Anomalous pH Effect of Blue Proteorhodopsin.
Proteorhodopsin (PR) is a light-driven proton pump found in marine bacteria, and thousands of PRs are classified into blue-absorbing PR (B-PR; λmax ≈ 490 nm) and green-absorbing PR (G-PR; λmax ≈ 525 nm). In this report, we present conversion of B-PR into G-PR using anomalous pH effect. B-PR in LC1-200, marine γ-proteobacteria, absorbs 497 and 513 nm maximally at pH 7 and 4, respectively, whose pH titration was reversible (pKa = 4.8). When pH was lowered from 4, the λmax was further red-shifted (528 nm at pH 2). This is unusual because blue shift occurs by chloride binding in the case of bacteriorhodopsin. Surprisingly, when pH was increased from 2 to 7, the λmax of this B-PR was further red-shifted to 540 nm, indicating that green-absorbing PR (PR540) is created only by changing pH. The present study reports the conformational flexibility of microbial rhodopsins, leading to the switch of absorbing color by a simple pH change.
†Department of Frontier Materials, Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555, Japan.
Keisuke
K
Yamada
†Department of Frontier Materials, Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555, Japan.
Akira
A
Kawanabe
‡Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa-shi, Chiba 277-8564, Japan.
Susumu
S
Yoshizawa
‡Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa-shi, Chiba 277-8564, Japan.
Kentaro
K
Inoue
‡Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa-shi, Chiba 277-8564, Japan.
Kazuhiro
K
Kogure
†Department of Frontier Materials, Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555, Japan.
Hideki
H
Kandori
color tuning; counterion; pH titlation; retinal; rhodopsin