Electrostatic Potential at the Retinal of Three Archaeal Rhodopsins: Implications for Their Different Absorption Spectra

ABSTRACT The color tuningmechanism of the rhodopsin protein family has been in the focus of research for decades. However, the structural basis of the tuning mechanism in general and of the absorption shift between rhodopsins in particular remains under discussion. It is clear that a major determinant for spectral shifts between different rhodopsins are electrostatic interactions between the chromophore retinal and the protein. Based on the Poisson-Boltzmann equation, we computed and compared the electrostatic potential at the retinal of three archaeal rhodopsins: bacteriorhodopsin (BR), halorhodopsin (HR), and sensory rhodopsin II (SRII) forwhichhigh-resolution structures are avail-able. These proteins are an excellent test case for understanding the spectral tuning of retinal. The absorption maxima of BR and HR are very similar, whereas the spectrum of SRII is considerably blue shifted—despite the structural similarity between these threeproteins. In agreementwith their absorp-tion maxima, we find that the electrostatic potential is similar in BR and HR, whereas significant differ-ences are seen for SRII. The decomposition of the electrostatic potential into contributions of indi-vidual residues, allowed us to identify seven resi-dues that are responsible for the differences in electrostatic potential between the proteins. Three of these residues are located in the retinal binding pocket and have in fact been shown to account for part of the absorption shift between BR and SRII by mutational studies. One residue is located close to the -ionone ring of retinal and the remaining three residues are more than 8 Å away from the retinal. These residues have not been discussed before, because they are, partly because of their location, no obvious candidates for the spectral shift among BR, HR, and SRII. However, their contribution to the differences in electrostatic potential is evident. The counterion of the Schiff base, which is fre-quently discussed to be involved in the spectral tuning, does not contribute to the dissimilarities between the electrostatic potentials. Proteins 2005