Figure 11.

(A) Cl⁻ transport pathway in HR and FR (NTQ rhodopsin). Cl⁻ binds near the Schiff base region, as is seen from the Cl⁻-dependent color change. The KR2 structure is used in which T is replaced by D116. (B) Na⁺ transport pathway in KR2 (NDQ rhodopsin). Na⁺ does not bind near the chromophore in the dark, while a light-induced structural alteration accompanies the uptake of Na⁺ upon formation of the O intermediate. This is the structure of KR2 (PDB: 3X3C, Kato et al., 2015).