Distortion and a Strong Hydrogen Bond in the Retinal Chromophore Enable Sodium-Ion Transport by the Sodium-Ion Pump KR2

We conducted a comprehensive time-resolved resonance Raman spectroscopy study of the structures of the retinal chromophore during the photocycle of the sodium-ion pump Krokinobacter rhodopsin 2 (KR2). We succeeded in determining the structure of the chromophore in the unphotolyzed state and in the K, L, M, and O intermediates, by overcoming the problem that only a small fraction of the M intermediate is accumulated in the KR2 photocycle. The Schiff base in the retinal chromophore forms a strong hydrogen bond in the unphotolyzed state and in the K, L, and O intermediates and is deprotonated in the M intermediate. Formation of this strong hydrogen bond facilitates deprotonation of the Schiff base, which is necessary for the sodium ion to move past the Schiff base. The polyene chain in the chromophore of KR2 is twisted in all of the states of the photocycle: the portion near the Schiff base is largely twisted in the unphotolyzed state and in the K intermediate, whereas the middle portion of the polyene chain becomes largely twisted in the L, M, and O intermediates. During the photocycle, the twisted structure of the polyene chain and strong hydrogen bond at the Schiff base are advantageous for transient relocation of the Schiff base proton. The obtained resonance Raman data clarified the unique structural features of the KR2 chromophore, which are not accessible by other methods.