Glucose Transporters Summary

Glucose is a primary energy source for living cells.
Its transport across cell membranes is vital for metabolic homeostasis.
Glucose cannot passively diffuse through the cell membrane's lipid bilayer because it is a polar molecule.
Glucose transporters (GLUTs) and sodium-glucose cotransporters (SGLTs) facilitate glucose transport into and out of cells.

Only monosaccharides are absorbed by the intestine.
The absorption rate varies: galactose (maximum), glucose (moderate), fructose (minimum).
Glucose absorption relies on specific transmembrane protein transporters.

GLUT1
- Present in: RBCs, brain, kidney, colon, retina, placenta.
- Function: Glucose uptake in most cells.
GLUT2
- Present in: Serosal surface of intestinal cells, liver, beta cells of the pancreas.
- Function: Low affinity for glucose; responsible for glucose uptake in the liver and acts as a glucose sensor in beta cells.
GLUT3
- Present in: Neurons, brain.
- Function: High affinity for glucose, ensuring glucose entry into brain cells.
GLUT4
- Present in: Skeletal muscle, heart muscle, adipose tissue.
- Function: Insulin-mediated glucose uptake.
GLUT5
- Present in: Small intestine, testis, sperms, kidney.
- Function: Primarily a fructose transporter, with limited ability to transport glucose.
GLUT7
- Present in: Liver endoplasmic reticulum.
- Function: Transports glucose from the ER to the cytoplasm.
SGLT
- Present in: Intestine, kidney.
- Function: Cotransport of glucose and sodium from the lumen into the cell.

Sodium-Dependent Glucose Transporter-1 (SGLT-1) mediates the process.
Absorption from the intestinal lumen into intestinal cells occurs via a cotransport mechanism (secondary active transport).
A membrane-bound carrier protein transports glucose along with sodium.
Sodium is subsequently expelled by the sodium pump, which requires energy.

Intestinal epithelial cells release glucose into the bloodstream via Glucose Transporter Type 2 (GLUT2) on the membrane facing capillaries.
GLUT2 is not dependent on sodium; it is a uniport, facilitated diffusion system.

Glucose binds to the transporter on one side of the membrane, causing a conformational change.
This change closes the first binding site and exposes a new binding site on the inner side of the membrane, releasing glucose.

GLUT2 (facilitated transport) facilitates absorption of glucose from the bloodstream to cells.
It is present in intestinal epithelial cells, liver cells, beta cells of the pancreas, and kidney cells.

GLUT2 has a high $K_m$ for glucose.
Its presence in beta cells enables the sensing of high glucose levels and subsequent insulin release which allows the pancreas to monitor glucose levels and adjust insulin secretion accordingly.

Glucose transporters ensure a continuous glucose supply to cells.
This supply is important for ATP production via glycolysis, the Krebs cycle, and oxidative phosphorylation.
The brain primarily relies on glucose as an energy source, GLUT1 and GLUT3 ensure glucose supply across the blood-brain barrier and into neurons.
GLUT4 is crucial for insulin-mediated glucose uptake, translocating from intracellular vesicles to the cell membrane upon insulin signaling to enhance glucose entry into muscle and fat cells.
SGLT2 prevents glucose loss in urine by reabsorbing glucose in the proximal tubules of the kidney which is critical for maintaining blood glucose levels.

Oral rehydration fluid contains glucose and sodium.
The presence of sodium and glucose together allows for:
- Absorption of sodium to replenish body sodium chloride levels.
- Glucose to provide energy.

Insulin promotes the translocation of intracellular GLUT4 molecules to the cell surface, increasing glucose uptake.
In Type 2 diabetes mellitus, membrane GLUT4 is reduced, causing insulin resistance in muscle and fat cells, reducing glucose entry into muscle cells to half of the normal rate.
In type 1 diabetes, the absence of insulin prevents GLUT4 translocation, which leads to reduced glucose uptake in muscle and adipose tissue.
SGLT2 inhibitors (e.g., canagliflozin, dapagliflozin) are used as antidiabetic drugs to enhance glucose excretion in urine.

Mutations in the GLUT1 gene impair glucose transport across the blood-brain barrier.
This leads to seizures, developmental delays, and neurological impairments.

Many cancers exhibit increased GLUT1 expression to meet the high energy demands of rapidly proliferating cells.
This phenomenon, known as the Warburg effect, is a target for cancer therapies.

Dysregulated glucose transport in the brain has been implicated in Alzheimer’s and Parkinson’s disease.
Impaired GLUT function may contribute to neuronal energy deficits.

Glucose and galactose are absorbed by the same transporter, SGLT.
It is an energy-dependent process, against a concentration gradient which causes absorption to be almost complete from the intestine.
Other monosaccharides are absorbed by carrier-mediated facilitated transport, thus absorption is not complete, and the remaining molecules in the intestine will be fermented by bacteria.

Glucose is the preferred energy source for most body tissues, especially brain cells.
Deranged glucose metabolism can lead to life-threatening conditions.
A minimum amount of glucose is always required for normal functioning.
Normal fasting plasma glucose level is 70 to 110 mg/dL.
After a heavy carbohydrate meal, in a normal person, glucose level stays below 150 mg/dL.