FIG. 9.

Proposed antiport activities of GerN. Two modes of GerN-mediated antiport are shown. (Left) GerN-mediated Na⁺/H⁺ antiport. The assignment of a stoichiometry of 2H⁺ entering in exchange for 1 Na⁺ effluxing is completely hypothetical and represents the apparent electrogenicity of the antiport, i.e., the number of total coupling ions translocated per turnover is greater than the number of effluxing Na⁺ ions, so that a net positive charge moves inward. The geometric figures surrounding the two coupling ions suggest that these ions have distinct binding sites. The pH profile of Na⁺/H⁺ antiport suggests that protons compete with Na⁺ on the cytoplasmic side of the membrane. (Right) Na⁺/H⁺-K⁺ antiport by GerN is supported by the the finding of GerN-mediated and Na⁺-dependent Rb⁺ (K⁺) translocation with K⁺ replacing some, but not all, of the H⁺ ions that are transported in antiport with Na⁺. In the diagram, K⁺ is shown, hypothetically, as able to compete with H⁺ at only one of the H⁺ binding sites, and the antiport is shown as still requiring the full complement of coupling ions in this mode. GerN-mediated Na⁺/H⁺-K⁺ antiport is much more rapid than Na⁺/H⁺ antiport; K⁺ increases the velocity of the antiport without affecting the K_m for Na⁺.