Figure 6: Model for sodium ion and proton pumpings by KR2.

KR2 is considered to pump a sodium ion in marine environment, and proton is pumped in the absence of sodium ion or lithium ion. R109 and carboxylates in BC-loop at residues E90, E91, D98 and D102 constitute binding site of the sodium ion, while H30 is important for proton pump.