Isomerization Of Rhodopsin Research Articles

A Simplified Model of Activation and Deactivation of Human Rod Phototransduction-An Electroretinographic Study.

To test the hypothesis that a simple model having properties consistent with activation and deactivation in the rod approximates the whole time course of the photoresponse. Routinely, an exponential of the form f = α·(1 - exp(-(τ·(t - teff)s-1))), with amplitude α, rate constant τ (often scaled by intensity), irreducible delay teff, and time exponent s-1, is fit to the early period of the flash electroretinogram. Notably, s (an integer) represents the three integrating stages in the rod amplification cascade (rhodopsin isomerization, transducin activation, and cGMP hydrolysis). The time course of the photoresponse to a 0.17 cd·s·m-2 conditioning flash (CF) was determined in 21 healthy eyes by presenting the CF plus a bright probe flash (PF) in tandem, separated by interstimulus intervals (ISIs) of 0.01 to 1.4 seconds, and calculating the proportion of the PF a-wave suppressed by the CF at each ISI. To test if similar kinetics describe deactivation, difference of exponential (DoE) functions with common α and teff parameters, respective rate constants for the initiation (I) and quenching (Q) phases of the response, and specified values of s (sI, sQ), were compared to the photoresponse time course. As hypothesized, the optimal values of sI and sQ were 3 and 2, respectively. Mean ± SD α was 0.80 ± 0.066, I was 7700 ± 2400 m2·cd-1·s-3, and Q was 1.4 ± 0.47 s-1. Overall, r2 was 0.93. A method, including a DoE model with just three free parameters (α, I, Q), that robustly captures the magnitude and time-constants of the complete rod response, was produced. Only two steps integrate to quench the rod photoresponse.

Quantum and Quantum-Classical Studies of the Photoisomerization of a Retinal Chromophore Model.

We report an in-depth analysis of the photo-induced isomerization of the 2-cis-penta-2,4-dieniminium cation: a minimal model of the 11-cis retinal protonated Schiff base chromophore of the dim-light photoreceptor rhodopsin. Based on recently developed three-dimensional potentials parametrized on ab initio multi-state multi-configurational second-order perturbation theory data, we perform quantum-dynamical studies. In addition, simulations based on various quantum-classical methods, among which Tully surface hopping and the coupled-trajectory approach derived from the exact factorization, allow us to validate their performance against vibronic wavepacket propagation and, therefore, a purely quantum treatment. Quantum-dynamics results uncover qualitative differences with respect to the two-dimensional Hahn-Stock potentials, widely used as model potentials for the isomerization of the same chromophore, due to the increased dimensionality and three-mode correlation. Quantum-classical simulations show, instead, that three-dimensional model potentials are capable of capturing a number of features revealed by atomistic simulations and experimental observations. In particular, a recently reported vibrational phase relationship between double-bond torsion and hydrogen-out-of-plane modes critical for rhodopsin isomerization efficiency is correctly reproduced.

Photosensitized Retinal Isomerization in Rhodopsin Mediated by a Triplet State

AbstractDirect 11‐cis to all‐trans retinal photoisomerization within rhodopsin is well known to be the initial chemical reaction triggering the process of vision in mammalians, such as bovine. Nevertheless, deep‐sea fish are known to use chlorophyll derivatives as photosensitizers in order to see deep‐red light, at wavelengths where retinal does not absorb. Also, some photodynamic therapy treatments were shown to enhance the vision of patients in dim light conditions. Energy transfer from the photosensitizer to rhodopsin was therefore proposed as a mechanism to populate the triplet state of the retinal chromophore. Herein, by means of hybrid quantum mechanics‐coupled‐molecular mechanics modeling techniques, we give insights into the possible energy mechanism and describe the retinal isomerization mediated by the lowest‐lying triplet state. Especially, we show how a few kcal/mol energy barrier separates a T1 minimum from a S0/T1 intersection region, hence proposing an equilibrium between phosphorescence and isomerization processes. Moreover, the eventual self‐production of singlet oxygen, constituting a potential danger for the integrity of rhodopsin, is discussed.

Engineering the vibrational coherence of vision into a synthetic molecular device

The light-induced double-bond isomerization of the visual pigment rhodopsin operates a molecular-level optomechanical energy transduction, which triggers a crucial protein structure change. In fact, rhodopsin isomerization occurs according to a unique, ultrafast mechanism that preserves mode-specific vibrational coherence all the way from the reactant excited state to the primary photoproduct ground state. The engineering of such an energy-funnelling function in synthetic compounds would pave the way towards biomimetic molecular machines capable of achieving optimum light-to-mechanical energy conversion. Here we use resonance and off-resonance vibrational coherence spectroscopy to demonstrate that a rhodopsin-like isomerization operates in a biomimetic molecular switch in solution. Furthermore, by using quantum chemical simulations, we show why the observed coherent nuclear motion critically depends on minor chemical modifications capable to induce specific geometric and electronic effects. This finding provides a strategy for engineering vibrationally coherent motions in other synthetic systems.

How Rhodopsin Tunes the Equilibrium between Protonated and Deprotonated Forms of the Retinal Chromophore.

Rhodopsin is a photoactive G-protein-coupled receptor (GPCR) that converts dim light into a signal for the brain, leading to eyesight. Full activation of this GPCR is achieved after passing through several steps of the protein's photoactivation pathway. Key events of rhodopsin activation are the initial cis-trans photoisomerization of the covalently bound retinal moiety followed by conformational rearrangements and deprotonation of the chromophore's protonated Schiff base (PSB), which ultimately lead to full activation in the meta II state. PSB deprotonation is crucial for achieving full activation of rhodopsin; however, the specific structural rearrangements that have to take place to induce this pKa shift are not well understood. Classical molecular dynamics (MD) simulations were employed to identify intermediate states after the cis-trans isomerization of rhodopsin's retinal moiety. In order to select the intermediate state in which PSB deprotonation is experimentally known to occur, the validity of the intermediate configurations was checked through an evaluation of the optical properties in comparison with experiment. Subsequently, the selected state was used to investigate the molecular factors that enable PSB deprotonation at body temperature to obtain a better understanding of the difference between the protonated and the deprotonated state of the chromophore. To this end, the deprotonation reaction has been investigated by applying QM/MM MD simulations in combination with thermodynamic integration. The study shows that, compared to the inactive 11-cis-retinal case, trans-retinal rhodopsin is able to undergo PSB deprotonation due to a change in the conformation of the retinal and a consequent alteration in the hydrogen-bond (HB) network in which PSB and the counterion Glu113 are embedded. Besides the retinal moiety and Glu113, also two water molecules as well as Thr94 and Gly90 that are related to congenital night blindness are part of this essential HB network.

Initial excited-state dynamics of an N-alkylated indanylidene-pyrroline (NAIP) rhodopsin analog.

N-Alkylated indanylidene-pyrroline-based molecular switches mimic different aspects of the light-induced retinal chromophore isomerization in rhodopsin: the vertebrate dim-light visual pigment. In particular, they display a similar ultrashort excited-state lifetime, subpicosecond photoproduct appearance time, and photoproduct vibrational coherence. To better understand the early light-induced dynamics of such systems, we measured and modeled the resonance Raman spectra of the Z-isomer of the N-methyl-4-(5'-methoxy-2',2'-dimethyl-indan-1'-ylidene)-5-methyl-2,3-dihydro-2H-pyrrolium (NAIP) switch in methanol solution. It is shown that the data, complemented with a <70 fs excited-state trajectory computation, demonstrate initial excited-state structural dynamics dominated by double-bond expansion and single-bond contraction stretches. This mode subsequently couples with the five-membered ring inversion and double-bond torsion. These results are discussed in the context of the mechanism of the excited-state photoisomerization of NAIP switches in solution and the 11-cis retinal in rhodopsin.

Incoherent control of the retinal isomerization in rhodopsin.

We propose to control the retinal photoisomerization yield through the backaction dynamics imparted by a nonselective optical measurement of the molecular electronic state. This incoherent effect is easier to implement than comparable coherent pulse shaping techniques, and is also robust to environmental noise. A numerical simulation of the quantum dynamics shows that the isomerization yield of this important biomolecule can be substantially increased above the natural limit.

QM/MM Trajectory Surface Hopping Approach to Photoisomerization of Rhodopsin and Isorhodopsin: The Origin of Faster and More Efficient Isomerization for Rhodopsin

The photoinduced cis-trans isomerization dynamics of rhodopsin and isorhodopsin are studied using a newly developed hybrid QM/MM trajectory surface hopping MD scheme based on the Zhu-Nakamura theory for nonadiabatic transitions. Rhodopsin and isorhodopsin have 11-cis and 9-cis forms of retinal as chromophore and the two proteins are isomerized to bathorhodopsin enclosing the all-trans form. The simulation reproduced faster and more efficient isomerization in rhodopsin than in isorhodopsin. In the excited state, rhodopsin shows a straightforward dynamics, whereas isorhodopsin dynamics is rather complicated and in a back-and-forth manner. The latter complicated dynamics would be mainly due to a narrow space near the active dihedral angle ═C8-C9═C10-C11═ (ϕ9) created by Thr 118 and Tyr 268 in opsin. Rhodopsin gives bathorhodopsin only while isorhodopsin yields a byproduct. The rigorous selectivity in rhodopsin would be another reason why rhodopsin is selected biologically. Comparison with our previous opsin-free investigations reveals that opsin tends to confine the twist of the active dihedral to only one direction and funnels transitions into the vicinity of minimum energy conical intersections (MECI). The twist-confinement totally blocks simultaneous twisting of ϕ9 and ϕ11 (═C10-C11═C12-C13═) and enhances the quantum yields. The opposite rotation of ϕ9 and ϕ11 ("wring-a-wet-towel" motion) takes place upon photoexcitation, which also does without opsin. The wring-a-wet-towel motion is dynamically enhanced in comparison with the one expected from locations of the MECI. The present simulation reveals that the Weiss-Warshel model for cis-trans photoisomerization is not applicable for rhodopsin because the branching ratio after transition is crucial.

Protein Gq Modulates Termination of Phototransduction and Prevents Retinal Degeneration

Appropriate termination of the phototransduction cascade is critical for photoreceptors to achieve high temporal resolution and to prevent excessive Ca(2+)-induced cell toxicity. Using a genetic screen to identify defective photoresponse mutants in Drosophila, we isolated and identified a novel Gα(q) mutant allele, which has defects in both activation and deactivation. We revealed that G(q) modulates the termination of the light response and that metarhodopsin/G(q) interaction affects subsequent arrestin-rhodopsin (Arr2-Rh1) binding, which mediates the deactivation of metarhodopsin. We further showed that the Gα(q) mutant undergoes light-dependent retinal degeneration, which is due to the slow accumulation of stable Arr2-Rh1 complexes. Our study revealed the roles of G(q) in mediating photoresponse termination and in preventing retinal degeneration. This pathway may represent a general rapid feedback regulation of G protein-coupled receptor signaling.

Tracking Ultrafast Energy Flow in Molecules Using Broadly Tunable Few-Optical-Cycle Pulses

Many (bio) molecules, following light absorption, undergo ultrafast nonradiative decay processes, taking place over timescales of tens to hundreds of femtoseconds. Ultrafast optical spectroscopy allows tracking in real time this energy flow using broadly tunable few-optical-cycle light pulses. Optical parametric amplifiers, thanks to their broad and tunable gain bandwidths, are powerful tools for the generation of such pulses. This paper first reviews our work on broadband optical parametric amplifiers, which led to the demonstration of few-optical-cycle sub-10-fs pulses continuously tunable from the visible (500 nm) to the near-IR (2 μm). We then present selected examples of applications to ultrafast spectroscopy of molecules, such as isomerization of rhodopsin, carotenoid-bacteriochlorophyll energy transfer in light-harvesting complexes, and polymer-fullerene electron transfer in blends for organic photovoltaics.

On the role of the triplet state in the cis/trans isomerization of rhodopsin: A CASPT2//CASSCF study of a model chromophore

AbstractThe possibility of population of the lowest‐lying triplet state (T1) in the early events of the photochemical isomerization process of a model chromophore of Rhodopsin (Rh) has been analyzed using multireference perturbation theory (CASPT2//CASSCF) methods. It is shown that the characteristics of the isomerization process namely small S1−T1 gap, presence of hydrogen out of plane active vibrational modes, and existence of a dense manifold of vibrational states, render possible the fulfilment of the conditions needed for the population of T1. The possible consequences for the photochemistry and photophysics of Rh are also discussed. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem 111:3431–3437, 2011

Thermal Stability of Rhodopsin and Implications for Retinitis Pigmentosa

Over 100 mutations in the dim-light photoreceptor rhodopsin are associated with retinitis pigmentosa (RP), a visual disorder characterized by progressive symptoms of night blindness, tunnel vision, and sometimes blindness. We studied the thermal stability of two mutations, S186W and D190N. We hypothesize that these mutations perturb an electrostatic interaction and hydrogen bonds in the active site and destabilize rhodopsin, which undermines the sensitivity to light and causes RP. We compared the rates of thermal decay, thermal isomerization, and Schiff base hydrolysis of WT, S186W, and D190N rhodopsin. Using UV-visible spectroscopy, we observed that the D190N mutant and WT rhodopsin do not decay over 24 hours at 37°C, whereas the S186W mutant decays with a half-life of 36 ± 4 min. We also measured the half-lives at 55°C, which are 70 ± 2 min for WT, 2.4 ± 0.2 for D190N, and 0.43 ± 0.03 min for S186W. Using HPLC and the acid denaturation assay, we measured the rates of thermal isomerization of 11-cis retinal and hydrolysis of the Schiff base linkage between retinal and opsin. We found that the mutations also increase these rates by 1-2 orders of magnitude. Because thermal isomerization of rhodopsin generates the same physiological response as photoisomerization, we suggest that the higher thermal isomerization rate in the mutants increases the level of dark noise, which desensitizes rhodopsin and causes the early symptom of night blindness. Because the drastic destabilizing effect of the mutations is likely correlated with the progressive deformation of the outer segment and subsequent loss of rod cells in RP, we propose that a future systematic study of the thermal stability of the RP-causing mutations can potentially provide more accurate predictions of the pace of vision loss in patients and guide strategies for treatment.

PROBING THE KEY VISION EVENT

RETURN TO ISSUEPREVNewsNEXTPROBING THE KEY VISION EVENTULTRAFAST SPECTROSCOPY: Rhodopsin isomerization owes speed to conical intersection between electronic statesCELIA ARNAUDCite this: Chem. Eng. News 2010, 88, 39, 11Publication Date (Print):September 27, 2010Publication History Published online27 March 2011Published inissue 27 September 2010https://doi.org/10.1021/cen-v088n039.p011Copyright © 2010 AMERICAN CHEMICAL SOCIETYArticle Views5Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (468 KB) SUBJECTS:Chemical reactions,Molecules,Photochemistry,Photoisomerization Get e-Alerts

Conical intersection dynamics of the primary photoisomerization event in vision

Ever since the conversion of the 11-cis retinal chromophore to its all-trans form in rhodopsin was identified as the primary photochemical event in vision, experimentalists and theoreticians have tried to unravel the molecular details of this process. The high quantum yield of 0.65 (ref. 2), the production of the primary ground-state rhodopsin photoproduct within a mere 200 fs (refs 3-7), and the storage of considerable energy in the first stable bathorhodopsin intermediate all suggest an unusually fast and efficient photoactivated one-way reaction. Rhodopsin's unique reactivity is generally attributed to a conical intersection between the potential energy surfaces of the ground and excited electronic states enabling the efficient and ultrafast conversion of photon energy into chemical energy. But obtaining direct experimental evidence for the involvement of a conical intersection is challenging: the energy gap between the electronic states of the reacting molecule changes significantly over an ultrashort timescale, which calls for observational methods that combine high temporal resolution with a broad spectral observation window. Here we show that ultrafast optical spectroscopy with sub-20-fs time resolution and spectral coverage from the visible to the near-infrared allows us to follow the dynamics leading to the conical intersection in rhodopsin isomerization. We track coherent wave-packet motion from the photoexcited Franck-Condon region to the photoproduct by monitoring the loss of reactant emission and the subsequent appearance of photoproduct absorption, and find excellent agreement between the experimental observations and molecular dynamics calculations that involve a true electronic state crossing. Taken together, these findings constitute the most compelling evidence to date for the existence and importance of conical intersections in visual photochemistry.

Arrestin Translocation Is Stoichiometric to Rhodopsin Isomerization and Accelerated by Phototransduction in Drosophila Photoreceptors

Upon illumination, visual arrestin translocates from photoreceptor cell bodies to rhodopsin and membrane-rich photosensory compartments, vertebrate outer segments or invertebrate rhabdomeres, where it quenches activated rhodopsin. Both the mechanism and function of arrestin translocation are unresolved and controversial. In dark-adapted photoreceptors of the fruitfly Drosophila, confocal immunocytochemistry shows arrestin (Arr2) associated with distributed photoreceptor endomembranes. Immunocytochemistry and live imaging of GFP-tagged Arr2 demonstrate rapid reversible translocation to stimulated rhabdomeres in stoichiometric proportion to rhodopsin photoisomerization. Translocation is very rapid in normal photoreceptors (time constant <10 s) and can also be resolved in the time course of electroretinogram recordings. Genetic elimination of key phototransduction proteins, including phospholipase C (PLC), Gq, and the light-sensitive Ca2+-permeable TRP channels, slows translocation by 10- to 100-fold. Our results indicate that Arr2 translocation in Drosophila photoreceptors is driven by diffusion, but profoundly accelerated by phototransduction and Ca2+ influx.

Light-Dependent Modulation of Shab Channels via Phosphoinositide Depletion in Drosophila Photoreceptors

The Drosophila phototransduction cascade transforms light into depolarizations that are further shaped by activation of voltage-dependent K+ (Kv) channels. In whole-cell recordings of isolated photoreceptors, we show that light selectively modulated the delayed rectifier (Shab) current. Shab currents were increased by light with similar kinetics to the light-induced current itself (latency approximately 20 ms), recovering to control values with a t(1/2) of approximately 60 s in darkness. Genetic disruption of PLCbeta4, responsible for light-induced PIP(2) hydrolysis, abolished this light-dependent modulation. In mutants of CDP-diaclyglycerol synthase (cds(1)), required for PIP(2) resynthesis, the modulation became irreversible, but exogenously applied PIP(2) restored reversibility. The modulation was accurately and reversibly mimicked by application of PIP(2) to heterologously expressed Shab channels in excised inside-out patches. The results indicate a functionally implemented mechanism of Kv channel modulation by PIP(2) in photoreceptors, which enables light-dependent regulation of signal processing by direct coupling to the phototransduction cascade.

Efficiency of Synaptic Transmission of Single-Photon Events from Rod Photoreceptor to Rod Bipolar Dendrite

A rod transmits absorption of a single photon by what appears to be a small reduction in the small number of quanta of neurotransmitter ( Q count) that it releases within the integration period (∼0.1 s) of a rod bipolar dendrite. Due to the quantal and stochastic nature of release, discrete distributions of Q count for darkness versus one isomerization of rhodopsin (R*) overlap. We suggested that release must be regular to narrow these distributions, reduce overlap, reduce the rate of false positives, and increase transmission efficiency (the fraction of R* events that are identified as light). Unsurprisingly, higher quantal release rates ( Q rates) yield higher efficiencies. Focusing here on the effect of small changes in Q rate, we find that a slightly higher Q rate yields greatly reduced efficiency, due to a necessarily fixed quantal-count threshold. To stabilize efficiency in the face of drift in Q rate, the dendrite needs to regulate the biochemical realization of its quantal-count threshold with respect to its Q count. These considerations reveal the mathematical role of calcium-based negative feedback and suggest a helpful role for spontaneous R*. In addition, to stabilize efficiency in the face of drift in degree of regularity, efficiency should be ≈50%, similar to measurements.

Deactivation of rhodopsin in the transition from the signaling state meta II to meta III involves a thermal isomerization of the retinal chromophore C[double bond]D.

Light-induced isomerization of rhodopsin's retinal chromophore to the activating all-trans geometry initializes the formation of the active receptor state, Meta II. In the absence of peripheral regulatory proteins, the activity of Meta II is switched off spontaneously by two independent pathways: either by hydrolysis of the retinal Schiff base and dissociation of the light receptor into apoprotein opsin plus free retinal or by formation of Meta III, an inactive species with intact retinal protonated Schiff base absorbing at 470 nm. By FTIR spectroscopy on rhodopsin reconstituted with isotopically labeled chromophores in combination with quantum mechanical DFT calculations, we show that the deactivating step during formation of Meta III involves a thermal isomerization of the chromophore C[double bond]N, such that the chromophore in Meta III is all-trans-15-syn. This isomerization step is catalyzed by the protein environment and proceeds via Meta I, as suggested by its dependence on pH and on properties of the lipid/detergent environment of the protein. In the long term, Meta III decays likewise to opsin and free retinal by slow hydrolysis of the Schiff base.

Resonance Raman Structural Evidence that the Cis-to-Trans Isomerization in Rhodopsin Occurs in Femtoseconds

Picosecond time-resolved resonance Raman spectroscopy is used to probe the structural changes of rhodopsin's retinal chromophore as the cis-to-trans isomerization reaction occurs that initiates vision. Room-temperature resonance Raman spectra of rhodopsin's photoproduct with time delays from -0.7 to 20.8 ps are measured using 2.2 ps, 480 nm pump and 1.5 ps, 600 nm probe pulses. Hydrogen-out-of-plane (HOOP) modes at 852, 871, and 919 cm(-1), fingerprint peaks at 1272, 1236, 1211, and 1166 cm(-1), and a broad red-shifted ethylenic band at 1530 cm(-1) are present at the earliest positive pump-probe time delay of 0.8 ps, indicating that the chromophore is already in a strained, all-trans configuration. Kinetic analyses of both the HOOP and ethylenic regions of the photoproduct spectra reveal that these features grow in with fast ( approximately 200 fs) and slow ( approximately 2-3 ps) components. These data provide the first structural evidence that photorhodopsin has a thermally unrelaxed, torsionally strained all-trans chromophore within approximately 1 ps, and possibly within 200 fs, of photon absorption. Following this ultrafast product formation, the all-trans chromophore cools and conformationally relaxes within a few picoseconds to form bathorhodopsin. This cooling process is revealed as an ethylenic frequency blue-shift of 6 cm(-1) (tau approximately 3.5 ps) as well as an ethylenic width narrowing (tau approximately 2 ps). The ultrafast production of photorhodopsin is likely accompanied by an impulsively driven, localized protein response. More delocalized protein modes are unable to relax on this ultrafast time scale enabling the chromophore-protein complex to store the large amounts of photon energy (30-35 kcal/mol) that are subsequently used to drive activating protein conformational changes.

Quantum-Mechanical Modeling of the Femtosecond Isomerization in Rhodopsin

The photoisomerization of retinal in rhodopsin is modeled by a vibronically coupled electronic two-state system, taking into account a collective reaction coordinate and the ethylenic stretch mode. The model qualitatively reproduces all available spectroscopic information on rhodopsin and accounts for its high reaction efficiency. Quantum simulations of femtosecond time-resolved experiments suggest that the prominent 60 cm-1 oscillations observed in experiments are due to nonadiabatic wave packet motion along the reaction coordinate. This indicates that the protein is capable of providing an almost friction-free environment for retinal up to ≈2 ps, thereby catalyzing the photoreaction.