Kinetic properties of NhaB, a Na⁺/H⁺ antiporter from Escherichia coli

NhaB, a Na⁺/H⁺ antiporter from Escherichia coli, was overproduced, purified, and reconstituted in a functional state, demonstrating that a single polypeptide, the product of the nhaB gene, can catalyze full activity. NhaB is a minor protein that accounts for less than 0.1% of the total membrane protein. The use of proteoliposomes made possible the determination of important kinetic and pharmacological properties in the absence of passive and mediated leaks. The activity of NhaB was found to have some pH dependence; the apparent K(m) for Na⁺ changes by 10-fold from 1.55 mM at pH 8.5 to 16.66 mM at pH 7.2, while the V(max) remains constant. It was demonstrated that NhaB is electrogenic and translocates more H⁺ than Na⁺ per cycle; the rate of sodium efflux from proteoliposomes was accelerated by a membrane potential, negative inside, and NhaB activity generated a membrane potential as monitored by two techniques. The stoichiometry of NhaB was estimated by a thermodynamic method in which the magnitude of Δψ generated by NhaB was measured at various Na⁺ gradients. A kinetic method, in which the electrophoretic movement of ⁸⁶Rb⁺ (in the presence of valinomycin) was monitored in parallel with measurements of NhaB-mediated ²²Na⁺ uptake, allowed us to determine a stoichiometry of 3H⁺/2Na⁺. The significance of the existence of two antiporters with different stoichiometries, NhaA and NhaB, active in the same cell, is discussed.