Color Vision: “OH-Site” Rule for Seeing Red and Green

Sekharan, Sivakumar; Katayama, Kota; Kandori, Hideki; Morokuma, Keiji

doi:10.1021/ja304820p

Cited by 56 publications

(63 citation statements)

References 59 publications

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“…The calculations are based on the two-layer ONIOM (our own n-layered integrated molecular orbital and molecular mechanics) scheme, [33][34][35][36] which has been previously used for studying related retinal proteins. 8,[37][38][39][40][41][42][43] These calculations provide interesting insights into the reaction pathways, as will be discussed in a future publication. The QM/MM results presented here focus on the isomerization pathway for which the method of identifying the transition state has already been established.…”

Section: Discussionmentioning

confidence: 90%

Probing the remarkable thermal kinetics of visual rhodopsin with E181Q and S186A mutants

Guo

Hendrickson

Videla

et al. 2017

The Journal of Chemical Physics

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We recently reported a very unusual temperature dependence of the rate of thermal reaction of wild type bovine rhodopsin: the Arrhenius plot exhibits a sharp "elbow" at 47 °C and, in the upper temperature range, an unexpectedly large activation energy (114 ± 8 kcal/mol) and an enormous prefactor (10 s). In this report, we present new measurements and a theoretical model that establish convincingly that this behavior results from a collective, entropy-driven breakup of the rigid hydrogen bonding networks (HBNs) that hinder the reaction at lower temperatures. For E181Q and S186A, two rhodopsin mutants that disrupt the HBNs near the binding pocket of the 11-cis retinyl chromophore, we observe significant decreases in the activation energy (∼90 kcal/mol) and prefactor (∼10 s), consistent with the conclusion that the reaction rate is enhanced by breakup of the HBN. The results provide insights into the molecular mechanism of dim-light vision and eye diseases caused by inherited mutations in the rhodopsin gene that perturb the HBNs.

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Section: Discussionmentioning

confidence: 90%

Probing the remarkable thermal kinetics of visual rhodopsin with E181Q and S186A mutants

Guo

Hendrickson

Videla

et al. 2017

The Journal of Chemical Physics

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“…for studying related retinal proteins (21)(22)(23)(24)(25)(26)(27). The isomerization energy, i.e., the energy of the TS when all H-bonds are intact, was calculated to be 40 kcal/mol, which increases to 106 kcal/mol on disrupting the H-bonds in the TS.…”

Section: Resultsmentioning

confidence: 99%

Unusual kinetics of thermal decay of dim-light photoreceptors in vertebrate vision

Guo

Sekharan

Liu

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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We present measurements of rate constants for thermal-induced reactions of the 11-cis retinyl chromophore in vertebrate visual pigment rhodopsin, a process that produces noise and limits the sensitivity of vision in dim light. At temperatures of 52.0-64.6°C, the rate constants fit well to an Arrhenius straight line with, however, an unexpectedly large activation energy of 114 ± 8 kcal/mol, which is much larger than the 60-kcal/mol photoactivation energy at 500 nm. Moreover, we obtain an unprecedentedly large prefactor of 10 72±5 s −1 , which is roughly 60 orders of magnitude larger than typical frequencies of molecular motions! At lower temperatures, the measured Arrhenius parameters become more normal: E a = 22 ± 2 kcal/mol and A pref = 10 9±1 s −1 in the range of 37.0-44.5°C. We present a theoretical framework and supporting calculations that attribute this unusual temperature-dependent kinetics of rhodopsin to a lowering of the reaction barrier at higher temperatures due to entropy-driven partial breakup of the rigid hydrogen-bonding network that hinders the reaction at lower temperatures.non-Arrhenius | dim-light vision | transition state theory | isomerization rate R hodopsin is a vertebrate dim-light photoreceptor. Molecular studies of rhodopsin in recent decades have largely focused on its photochemistry and photoactivation (1-3). However, complete understanding of rhodopsin's function requires characterization of its thermal properties because thermal isomerization of the 11-cis retinyl chromophore can trigger the same physiological response as photo-isomerization, generating false visual signals as dark noise that jeopardizes photosensitivity (4-6). To enhance dim-light vision, rhodopsin has evolved to acquire remarkable thermal stability with a half-life of 420 y as determined by electrophysiological experiments using the outer segments of rod cells at 36°C (4). However, the molecular mechanism for the thermal stability has remained unclear. Here, we have addressed this by exploring the temperature dependence of thermal decay rate constants k(T) associated with isomerization of the retinyl chromophore and hydrolysis of the chromophore protonated Schiff-base (PSB) linkage, for temperatures ranging from the physiological temperature of 37.0°C to 64.6°C. In the upper range of temperatures from 52.0°C to 64.6°C, we find that the rate constants determined by UV-visible spectroscopy follow a linear Arrhenius model, k(T) = A pref exp(−E a /k B T), where k B is the Boltzmann constant (Fig. 1A). The slope, however, is very steep, giving an elevated activation energy, E a = 114 ± 8 kcal/mol. Surprisingly, this value is much higher than the photoactivation energy at visible wavelengths (60 kcal/mol at 500 nm). E a is also much higher than the reaction enthalpy change (32-35 kcal/mol) (7, 8) (Fig. 1B). In the lower temperature range of our rate constant measurements, 37.0-44.5°C, the slope of the Arrhenius plot decreases abruptly, as shown in Fig. 1A. Fitting a straight line through the low temperature points prod...

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“…Spectral analysis of the tuning induced by the protein environment has led to a debate about the nature of the most important contribution to the observed large shifts. While some work has identified the geometric effect (torsion of the retinal or change in the bond length alternation) as the major source for tuning [11], other work attributed it to the electrostatic effect of the counterion [5,6] or the sum of interactions from the binding pocket [10,12,13].…”

Section: Introductionmentioning

confidence: 99%

Counterion-controlled spectral tuning of the protonated Schiff-base retinal

et al. 2018

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Color vision is based on the ability of different Opsin proteins to tune the absorption band of their chromophore: the retinal protonated Schiff base (RPSB). Two main mechanisms proposed for this tunability are geometric and electrostatic. Here we probe the latter effect experimentally and by a quantum chemical calculation of the absorption by an isolated complex containing the retinal chromophore and molecules with a strong dipole moment. Betaine complexation causes an anomalously large blue shift. The shift provides direct evidence that the strong charge-transfer character of the electronic transition is the cause of the opsin shift, and shows that the electric field of the counterion is responsible for the color tuning which allows absorption of light in the blue region of the visible spectrum by opsin proteins.

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Color Vision: “OH-Site” Rule for Seeing Red and Green

Cited by 56 publications

References 59 publications

Probing the remarkable thermal kinetics of visual rhodopsin with E181Q and S186A mutants

Probing the remarkable thermal kinetics of visual rhodopsin with E181Q and S186A mutants

Unusual kinetics of thermal decay of dim-light photoreceptors in vertebrate vision

Counterion-controlled spectral tuning of the protonated Schiff-base retinal

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