Membrane transport protein

A membrane transport protein (or simply transporter) is a membrane protein^[1] 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.^[2]

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.

Types

Facilitated diffusion

(Grouped by Transporter Classification database categories)

1: Channels/pores

• α-helical protein channels such as voltage-gated ion channel (VIC), ligand-gated ion channels(LGICs)
• β-barrel porins such as aquaporin
• channel-forming toxins, including colicins, diphtheria toxin, and others
• Nonribosomally synthesized channels such as gramicidin
• Holins; which function in export of enzymes that digest bacterial cell walls in an early step of cell lysis.

Facilitated diffusion occurs in and out of the cell membrane via channels/pores and carriers/porters.

Note:

• Channels:

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:

Pores are continuously open to these both environment, because they do not undergo conformational changes. They are always open and active.

2: Electrochemical potential-driven transporters

• 2.A: Porters ( uniporters, symporters, antiporters)
  • Glucose transporter
  • Monoamine transporters, including:
    • Dopamine transporter (DAT)
    • Norepinephrine transporter (NET)
    • Serotonin transporter (SERT)
    • Vesicular monoamine transporters (VMAT)
  • Adenine nucleotide translocator (ANT)
• 2.B: Nonribosomally synthesized porters, such as
  • The Nigericin (Nigericin) Family
  • The Ionomycin (Ionomycin) Family
• 2.C: Ion-gradient-driven energizers

3: Primary active transporters

• 3.A: P-P-bond-hydrolysis-driven transporters :
  • ATP-binding cassette transporter (ABC transporter), such as MDR, CFTR
  • V-type ATPase ; ( "V" related to vacuolar ).
  • P-type ATPase ; ( "P" related to phosphorylation), such as :
    • Na⁺/K⁺-ATPase
    • Plasma membrane Ca²⁺ ATPase
    • Proton pump
  • F-type ATPase; ("F" related to factor), including: mitochondrial ATP synthase, chloroplast ATP synthase1
• 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.

See also

• Carrier protein
• Cotransport
• Cotransporter
• Ion channel
• P-loop
• Solute carrier family (classification)
• TC number (classification)
• Vesicular transport protein

References

External links

• "Transport protein" at Dorland's Medical Dictionary

• DDI Regulatory Guidance Request a guide to drug-drug interaction regulatory recommendations.