FIG. 12.

Circulation of H⁺, Na⁺, and K⁺ in bacteria. (A) E. coli. The proton current is generated by the electrogenic extrusion of protons through the respiratory chain. Protons return to the cytosol by one primary system, F_OF₁-ATP synthase, and several secondary systems. Among these are Trk (K⁺ uptake), Nha Na⁺/H⁺ antiporters (Na⁺ extrusion), the flagellar motor, and various proton-linked metabolite porters. Some porters are coupled to the Na⁺ circulation; the K⁺ transport systems Kup and Kef are not shown. (B) Alkaliphilic bacteria. Ion movements are similar to those of E. coli. However, flagellar motion and many secondary-transport systems are energized by Na⁺ circulation; K⁺ transport has been little studied. (C) E. hirae. At acidic pH, the proton current is generated by the F_OF₁-ATPase and completed by several secondary pathways that perform useful work: KtrI (K⁺ uptake), NapA (Na⁺ extrusion), the flagellar motor, and various proton-linked transport systems. At alkaline pH, there is no obvious proton circulation. Ion transport relies on ATP-driven systems including the vacuolar Na⁺-ATPase, Kep (K⁺/H⁺ exchange), and K⁺ uptake via KtrII.