Isorhodopsin: An Undervalued Visual Pigment Analog

Colorants, vol. 1, pp. 256–279

Abstract

Rhodopsin, the first visual pigment identified in the animal retina, was shown to be a photosensitive membrane protein containing covalently bound retinal in the 11-cis configuration, as a chromophore. Upon photoexcitation the chromophore isomerizes in femtoseconds to all-trans, which drives the protein into the active state. Soon thereafter, another geometric isomer—9-cis retinal—was also shown to stably incorporate into the binding pocket, generating a slightly blue-shifted photosensitive protein. This pigment, coined isorhodopsin, was less photosensitive, but could also reach the active state. However, 9-cis retinal was not detected as a chromophore in any of the many animal visual pigments studied, and isorhodopsin was passed over as an exotic and little-relevant rhodopsin analog. Consequently, few in-depth studies of its photochemistry and activation mechanism have been performed. In this review, we aim to illustrate that it is unfortunate that isorhodopsin has received little attention in the visual research and literature. Elementary differences in photoexcitation of rhodopsin and isorhodopsin have already been reported. Further in-depth studies of the photochemical properties and pathways of isorhodopsin would be quite enlightening for the initial steps in vision, as well as being beneficial for biotechnological applications of retinal proteins.

Authors 2

1. Willem J. DeGrip corresponding Aachen Leiden Institute of Chemistry Department of Biophysical Organic Chemistry

   Leiden University · Radboud University Medical Center · Radboud University Nijmegen · Radboud Institute for Molecular Life Sciences

   Affiliation as printed

   Leiden Institute of Chemistry, Department of Biophysical Organic Chemistry, Leiden University, 2300 RA Leiden, The Netherlands

   Radboud Institute for Molecular Life Sciences, Radboud University Medical Center, 6500 HB Nijmegen, The Netherlands

2. Johan Lugtenburg Aachen Leiden Institute of Chemistry Department of Biophysical Organic Chemistry

   Leiden University

   Affiliation as printed

   Leiden Institute of Chemistry, Department of Biophysical Organic Chemistry, Leiden University, 2300 RA Leiden, The Netherlands