Thermal Decay of Rhodopsin: Role of Hydrogen Bonds in Thermal Isomerization of 11-<i>cis</i> Retinal in the Binding Site and Hydrolysis of Protonated Schiff Base

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

doi:10.1021/ja903154u

Cited by 27 publications

(61 citation statements)

References 16 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…This network is thought to stabilize rhodopsin (2,18,19) and suppress thermal isomerization, conferring rhodopsin with extraordinary thermal stability to lower the dark noise and enhance photosensitivity. This hypothesis is supported by the observation that the rates of thermal reactions are slowed down threefold in deuterated water, consistent with a rate-determining step involving water interactions (20). In addition, the mutant S186A where the H-bonds in the retinyl binding site are disrupted (Fig.…”

Section: Resultssupporting

confidence: 56%

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.

Self Cite

View full text Add to dashboard Cite

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...

show abstract

Section: Resultssupporting

confidence: 56%

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.

Self Cite

View full text Add to dashboard Cite

show abstract

“…Thermal Decay-UV-visible spectroscopy was used to monitor thermal decay as described (32). All measurements were made in the dark on a UV-visible spectrophotometer (Shimadzu UV-2450).…”

Section: Methodsmentioning

confidence: 99%

“…Each sample was immediately frozen in a glass vial precooled by dry ice and stored on ice to quench any thermal process. These samples were divided equally into two parts for measuring the rates of thermal isomerization and hydrolysis of the SB (32).…”

Section: Methodsmentioning

confidence: 99%

See 1 more Smart Citation

Thermal Properties of Rhodopsin

Liu

Nguyen

et al. 2011

Journal of Biological Chemistry

Self Cite

View full text Add to dashboard Cite

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 ϳ20 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.Rhodopsin is a widely studied G protein-coupled receptor responsible for generating visual signals in dim-light environments (1-11). It is expressed in the disc membranes in the outer segment of rod photoreceptor cells (1-3). It has a seven-helical transmembrane structure and incorporates the 11-cis-retinal chromophore via a protonated Schiff base (SB) 6 in the transmembrane region (3,8,9). Upon absorption of a photon, the retinal chromophore isomerizes from the 11-cis-to all-trans-configuration to form the primary photoproduct bathorhodopsin (12, 13), which then evolves into a number of photointermediates on various time scales (14, 15), forming metarhodopsin II (Meta II) in milliseconds. In Meta II, the SB linkage becomes deprotonated, and the absorption changes from the visible to the UV (380 nm) region. Subsequently, Meta II couples to the G protein transducin to induce an exchange of GDP for GTP in the ␣-subunit of transducin (16). The ␣-subunit dissociates from the ␤␥-subunit and activates phosphodiesterase, which catalyzes hydrolysis of cGMP. A decrease in the concentration of cGMP closes the sodium channels of the rod photoreceptor cells and generates hyperpolarization for a visual signal.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 imp...

show abstract

“…At 37ºC, G90V bicelles and N55K bicelles showed enhanced thermal stability when compared to the DM-solubilized samples. Thermal decay process involves protein conformational changes, retinal isomerization and eventually hydrolysis of SB and chromophore release 20,38,39 . We find that t 1/2 of G90V bicelles and N55K bicelles suffers a 3 and 4 fold increase respectively when compared to the detergentsolubilized samples (Figure 6a and Table 2).…”

Section: Thermal Stability Of Wt and G90v N55k In Dm And In Dmpc/dhpmentioning

confidence: 99%

Phospholipid Bicelles Improve the Conformational Stability of Rhodopsin Mutants Associated with Retinitis Pigmentosa

et al. 2015

View full text Add to dashboard Cite

Mutations in the visual photoreceptor rhodopsin are the cause of the retinal degenerative disease retinitis pigmentosa. Some naturally occurring mutations can lead to protein conformational instability. Two such mutations, N55K and G90V, in the first and second transmembrane helices of the receptor, have been associated with sector and classical retinitis pigmentosa, respectively, and showed enhanced thermal sensitivity. We have carefully analyzed the effect of phospholipid bicelles on the stability and ligand binding properties of these two mutants and compared it with those of the detergent-solubilized samples. We have used a phospholipid bilayer consisting of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) and 1,2dihexanoyl-sn-glycero-3-phosphocholine (DHPC). We find that DMPC/DHPC bicelles dramatically increase the thermal stability of the rhodopsin mutants G90V and N55K. The chromophore stability and regeneration of the mutants were also increased in bicelles when compared to their behavior in a dodecyl maltoside detergent solution. The retinal release process was slowed in bicelles, and chromophore entry, after illumination, was improved for the G90V mutant but not for N55K. Furthermore, fluorescence spectroscopy measurements showed that bicelles allowed more exogenous retinal binding to the photoactivated G90V mutant than in a detergent solution. In contrast, N55K could not reposition any chromophore either in the detergent or in bicelles. The results demonstrate that DMPC/DHPC bicelles can counteract the destabilizing effect of the disease-causing mutations and can modulate the structural changes in the ensuing receptor photoactivation in a distinct specific manner for different retinitis pigmentosa mutant phenotypes.

show abstract

Thermal Decay of Rhodopsin: Role of Hydrogen Bonds in Thermal Isomerization of 11-cis Retinal in the Binding Site and Hydrolysis of Protonated Schiff Base

Cited by 27 publications

References 16 publications

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

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

Thermal Properties of Rhodopsin

Phospholipid Bicelles Improve the Conformational Stability of Rhodopsin Mutants Associated with Retinitis Pigmentosa

Contact Info

Product

Resources

About