2. DEFINITION
The cells of the body use glucose for energy, and the
brain must have glucose to function.
Glucose attracts water, but because cell
membranes are made of lipids, or fats.
So, to pass from the cell membrane, glucose needs a
protein to carry it. These are the glucose transporters.
There are 13 of them, but first four are well
explained.
3. Types
13 members
On the basis of sequence similarities, the GLUT family has been divided into three
subclasses.
Class I
Class I comprises GLUT1-GLUT4
Class II:
GLUT5 (SLC2A5), a fructose transporter
GLUT7 - SLC2A7 - (SLC2A7), transporting glucose out of the endoplasmic reticulum.
GLUT9 – SLC2A9
GLUT11 (SLC2A11)
Class III comprises:
GLUT6 (SLC2A6),
GLUT8 (SLC2A8),
GLUT10 (SLC2A10),
GLUT12 (SLC2A12), and
the H+/myoinositol transporter HMIT (SLC2A13)
4. Name Distribution Notes
GLUT1
In the adult, it expressed at highest in erythrocytes,
& also in endothelial cells of blood-brain barrier.
Levels in cell membranes are increased
by reduced glucose levels and
decreased by increased glucose levels.
GLUT2
Is a Bielateral transporter allowing glucose to flow in
2 direction . Is expressed by renal tubular cells, small
intestinal epithelial cells, liver cells and pancreatic beta
cells.
High-capacity & low-affinty isoform.
The GLUT 1&3 are the functional
transporter in beta cells.
GLUT3 Expressed mostly in neurons and in the placenta.
high-affinity isoform, allowing it to
transport even in times of low glucose
concentrations.
GLUT4 Found in adipose tissues and striated muscle.
The insulin-regulated glucose
transporter. Responsible for insulin-
regulated glucose storage.
5. GLUT5 (SLC2A5)
GLUT5 exhibits no transport activity for glucose, but
mediates fructose transport.
Primarily expressed in the jejunal region.
In the small intestine, GLUT5 is localized to both the apical
and basolaterial membranes mediates fructose absorption.
6. GLUT6 (SLC2A6)
Is composed of 507 amino acids.
Its mRNA is predominantly expressed in the brain, spleen
and peripheral leucocytes.
No translocation of GLUT6 can be stimulated by insulin.
7. GLUT7 (SLC2A7)
Comprised of 524 amino acids.
It does not transport galactose & xylose
The tissue distribution of GLUT7 mRNA is the small
intestine, colon, testis and prostate .
8. GLUT8 (SLC2A8)
It is homology to GLUT 1-4, but is more closely related to
hexose transporters present in plants and bacteria .
Also differs from other GLUT’s at the C-terminal
cytoplasmic tail, which is quite short at 20 amino acids in
length.
9. GLUT9 (SLC2A9)
The major isoform GLUT9 is comprised of 540
amino acids.
It is expressed mainly in the kidney and liver.
10. GLUT10 (SLC2A10)
It is composed of 541 amino acids in humans.
Its mRNA is expressed at highest levels in the liver
and pancreas.
It’s deficiency has recently been found in arterial
tortuosity syndrome .
11. GLUT11 (SLC2A11)
These three isoforms are expressed in a tissue specific
manner.
GLUT11A is present in heart, skeletal muscle and kidney .
GLUT11B is present in placenta, adipose tissue, and kidney.
GLUT11C present in adipose tissue, heart, skeletal muscle
and pancreas .
12. GLUT12 (SLC2A12)
Same family and contains a number of similar
features to GLUT4.
In adult tissues, GLUT12 expression appears to be
restricted to insulin-sensitive skeletal muscle, heart
and fat.
13. SGLT1
The sodium-dependent glucose transporter expressed in
the small intestinal mucosa.
It is the small intestinal brush-border transporter .
Thus plays a major role in absorption of dietary D-glucose
and D-galactose from the intestinal lumen.
14. SGLT2 (SLC5A2)
SGLT2 is a low-affinity glucose transporter.
It is predominantly located in the S1 and S2 segments of
renal convoluted proximal tubules .
It is associated with autosomal recessive renal glycosuria in
human patients.
15. Clinical significance OF GLUT2
Defects in the SLC2A2 gene with a particular type
of glycogen storage disease called Fanconi-Bickel
syndrome.
GLUT2 preventing edema-induced stroke, coma.