 NhaA

NhaA is a sodium/proton antiporter exemplified by the Escherichia coli membrane protein. It couples the movement of sodium ions to proton movement in the opposite direction across a cell membrane, contributing to ion and pH regulation. The name of this particular protein should not be confused with every sodium/proton exchanger.

A 1993 experiment purified NhaA and placed it in sodium-loaded proteoliposomes: artificial membrane vesicles containing the transport protein. Parallel sodium-efflux and proton-influx measurements supported two protons per sodium ion over the tested pH range 7.2–8.1. A second determination used membrane electric potential difference and equilibrium gradients, under assumptions of coupled transport.^[ref]

For an illustrative cycle with two protons entering and one sodium ion leaving, the inward transfer of electric charge is $2e-e=e$, where $e$ is the elementary charge. Reversing the cycle reverses that transfer. Opposite transport directions therefore do not imply electrical neutrality, nor do they alone determine which direction is energetically favored; the electrochemical gradients matter.

A 2005 X-ray crystallography study supplied a different kind of evidence: a structure of downregulated NhaA, including a cytoplasm-facing funnel toward a putative ion-binding region. The authors proposed a mechanism in which access changes between the two sides. A static structure helps constrain such a mechanism but does not directly measure the transported ratio or film a working exchange cycle.^[ref]