Biology: Glucose Transport Mechanism

Study Notes

Glucose is transported against its concentration gradient, from low extracellular concentrations to higher intracellular concentrations, while Na+ is transported down its electrochemical gradient.
This process requires the concurrent uptake of Na+, making the transporter a sodium-dependent glucose cotransporter (SGLT).

Glycolysis is a metabolic pathway that breaks down glucose to provide energy (in the form of ATP) and intermediates for other metabolic pathways.
It is employed by all tissues and is at the hub of carbohydrate metabolism, as virtually all sugars can be converted to glucose.

The fate of glucose varies with physiological conditions, tissues, and organisms.
Exercising muscle cells, for example, use glucose for energy production.

Aerobic glycolysis occurs in cells with mitochondria and an adequate supply of oxygen.
It produces pyruvate as the end product, which is then converted to acetyl CoA for the citric acid cycle.

Phosphorylation of glucose is an irreversible process that traps the sugar in the cell, committing it to further metabolism.
Hexokinase is the enzyme that catalyzes the phosphorylation of glucose to glucose 6-phosphate.

There are four isozymes of hexokinase: hexokinases I-III and hexokinase IV (also known as glucokinase).
Hexokinases I-III have low Michaelis constant (Km) and low maximal velocity (Vmax), allowing for efficient phosphorylation of glucose even at low tissue concentrations.
Hexokinase IV has a high Km and high Vmax, with low affinity for glucose, and is found in liver parenchymal cells and pancreatic β cells.

There are two ways to transport glucose into cells: a sodium (Na+) and ATP-independent transport system, and a (Na+) and ATP-dependent cotransport system.
The sodium-independent system is mediated by a family of at least fourteen glucose transporters (GLUT-1 to GLUT-14) in cell membranes.
The sodium-dependent system involves the cotransport of glucose with sodium and is found in the epithelial cells of the intestine, the renal tubules, and the choroid plexus.

Glucose transporters display a tissue-specific pattern of expression, with different isoforms found in different tissues.
For example, GLUT-3 is the primary glucose transporter in neurons, while GLUT-1 is abundant in erythrocytes and brain, but low in adult muscle.