Figure 5.

Scheme illustrating properties of the NqrB-D397N mutant compared to those of the wild-type enzyme. In red are shown the acidic residues involved in Na⁺ uptake. In the wild-type enzyme (top row, left to right), (i) electron flow is coupled to Na⁺ pumping, K⁺ binds to a different, allosteric site (bottom left corner) and accelerates the reaction (ii) in the absence of Na⁺, and electron flow is inhibited, confirming that the redox and pumping processes are coupled. In the NqrB-D397N mutant (bottom row, left to right), (iii) in the absence of K⁺, electron flow is still coupled to Na⁺ pumping; (iv) in the presence of K⁺ (in the absence of Na⁺), electron flow proceeds, but without ion pumping, meaning that coupling has been disrupted; and (v) K⁺ causes the mutant to run uncoupled, even in the presence of Na⁺. Of the acidic residues involved in cation uptake, shown in bold, only NqrB-D397 is associated with the modified binding site.