Membrane transport protein

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Overview

A membrane transport protein (or simply transporter) is a protein involved in the movement of ions, small molecules, or macromolecules, such as another protein across a biological membrane. Transport proteins are integral membrane proteins; that is they exist within and span the membrane across which they transport substances. The proteins may assist in the movement of substances by facilitated diffusion or active transport.

Facilitated diffusion

A facilitated diffusion protein speeds the movement of a chemical through a membrane in the absence of energy input; therefore, the transported chemical can only move down a concentration gradient. This can be accomplished by the formation of a high-specificity pore or channel that spans the membrane. These polar "holes" through the membrane are lined by specific amino acids residues which lower the energy barrier to the movement of polar molecules.

Active transport

Transport proteins are also used in active transport, which by definition does require an energy input.

Chemiosmotic transport utilizes electrochemical gradients to drive transport. As the creation and maintenance of chemiosmotic gradients require energy input from the cell, this is a form of active transport. Prokaryotes typically use hydrogen ions as the driving force for chemiosmotic transport, while eukaryotes typically use sodium ions. A symporter/coporter transports a chemical in the same direction as the electrochemical gradient, while an antiporter moves the target chemical in a direction opposite to the gradient.

The uniporter is also often included as a category of chemiosmotic transporter, although a uniporter can also be considered a facilitated diffusion protein on the basis of function.

Binding dependent active transport

Binding dependent active transport also moves the targeted chemical against a concentration gradient, but uses stored chemical energy, typically in the form of adenosine triphosphate, to power the transport. Generally speaking, a binding dependent transport system consists of a membrane spanning component with a high degree of specifity. The membrane spanning component changes configuration with the aid of chemical energy input (often through the use of an associated ATPase protein), thus translocating the chemical from one side of the membrane to the other.

By some definitions, proteins that catalyze the ligation of phosphate or coenzyme groups to a catabolized chemical can be considered active transport proteins in that they drive the uptake of a chemical by maintaining a steep functional concentration gradient. This pheonomenon is termed group translocation in the case of sugar phosphorylation and vectoral acylation or vectoral esterification in the case of fatty acid coenzyme A ligation.

Classification and examples

Classification of transmembrane transporters according to TCDB and examples of transporters with known 3D structure:

1. Channels/Pores

2. Electrochemical Potential-driven transporters

    • Mitochondrial carrier proteins [9]
    • Major Facilitator Superfamily (Glycerol-3-hosphate transporter, Lactose permease, and Multidrug transporter EmrD) [10]
    • Resistance-nodulation-cell division (multidrug efflux transporter AcrB, see multidrug resistance)[11]
    • Dicarboxylate/amino acid:cation symporter (proton glutamate symporter) [12]
    • Monovalent cation/proton antiporter (Sodium/proton antiporter 1 NhaA) [13]
    • Neurotransmitter sodium symporter [14]
    • Ammonia transporters [15]
    • Drug/Metabolite Transporter (small multidrug resistance transporter EmrE - the structures are retracted as erroneous) [16]

3. Primary Active Transporters

4. Group Translocators

5. Transport Electron Carriers

    • Disulfide bond formation protein B (DsbB) [26]

8. Accessory Factors Involved in Transport

More examples

See also

External links


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