= Antiporter
An <antiporter> is a <membrane protein> that couples movement of different substances in opposite directions across a <cell membrane>. In an idealized exchange cycle, one substrate moves inward while another moves outward. The ratio transported, or <stoichiometry>, depends on the transporter; opposite directions do not imply equal numbers or zero net electric charge.
<NhaA> provides a measured example. In a 1993 study, purified <Escherichia coli> <NhaA> was reconstituted in sodium-loaded membrane vesicles. Parallel measurements of sodium efflux and <proton> influx supported exchange of two <protons> for each sodium ion. An independent method based on the generated membrane <electric potential difference> and ion gradients agreed under the stated coupling assumptions.<Proton-sodium stoichiometry of NhaA, an electrogenic antiporter from Escherichia coli by D. Taglicht and others>[Abstract: two independent stoichiometry measurements]{ref}
The charge accounting is revealing. In the tested direction, two inward singly positive ions and one outward singly positive ion produce one net positive charge moving inward per exchange cycle. Thus
$$
q_{\mathrm{net,in}}=2e-e=e,
$$
where $e$ is the magnitude of the <elementary charge>. An <antiporter> can therefore be electrogenic. Directional exchange, charge balance and the energy available from an <electrochemical gradient> must be assessed separately.
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