Membrane transport protein

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
 * Voltage-gated ion channel like, including potassium channels KcsA and KvAP, and inward-rectifier potassium ion channel Kirbac
 * Large-conductance mechanosensitive channel, MscL
 * Small-conductance mechanosensitive ion channel (MscS)
 * CorA metal ion transporters
 * Ligand-gated ion channel of neurotransmitter receptors (acetylcholine receptor)
 * Aquaporins
 * Chloride channels
 * Outer membrane auxiliary proteins (polysaccharide transporter)

2. Electrochemical Potential-driven transporters 3. Primary Active Transporters
 * Mitochondrial carrier proteins
 * Major Facilitator Superfamily (Glycerol-3-hosphate transporter, Lactose permease, and Multidrug transporter EmrD)
 * Resistance-nodulation-cell division (multidrug efflux transporter AcrB, see multidrug resistance)
 * Dicarboxylate/amino acid:cation symporter (proton glutamate symporter)
 * Monovalent cation/proton antiporter (Sodium/proton antiporter 1 NhaA)
 * Neurotransmitter sodium symporter
 * Ammonia transporters
 * Drug/Metabolite Transporter (small multidrug resistance transporter EmrE - the structures  are retracted as erroneous)
 * Light absorption-driven transporters:
 * Bacteriorhodopsin-like proteins including rhodopsin (see also opsin)
 * Bacterial photosynthetic reaction centres and photosystems I and II
 * Light harvesting complexes from bacteria and chloroplasts
 * Oxidoreduction-driven transporters
 * Transmembrane cytochrome b-like proteins : coenzyme Q - cytochrome c reductase (cytochrome bc1 ); cytochrome b6f complex; formate dehydrogenase, respiratory nitrate reductase; succinate - coenzyme Q reductase (fumarate reductase); and succinate dehydrogenase.  See electron transport chain.
 * Cytochrome c oxidases from bacteria and mitochondria
 * Electrochemical potential-driven transporters
 * Proton or sodium translocating F-type and V-type ATPases
 * P-P-bond hydrolysis-driven transporters
 * P-type calcium ATPase (five different conformations)
 * Calcium ATPase regulators phospholamban and sarcolipin
 * ABC transporters: BtuCD, multidrug transporter, and molybdate uptake transporter
 * General secretory pathway (Sec) translocon (preprotein translocase SecY)

4. Group Translocators

5. Transport Electron Carriers 8. Accessory Factors Involved in Transport
 * Disulfide bond formation protein B (DsbB)

More examples

 * Karyopherin
 * Mitochondrial membrane transport protein
 * Glucose transporter
 * ATP-binding cassette transporter genes
 * P-glycoprotein
 * CD98
 * CD36
 * Neurotransmitter transporters
 * Glutamate transporter
 * Norepinephrine transporter
 * Vesicular monoamine transporter
 * V-ATPase
 * Ion transporters
 * Na+/K+-ATPase
 * Plasma membrane Ca2+ ATPase
 * Proton pump
 * Hydrogen potassium ATPase
 * Sodium-chloride symporter