Thermal Properties of Rhodopsin: INSIGHT INTO THE MOLECULAR MECHANISM OF DIM-LIGHT VISION

Jian Liu,Monica Yun Liu,Jennifer B Nguyen,Aditi Bhagat,Victoria Mooney,Elsa C.Y Yan

doi:10.1074/jbc.m111.233312

Abstract

Rhodopsin has developed mechanisms to optimize its sensitivity to light by suppressing dark noise and enhancing quantum yield. We propose that an intramolecular hydrogen-bonding network formed by ∼20 water molecules, the hydrophilic residues, and peptide backbones in the transmembrane region is essential to restrain thermal isomerization, the source of dark noise. We studied the thermal stability of rhodopsin at 55 °C with single point mutations (E181Q and S186A) that perturb the hydrogen-bonding network at the active site. We found that the rate of thermal isomerization increased by 1-2 orders of magnitude in the mutants. Our results illustrate the importance of the intact hydrogen-bonding network for dim-light detection, revealing the functional roles of water molecules in rhodopsin. We also show that thermal isomerization of 11-cis-retinal in solution can be catalyzed by wild-type opsin and that this catalytic property is not affected by the mutations. We characterize the catalytic effect and propose that it is due to steric interactions in the retinal-binding site and increases quantum yield by predetermining the trajectory of photoisomerization. Thus, our studies reveal a balancing act between dark noise and quantum yield, which have opposite effects on the thermal isomerization rate. The acquisition of the hydrogen-bonding network and the tuning of the steric interactions at the retinal-binding site are two important factors in the development of dim-light vision.

Highlights

Rhodopsin is a widely studied G protein-coupled receptor responsible for generating visual signals in dim-light environments [1,2,3,4,5,6,7,8,9,10,11]
We show that thermal isomerization of 11-cis-retinal in solution can be catalyzed by wild-type opsin and that this catalytic property is not affected by the mutations
Sample Preparation—UV-visible spectra of dark-state WT, E181Q, and S186A rhodopsins were recorded at room temperature following purification

Summary

Introduction

Rhodopsin is a widely studied G protein-coupled receptor responsible for generating visual signals in dim-light environments [1,2,3,4,5,6,7,8,9,10,11] It is expressed in the disc membranes in the outer segment of rod photoreceptor cells [1,2,3]. We asked what molecular properties allow rhodopsin to gain photosensitivity for dim-light vision; rhodopsin has the ability to handle intensities of light spanning orders of magnitude and yet the capacity to detect single photons [16, 17] These properties must be related to the optimization of amplification, quantum yield, and dark noise level, three important criteria for evaluating the sensitivity of any light detector, both man-made and biological. We aimed to link the steric interactions in the retinal-binding site to the ability of opsin to catalyze cisto-trans-isomerization of retinal to explain the high quantum yield of rhodopsin

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Conclusion