NhaA (source code)
= NhaA
{c}
<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.<Proton-sodium stoichiometry of NhaA, an electrogenic antiporter from Escherichia coli by D. Taglicht and others>[Abstract: reconstitution, two stoichiometry methods and tested pH range]{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.<Structure of a Na+ H+ antiporter and insights into mechanism of action and regulation by pH by Carola Hunte and others>[Abstract; Figures2–3 and proposed mechanism]{ref}
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