uniporters

Uniporters are membrane proteins that transport a single substrate across a biological membrane. They function mainly by facilitated diffusion, moving their substrate down its concentration or electrochemical gradient without coupling to another molecule. This distinguishes them from symporters and antiporters, which move two or more substrates in a coordinated fashion.

The transport cycle of a uniporter is typically saturable and follows Michaelis-Menten–like kinetics. A substrate binds to the carrier on one side of the membrane, the protein undergoes a conformational change that exposes the binding site to the opposite membrane face, the substrate is released, and the carrier returns to its original state. Because the process depends on the substrate gradient, uniporters do not provide energy for transport.

In humans, uniporters are exemplified by the GLUT family of glucose transporters (SLC2), which mediate facilitated diffusion of glucose and related hexoses across the plasma membrane. Different isoforms show tissue-specific expression and kinetic properties, supporting processes such as basal glucose uptake in the brain and insulin-stimulated uptake in muscle and adipose tissue (for example, GLUT1, GLUT3, and GLUT4). While GLUTs are the best-known example, many other carriers exist that transport a single substrate, contributing to the cell’s ability to regulate intracellular concentrations of nutrients and metabolites.

Genetic or functional defects affecting uniporters can disrupt substrate homeostasis and contribute to disease. For instance, mutations in SLC2A1, the gene encoding GLUT1, can cause GLUT1 deficiency syndrome, characterized by impaired brain glucose transport and neurological symptoms.