A membrane transport protein (or simply transporter) is a membrane protein involved in the movement of ions, small molecules, or macromolecules, such as another protein, across a biological membrane. Transport proteins are integral transmembrane proteins; that is they exist permanently 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. These mechanisms of action are known as carrier-mediated transport.
Difference between protein channels and protein carriers
- Cavity in a carrier protein is not open simultaneously to the both environment (extracellular and intracellular). Either its inner gate is open, or outer gate is open, Carrier has binding sites, but porins and channel proteins have not. When a channel is opened, thousands to millions of ions can pass through the membrane in one time, but only one or a small amount of molecules can pass through a carrier molecule.
(Grouped by Transporter Classification database categories)
Facilitated diffusion occurs in and out of the cell membrane via channels/pores and carriers/porters.
Channels are either in open state or closed state. When a channel is opened with a slight conformational switch, it is open to both environment simultaneously (extracellular and intracellular)
Pores are continuously open to these both environment, because they do not undergo conformational changes. They are always open.
2: Electrochemical potential-driven transporters
3: Primary active transporters
- 3.A: P-P-bond-hydrolysis-driven transporters :
- 3.B: Decarboxylation-driven transporters
- 3.C: Methyltransfer-driven transporters
- 3.D: Oxidoreduction-driven transporters
- 3.E: Light absorption-driven transporters, such as rhodopsin
4: Group translocators
The group translocators provide a special mechanism for the phosphorylation of sugars as they are transported into bacteria (PEP group translocation)
5: Electron carriers
The transmembrane electron transfer carriers in the membrane include two-electron carriers, such as the disulfide bond oxidoreductases (DsbB and DsbD in E. coli) as well as one-electron carriers such as NADPH oxidase. Often these redox proteins are not considered transport proteins.
|F-, V-, and A-type ATPase (3.A.2)|
|P-type ATPase (3.A.3)|
- 3.A.3.1.4: H+/K+ transporting, nongastric: ATP12A