Influence of Halide Binding
on the Hydrogen Bonding
Network in the Active Site of <i>Salinibacter</i> Sensory
Rhodopsin I
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Abstract
In nature, organisms are subjected to a variety of environmental
stimuli to which they respond and adapt. They can show avoidance or
attractive behaviors away from or toward such stimuli in order to
survive in the various environments in which they live. One such stimuli
is light, to which, for example, the receptor sensory rhodopsin I
(SRI) has been found to respond by regulating both negative and positive
phototaxis in, e.g., the archaeon <i>Halobacterium salinarum</i>. Interestingly, to date, all organisms having SRI-like proteins
live in highly halophilic environments, suggesting that salt significantly
influences the properties of SRIs. Taking advantage of the discovery
of the highly stable SRI homologue from <i>Salinibacter ruber</i> (<i>Sr</i>SRI), which maintains its color even in the
absence of salt, the importance of the chloride ion for the color
tuning and for the slow M-decay, which is thought to be essential
for the phototaxis function of SRIs, has been reported previously
[Suzuki, D., et al. (2009) <i>J. Mol. Biol.</i> <i>392</i>, 48–62]. Here the effects of the anion binding
on the structure and structural changes of SRI during its photocycle
are investigated by means of Fourier transform infrared (FTIR) spectroscopy
and electrochemical experiments. Our results reveal that, among other
things, the structural change and proton movement of a characteristic
amino acid residue, Asp102 in <i>Sr</i>SRI, is suppressed
by the binding of an anion in its vicinity, both in the K- and M-intermediate.
The presence of this anion also effects the extent of chromophore
distrotion, and tentative results indicate an influence on the number
and/or properties of internal water molecules. In addition, a photoinduced
proton transfer could only be observed in the absence of the bound
anion. Possible proton movement pathways, including the residues Asp102
and the putative Cl binding site His131, are discussed. In conclusion,
the results show that the anion binding to SRI is not only important
for the color tuning, and for controlling the photocycle kinetics,
but also induces some structural changes which facilitate the observed
properties