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The alternating access model describes membrane transport by a carrier protein whose binding region is exposed successively to opposite sides of a cell membrane. A solute can bind on one side, remain associated while the carrier protein changes conformation, and be released on the other. Unlike an open ion channel, this mechanism need not provide a continuous aqueous passage across the cell membrane.^[ref]

The model explains a route, while thermodynamics determines the possible net direction. For an uncharged ideal dilute solute at uniform temperature, with equal standard chemical potentials on the two sides, the molar free energy change for inward transfer isHere $R$ is the molar gas constant. At $298.15\mathrm{K}$, accumulating that solute to ten times its outside concentration costs approximately $5.71\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$. Changing which side can reach a binding site does not supply that free energy. Sustained uphill transport requires coupling to another energetically favourable process; at equilibrium, detailed balance excludes a persistent directed pumping cycle.

Structural experiments help test how such access could change. The 2005 NhaA antiporter structure revealed a cytoplasmic funnel and an internal region implicated in ion binding in a pH-downregulated state. The authors proposed movements connecting access and regulation. A structure of one state constrains a transport mechanism, but is not a direct movie of its complete cycle.^[ref] The neutral-solute calculation above illustrates the energetic distinction; the full NhaA cycle also involves coupled charged ions.

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.^[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. Thuswhere $e$ 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.

Carola Hunte is a structural biologist and first author of a 2005 study of NhaA, the sodium/proton antiporter of Escherichia coli. The team determined a crystal structure in a state downregulated by pH, revealing a path from the cytoplasm toward a proposed ion-binding region within the cell membrane.^[ref]

The arrangement of short helices and charged groups motivated an alternating access model: changes in the transporter would expose the relevant region toward different sides of the membrane. The paper distinguished the observed structure from the proposed movements and pH-regulatory mechanism.^[ref]

That distinction connects two kinds of experiment. Transport measurements count movement of ions; a protein structure supplies candidate routes and interactions. Seeing a possible route in one conformation does not by itself measure transport rate or establish the complete cycle.

D. Taglicht is the first author of a 1993 measurement of the transport stoichiometry of NhaA, an antiporter from Escherichia coli. Purified NhaA was reconstituted into sodium-loaded proteoliposomes, allowing transport to be studied in a defined membrane preparation.^[ref]

Two approaches converged on two protons exchanged for each sodium ion over the tested pH interval $7.2$–$8.1$. One compared opposing ion fluxes; the other related the membrane potential to imposed concentration gradients under coupling and equilibrium assumptions.^[ref]

The ratio explains the word electrogenic: moving two singly positive ions one way and one singly positive ion the other transfers a net unit of positive charge per completed exchange cycle. It also separates a transporter's rate from its stoichiometry: faster exchange need not mean a different number of ions per cycle.

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]

A proteoliposome is a liposome containing one or more reconstituted membrane proteins in its lipid bilayer. It combines a controlled membrane boundary with a chosen molecular machine. A solution containing detergent-solubilized protein is not automatically a proteoliposome: a closed vesicle and a membrane-incorporated protein must actually be formed.

The artificial compartment is useful because material can be placed on one side of a lipid bilayer and transport measured across it. In an originating experiment on the NhaA sodium/proton antiporter, purified protein was reconstituted into sodium-loaded proteoliposomes. Parallel sodium and proton flux measurements, together with an independent membrane-potential equilibrium method, supported a stoichiometry of two protons per sodium ion over the tested pH range 7.2–8.1.^[ref]

For a bookkeeping example,100 sodium ions moved outward through a perfectly coupled $2{\mathrm{H}}^{+}/\mathrm{N}{\mathrm{a}}^{+}$ exchanger would accompany 200 protons moving inward. The net movement is 100 positive elementary charges inward, which explains the word electrogenic. These invented counts illustrate coupling, not a measured single-vesicle trace.

Interpreting a proteoliposome assay still requires knowing which way the membrane protein faces, whether the vesicles leak, and how much protein is incorporated. An otherwise matched empty liposome helps estimate membrane background. Reconstitution simplifies a biological system; it does not automatically eliminate every alternative source of the measured flux.