efflux

Efflux refers to the active transport of molecules out of a cell or organelle, mediated by membrane-embedded transporter proteins. This process helps maintain intracellular homeostasis, remove unwanted or toxic compounds, and influence the distribution and elimination of nutrients and drugs.

Most efflux systems are energy dependent. Primary active transport uses ATP hydrolysis, as in ATP-binding cassette (ABC) transporters. Secondary active transport relies on electrochemical ion gradients, such as proton or sodium gradients, and includes families like the major facilitator superfamily (MFS), resistance-nodulation-division (RND), and multidrug and toxic compound extrusion (MATE) transporters. Some efflux-like processes may involve facilitated diffusion or exchange mechanisms, but effective efflux generally requires energy input.

Major transporter families include the ABC transporters that hydrolyze ATP to move substrates, the MFS that couple substrate movement to ion gradients, the RND family prominent in Gram-negative bacteria, the MATE family that uses ions to drive export, and the SMR family composed of small, efficient pumps. The substrates of efflux pumps are diverse, ranging from nutrients to xenobiotics, drugs, and metabolic byproducts.

Biological roles are varied. In microbes and fungi, efflux pumps contribute to intrinsic and acquired drug resistance, solvent tolerance, and virulence. In human cells, efflux pumps regulate absorption, distribution, and excretion of xenobiotics and metabolites, affecting pharmacokinetics and drug disposition. Examples include P-glycoprotein (ABCB1) in human tissues, AcrB in Escherichia coli, and MexAB-OprM in Pseudomonas aeruginosa.

Clinical and environmental relevance centers on multidrug resistance and the ongoing study of efflux inhibitors to restore drug efficacy and to understand the role of pumps in physiology and disease.