Abstract
Anabaena sensory rhodopsin (ASR) is an archaeal-type rhodopsin found in eubacteria. The gene encoding ASR forms a single operon with ASRT (ASR transducer) which is a 14 kDa soluble protein, suggesting that ASR functions as a photochromic sensor by activating the soluble transducer. This article reviews the detailed photoreaction processes of ASR, which were studied by low-temperature Fourier-transform infrared (FTIR) and UV-visible spectroscopy. The former research reveals that the retinal isomerization is similar to bacteriorhodopsin (BR), but the hydrogen-bonding network around the Schiff base and cytoplasmic region is different. The latter study shows the stable photoproduct of the all-trans form is 100% 13-cis, and that of the 13-cis form is 100% all-trans. These results suggest that the structural changes of ASR in the cytoplasmic domain play important roles in the activation of the transducer protein, and photochromic reaction is optimized for its sensor function.
Highlights
Photosynthesis is one of the most important chemical reactions in living cells because almost all energy spent by living things on Earth originates from it
The N-D stretching of the Schiff base was observed at 2,163(−) and 2,125(−) cm-1, while the O-D stretchings of water molecules were observed in the region >2,500 cm-1. These results indicate that the protonated Schiff base forms a strong hydrogen bond with a water molecule, which is connected to Asp75 with a weak hydrogen bond
On the basis of our Fourier-transform infrared (FTIR) studies of BR mutants and other rhodopsins, we have found an interesting correlation between strongly hydrogen bonded water molecules and proton pump activity
Summary
Photosynthesis is one of the most important chemical reactions in living cells because almost all energy spent by living things on Earth originates from it. Four archaeal type rhodopsins [Bacteriorhodopsin (BR), Halorhodopsin (HR), Sensory rhodopsin I (SRI), and Sensory rhodopsin II (SRII); called phoborhodopsin (pR)] were discovered in the cytoplasmic membrane of Halobacterium salinarum [1,2,3,4] The former two (BR and HR) function as light-driven proton and chloride pumps, respectively, while the latter two (SRI and SRII) are responsible for attractive and repellent phototaxis, respectively (Figure 1). ASR accommodates both all-trans and 13-cis retinal in the ground state, which can be interconverted by illumination with blue (480 nm) or orange (590 nm) light (Figure 3) [16] Such photochromic behavior has never been observed in other archaeal rhodopsins such as BR, HR, SRI and SRII, being characteristic to ASR.
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