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pH-dependent transitions in xanthorhodopsin.

Eleonora S Imasheva¹, Sergei P Balashov, Jennifer M Wang

¹Department of Physiology and Biophysics, University of California, Irvine, California 92697, USA.

Photochemistry and Photobiology

|May 3, 2006

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Xanthorhodopsin (XR), a light-driven proton pump, shows unique pH-dependent properties, including a high Schiff base pKa and altered carotenoid antenna behavior. Its photocycle reveals distinct pKa values influencing recovery rates.

Area of Science:

Biochemistry
Microbiology
Spectroscopy

Background:

Xanthorhodopsin (XR) is a light-driven proton pump from Salinibacter ruber, sharing homology with bacteriorhodopsin (BR) and proteorhodopsin (PR).
Unlike BR and PR, XR possesses a light-harvesting carotenoid antenna, salinixanthin, in addition to retinal.

Purpose of the Study:

To investigate the pH-dependent properties of xanthorhodopsin (XR).
To characterize the influence of pH on XR's retinal Schiff base, carotenoid antenna, and photocycle.

Main Methods:

Spectroscopic analysis of XR across a range of pH values.
Examination of XR's photocycle and intermediate states under varying pH conditions.

Main Results:

XR exhibits a high retinal Schiff base pKa (> or =12.4), similar to BR.

Related Experiment Videos

Alkaline denaturation causes significant changes in the salinixanthin antenna spectrum.

The pKa of the Schiff base counterion in detergent-solubilized XR is approximately 6, and the photocycle recovery shows pKas of 6.0 and 9.3.

Conclusions:

XR's unique structure, including the carotenoid antenna, leads to distinct pH-dependent behaviors compared to BR and PR.
The study elucidates the roles of specific residues, like Glu94, in XR's proton pumping mechanism and photocycle dynamics.