Uniporter

Uniporter is a membrane transport protein that carries a single substrate across a biological membrane by facilitated diffusion, moving down its electrochemical gradient without direct energy input. Uniporters are a subset of carrier proteins that bind a substrate on one side, undergo a conformational change, and release it on the opposite side. Because movement relies on the substrate gradient, uniporter activity is bidirectional and saturable, characterized by kinetic parameters such as Km and Vmax.

Glucose transporters of the GLUT/SLC2 family are the best-known examples of uniporters. GLUT1 is widely expressed and supports basal glucose uptake; GLUT3 in neurons; GLUT4 in adipose and muscle is insulin-responsive. Other uniporters transport fructose, galactose, amino acids, or nucleosides, depending on substrate specificity. In many cases, transport is bidirectional, equilibrating intracellular and extracellular concentrations when gradients permit.

Uniporters differ from channels, which provide a continuous pore, and from cotransporters (symporters) and exchangers (antiporters), which move substrates together with or in exchange for other ions or molecules and may consume energy indirectly. Clinically, defects in uniporter genes can disrupt metabolite uptake; for example, mutations in SLC2A1 encoding GLUT1 cause GLUT1 deficiency syndrome, with neurological symptoms due to reduced cerebral glucose transport.

Research uses kinetic models such as alternating-access to describe their function, and some therapeutics seek to modulate uniporter activity to alter substrate availability in tissues.