solute carrier family 2, facilitated glucose transporter member 11 (SLC2A11) also known as glucose transporter type 10/11 (GLUT-10/11) is a protein that in humans is encoded by the SLC2A11 gene In biology, the word gene (from , ; "...Wilhel ...

() Class III comprises: *

GLUT6 Solute carrier family 2, facilitated glucose transporter member 6 is a protein that in humans is encoded by the ''SLC2A6'' gene. Function Hexose transport into mammalian cells is catalyzed by a family of membrane proteins, including SLC2A6, tha ...

(), *

GLUT8 GLUT8 also known as SLC2A8 is the eighth member of glucose transporter superfamily. It is characterized by the presence of two leucine residues in its N-terminal intracellular domain, which influences intracellular trafficking. Discovery GL ...

(), * GLUT10 (), *

GLUT12 Solute carrier family 2, facilitated glucose transporter member 12 is a protein that in humans is encoded by the ''SLC2A12'' gene. See also * Glucose transporter * Solute carrier family References Further reading

* * * * * * * * Solut ...

(), and *

GLUT13 Proton ''myo''-inositol cotransporter, also known as solute carrier family 2 member 13 is a protein that in humans is encoded by the ''SLC2A13'' gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to descri ...

, also H+/ ''myo''-inositol transporter HMIT (), primarily expressed in brain. 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 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 Robert Kellogg Crane (December 20, 1919 – October 31, 2010) was an American biochemist best known for his discovery of sodium–glucose cotransport. Biography Crane was born on December 20, 1919 in Palmyra, New Jersey, to Wilbur Fiske Crane, J ...

presented for the first time his discovery of the sodium-glucose cotransport as the mechanism for intestinal glucose absorption.

Crane Crane or cranes may refer to: Common meanings * Crane (bird), a large, long-necked bird * Crane (machine), industrial machinery for lifting ** Crane (rail), a crane suited for use on railroads People and fictional characters * Crane (surname) ...

's discovery of cotransport was the first ever proposal of flux coupling in biology.

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 scoupled to downhill Na+ transport cross the brush border. This hypothesis was rapidly tested, refined, and extended oencompass the active transport of a diverse range of molecules and ions into virtually every cell type.

References

External links

* {{Sodium-glucose transporter modulators Transport proteins Integral membrane proteins Solute carrier family