Membrane transport protein

A membrane transport protein (or simply transporter) is a 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 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 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 facilitated diffusion protein on the basis of function.

Binding dependent active transport

Binding dependent active transport also moves the targeted chemical against a ATPase protein), thus translocating the chemical from one side of the membrane to the other.

By some definitions, proteins that coenzyme A ligation.

Classification and examples

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

1. Channels/Pores

- inward-rectifier potassium ion channel Kirbac [1]
- Large-conductance mechanosensitive channel, MscL [2]
- Small-conductance mechanosensitive ion channel (MscS) [3]
- CorA metal ion transporters [4]
- acetylcholine receptor) [5]
- Aquaporins [6]
- Chloride channels [7]
- Outer membrane auxiliary proteins (polysaccharide transporter) [8]

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 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

Light absorption-driven transporters:
- opsin)[17]
- Bacterial photosystems I and II [18]
- Light harvesting complexes from bacteria and chloroplasts [19]
Oxidoreduction-driven transporters
- Transmembrane cytochrome b-like proteins [20]: electron transport chain.
- mitochondria
Electrochemical potential-driven transporters
- Proton or sodium translocating F-type and V-type ATPases [22]
P-P-bond hydrolysis-driven transporters
- P-type calcium ATPase (five different conformations) [23]
- Calcium ATPase regulators phospholamban and sarcolipin[24]
- ABC transporters: BtuCD, multidrug transporter, and molybdate uptake transporter
- General secretory pathway (Sec) translocon (preprotein translocase SecY) [25]

4. Group Translocators

5. Transport Electron Carriers

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

8. Accessory Factors Involved in Transport

More examples

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