Intrinsic, apparent, and effective affinities of co- and countertransport systems

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Abstract
Solutions to kinetic schemes for the simple carrier, the countertransporter (antiport, exchanger), and the rapid equilibrium cases of the cotransporter (symport) and co-chemiporter (cation-dependent ATPase) are listed. A distinction is made between the intrinsic, apparent, and effective affinities of the transporters for their substrates. Effective pumping requires that the active transporter binds the pumped substrate, at high affinity, realized at the “whence side” (from which pumping takes place) and, at low affinity, at the “whither side” (to which pumping takes place). It is demonstrated how effective pumping might be achieved by appropriate design of the transporter or chemiporter in terms of the energies of the intrinsic binding sites, the energies of the conformation changes that the pump protein undergoes, the dissociation constant of the chemical reaction that drives the co-chemiport, and the order of binding of the cosubstrates, appropriate at different prevailing levels of the driving substrate.