jp9b00928_si_001.pdf (1.32 MB)
Distortion and a Strong Hydrogen Bond in the Retinal Chromophore Enable Sodium-Ion Transport by the Sodium-Ion Pump KR2
journal contribution
posted on 2019-04-04, 00:00 authored by Nao Nishimura, Misao Mizuno, Hideki Kandori, Yasuhisa MizutaniWe 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.