Cotransporter

A cotransporter is a membrane protein that couples the movement of different solutes in a transport cycle. In common same-direction cotransport, a symporter moves the coupled species toward the same side; an antiporter exchanges species moving in opposite directions. Coupling lets a favorable electrochemical gradient for one solute help drive another against its own gradient, providing secondary active transport.

NKCC2 illustrates same-direction coupling: one sodium ion, one potassium ion and two chloride ions move together in the kidney transport system. The net transported charge per cycle is $1+1-2=0$. Electrically silent transport can therefore involve substantial ion movement; zero net charge is not zero flux.^[ref]

For an idealized one-to-one symporter carrying a monovalent cation $A$ and a neutral solute $B$ inward,Here $a$ denotes chemical activity and $\psi$ electric potential. With zero electric potential difference, an activity ratio of $0.01$ for $A$ and $10$ for $B$ gives a logarithm argument $0.1$: inward cotransport is thermodynamically favorable even though $B$ is more concentrated inside. This example assumes the stated coupling and stoichiometry; it is not an assignment of that stoichiometry to every transporter.

Cloning followed by expression in frog oocytes helped identify the kidney transporters experimentally. Comparing transport, ion dependence and inhibition distinguished related sequences with different functions. A cotransporter is thus defined by coupled movement, rather than merely by sequence resemblance to another carrier.^[ref]