Structural Biochemistry/Membrane Proteins/Uniporters

Uniporters
Uniporters are able to transport a specific species like ion channels in either direction governed only by concentrations of that species on either side of the membrane. Noted as either a channel or carrier protein, uniporter is an integral membrane protein involved in facilitated diffusion.

Uniporter carrier proteins bind to one molecule of solute at a time and transport it with the solute gradient. Uniporter channels open in response to a stimulus and allow the free flow of specific molecules. Since uniporters may not utilize energy other than the solute gradient, they may only transport molecules along with the solute gradient, and not against it. It is good to note that with uniporters the rate of movement is much higher than passive diffusion because the molecule never comes in contact with hydrophobic core of the membrane.



By definition, a uniporter is a channel protein that transfers only a single substrate at a time through the membrane. Multiple uniporters are found in the plasma membrane of most cells, where they allow small molecules such as amino acids, nucleosides, and sugars to enter and exit the cells down their concentration gradients. The movement often known as facilitated diffusion is when a substance moves through a membrane down its concentration gradient, and has the same negative ΔG value regardless of whether a protein transporter is included. This transport is fueled by the potential energy of a concentration gradient, and therefore does not need the assistance of metabolic energy.

Uniporters can be better understood by comparing and contrasting them to enzymes. They are similar to enzymes because they accelerate a reaction that is already thermodynamically favored. On the other hand, they are different from enzymes because the transported substance experience no chemical change when traveling through a membrane.

Uniporters are classified by two categories: channels and carriers. These two categories determine the kind of transmembrane solute transfers by different mechanisms. Channel proteins have hydrophilic pores that expand the lipid bilayer and are used to transport water-soluble solutes from both sides of the membrane at the same time. Carriers also expand the lipid bilayer, but unlike the channel proteins, their substrate binding locations cannot be simultaneously used on both sides of the membrane. Due to this mechanism difference, carriers are able to mediate counter-transports.

Regulation of Uniporter Channels
There are several ways in which the opening of uniporter channels may be regulated:

Voltage - Regulated by the difference in voltage across the membrane

Stress - Regulated by physical pressure on the transporter

Ligand - Regulated by the binding of a ligand to either the intracellular or extracellular side of the cell

Examples
Involved in many biological processes, Uniporters use voltage-gated sodium channels in the propagation of nerve impulse across neurons. As the signal is transmitted from one neuron to the next, calcium is transported into the presynaptic neuron by voltage-gated calcium channels. The presynaptic neuron calcium is bound to a ligand-gated calcium channel in the postsynaptic neuron to stimulate an impulse within the neuron.

Potassium leak channels are also regulated by voltage and help restore the resting membrane potential after impulse transmission.

Sound waves in the ear cause stress-regulated channels in the ear to open vestibulocochlear nerve by sending impulses.

GLUT1 – widely distributed glucose transporter Glucose uniporter shuttles between two conformational states. The net flow is reversed when the concentration of glucose changes.

GLUT4- primary insulin regulated glucose transporter in muscle and adipose tissue

UCP - uncoupler of proton gradient in mitochondria