Antiporter

An antiporter is a membrane transport protein that moves two or more different substrates in opposite directions across a biological membrane. It is a form of secondary active transport because the movement of one substrate down its electrochemical gradient provides the energy to move the other substrate against its gradient. This contrasts with symporters, which move substrates in the same direction, and uniporters, which transport a single substrate without coupling.

Common examples include the Na+/H+ exchanger, which exports protons in exchange for extracellular sodium; the Na+/Ca2+ exchanger, which uses Na+ influx to remove Ca2+; and the Cl−/HCO3− exchangers, which swap chloride for bicarbonate. Antiporters can be electroneutral (net zero charge transfer when substrates have balanced charges) or electrogenic (net charge transfer across the membrane, producing current).

Mechanistically, most antiporters function via an alternating-access model. Substrates bind on one side of the membrane, a conformational change exposes binding sites to the opposite side, and release occurs, effectively exchanging substrates across the membrane. The transport cycle is coupled to the electrochemical gradient of the driving substrate, which is often established by primary active transporters elsewhere in the membrane.

Physiological roles include regulation of intracellular pH, cell volume, and ion homeostasis, particularly for Na+, H+, Ca2+, and Cl−. Antiporters contribute to acid-base balance in tissues such as kidney and intestine and participate in osmoregulation in microorganisms. Their activity can be regulated by pH, phosphorylation, and interaction with other proteins.

Malfunction or dysregulation of antiporters can disturb ion balance and pH homeostasis, contributing to physiological disorders. They are subjects of ongoing research in physiology, biophysics, and pharmacology, with interest in targeting them for therapeutic purposes.