Resonance Raman studies of the HOOP modes in octopus bathorhodopsin with deuterium-labeled retinal chromophores

Photooxidation mediated by 11-cis and all-trans retinal in single isolated mouse rod photoreceptors

Retinal, the vitamin A aldehyde, is a potent photosensitizer that plays a major role in light-induced damage to vertebrate photoreceptors. 11-Cis retinal is the light-sensitive chromophore of rhodopsin, the photopigment of vertebrate rod photoreceptors. It is isomerized by light to all-trans, activating rhodopsin and beginning the process of light detection. All-trans retinal is released by activated rhodopsin, allowing its regeneration by fresh 11-cis retinal continually supplied to photoreceptors. The released all-trans retinal is reduced to all-trans retinol in a reaction using NADPH. We have examined the photooxidation mediated by 11-cis and all-trans retinal in single living rod photoreceptors isolated from mouse retinas. Photooxidation was measured with fluorescence imaging from the oxidation of internalized BODIPY C11, a fluorescent dye whose fluorescence changes upon oxidation. We found that photooxidation increased with the concentration of exogenously added 11-cis or all-trans retinal to metabolically compromised rod outer segments that lacked NADPH supply. In dark-adapted metabolically intact rod outer segments with access to NADPH, there was no significant increase in photooxidation following exposure of the cell to light, but there was significant increase following addition of exogenous 11-cis retinal. The results indicate that both 11-cis and all-trans retinal can mediate light-induced damage in rod photoreceptors. In metabolically intact cells, the removal of the all-trans retinal generated by light through its reduction to retinol minimizes all-trans retinal-mediated photooxidation. However, because the enzymatic machinery of the rod outer segment cannot remove 11-cis retinal, 11-cis-retinal-mediated photooxidation may play a significant role in light-induced damage to photoreceptor cells.

The evolution of eyes and visually guided behaviour

The morphology and molecular mechanisms of animal photoreceptor cells and eyes reveal a complex pattern of duplications and co-option of genetic modules, leading to a number of different light-sensitive systems that share many components, in which clear-cut homologies are rare. On the basis of molecular and morphological findings, I discuss the functional requirements for vision and how these have constrained the evolution of eyes. The fact that natural selection on eyes acts through the consequences of visually guided behaviour leads to a concept of task-punctuated evolution, where sensory systems evolve by a sequential acquisition of sensory tasks. I identify four key innovations that, one after the other, paved the way for the evolution of efficient eyes. These innovations are (i) efficient photopigments, (ii) directionality through screening pigment, (iii) photoreceptor membrane folding, and (iv) focusing optics. A corresponding evolutionary sequence is suggested, starting at non-directional monitoring of ambient luminance and leading to comparisons of luminances within a scene, first by a scanning mode and later by parallel spatial channels in imaging eyes.

Photoreverse reaction dynamics of octopus rhodopsin

Photoreverse reactions of octopus rhodopsin (Rh) from acid-metarhodopsin (Acid-Meta), which is the final product of the photoreaction of Rh, to Rh were studied by the time-resolved transient absorption and transient grating methods. The time course of the absorption signal showed a rapid change within 500 ns followed by one phase with a time constant of approximately 470 micros, whereas the transient grating signal indicates three phases with time constants of <500 ns, approximately 490 micros, and 2.6 ms. The faster two phases indicate the conformational change in the vicinity of the chromophore, and the slowest one represents conformational change far from the chromophore. The absorption spectrum of the first intermediate created just after the laser excitation (<500 ns) is already very similar to the final product, Rh. This behavior is quite different from that of the forward reaction from Rh to Acid-Meta, in which several intermediates with different absorption spectra are involved within 50 ns-500 micros. This result indicates that the conformation around the chromophore is easily adjusted from all-trans to 11-cis forms compared with that from 11-cis to all-trans forms. Furthermore, it was found that the protein energy is quickly relaxed after the excitation. One of the significantly different properties between Rh and Acid-Meta is the diffusion coefficient (D). D is reduced by about half the transformation from Rh to Acid-Meta. This large reduction was interpreted in terms of the helix opening of the Rh structure.

Resonance Raman analysis of the mechanism of energy storage and chromophore distortion in the primary visual photoproduct

The vibrational structure of the chromophore in the primary photoproduct of vision, bathorhodopsin, is examined to determine the cause of the anomalously decoupled and intense C(11)=C(12) hydrogen-out-of-plane (HOOP) wagging modes and their relation to energy storage in the primary photoproduct. Low-temperature (77 K) resonance Raman spectra of Glu181 and Ser186 mutants of bovine rhodopsin reveal only mild mutagenic perturbations of the photoproduct spectrum suggesting that dipolar, electrostatic, or steric interactions with these residues do not cause the HOOP mode frequencies and intensities. Density functional theory calculations are performed to investigate the effect of geometric distortion on the HOOP coupling. The decoupled HOOP modes can be simulated by imposing approximately 40 degrees twists in the same direction about the C(11)=C(12) and C(12)-C(13) bonds. Sequence comparison and examination of the binding site suggests that these distortions are caused by three constraints consisting of an electrostatic anchor between the protonated Schiff base and the Glu113 counterion, as well as steric interactions of the 9- and 13-methyl groups with surrounding residues. This distortion stores light energy that is used to drive the subsequent protein conformational changes that activate rhodopsin.

Energetics and volume changes of the intermediates in the photolysis of octopus rhodopsin at a physiological temperature

Enthalpy changes (Delta H) of the photointermediates that appear in the photolysis of octopus rhodopsin were measured at physiological temperatures by the laser-induced transient grating method. The enthalpy from the initial state, rhodopsin, to bathorhodopsin, lumirhodopsin, mesorhodopsin, transient acid metarhodopsin, and acid metarhodopsin were 146 +/- 15 kJ/mol, 122 +/- 17 kJ/mol, 38 +/- 8 kJ/mol, 12 +/- 5 kJ/mol, and 12 +/- 5 kJ/mol, respectively. These values, except for lumirhodopsin, are similar to those obtained for the cryogenically trapped intermediate species by direct calorimetric measurements. However, the Delta H of lumirhodopsin at physiological temperatures is quite different from that at low temperature. The reaction volume changes of these processes were determined by the pulsed laser-induced photoacoustic method along with the above Delta H values. Initially, in the transformation between rhodopsin and bathorhodopsin, a large volume expansion of +32 +/- 3 ml/mol was obtained. The volume changes of the subsequent reaction steps were rather small. These results are compared with the structural changes of the chromophore, peptide backbone, and water molecules within the membrane helixes reported previously.

Movement of retinal along the visual transduction path

Movement of the ligand/receptor complex in rhodopsin (Rh) has been traced. Bleaching of diazoketo rhodopsin (DK-Rh) containing 11-cis-3-diazo-4-oxo-retinal yields batho-, lumi-, meta-I-, and meta-II-Rh intermediates corresponding to those of native Rh but at lower temperatures. Photoaffinity labeling of DK-Rh and these bleaching intermediates shows that the ionone ring cross-links to tryptophan-265 on helix F in DK-Rh and batho-Rh, and to alanine-169 on helix D in lumi-, meta-I-, and meta-II-Rh intermediates. It is likely that these movements involving a flip-over of the chromophoric ring trigger changes in cytoplasmic membrane loops resulting in heterotrimeric guanine nucleotide-binding protein (G protein) activation.

Rhodopsin and phototransduction

Recent studies on rhodopsin structure and function are reviewed and the properties of vertebrate as well as invertebrate rhodopsin described. Open issues such as the 'red shift' of the absorbance spectra are emphasized in the light of the present model of the retinal-binding pocket. The processes that restore the rhodopsin content in photoreceptors are also presented with a comparison between vertebrate and invertebrate visual systems. The central role of rhodopsin in the phototransduction cascade becomes evident by examining the main reports on light-activated conformational changes of rhodopsin and its interaction with transducin. Shut-off mechanisms are considered by reporting the studies on the sites of rhodopsin phosphorylation and arrestin binding. Furthermore, recent findings on the energetics of phototransduction point out that the ATP needed for photoreception in vertebrates is synthesized in the outer segments where phototransduction events take place.

Vibrational spectrum of the lumi intermediate in the room temperature rhodopsin photo-reaction

The vibrational spectrum (650-1750 cm(-1)) of the lumi-rhodopsin (lumi) intermediate formed in the microsecond time regime of the room-temperature rhodopsin (RhRT) photoreaction is measured for the first time using picosecond time-resolved coherent anti-Stokes Raman spectroscopy (PTR/CARS). The vibrational spectrum of lumi is recorded 2.5 micros after the 3-ps, 500-nm excitation of RhRT. Complementary to Fourier transform infrared spectra recorded at Rh sample temperatures low enough to freeze lumi, these PTR/CARS results provide the first detailed view of the vibrational degrees of freedom of room-temperature lumi (lumiRT) through the identification of 21 bands. The exceptionally low intensity (compared to those observed in bathoRT) of the hydrogen out-of-plane (HOOP) bands, the moderate intensity and absolute positions of C-C stretching bands, and the presence of high-intensity C==C stretching bands suggest that lumiRT contains an almost planar (nontwisting), all-trans retinal geometry. Independently, the 944-cm(-1) position of the most intense HOOP band implies that a resonance coupling exists between the out-of-plane retinal vibrations and at least one group among the amino acids comprising the retinal binding pocket. The formation of lumiRT, monitored via PTR/CARS spectra recorded on the nanosecond time scale, can be associated with the decay of the blue-shifted intermediate (BSI(RT)) formed in equilibrium with the bathoRT intermediate. PTR/CARS spectra measured at a 210-ns delay contain distinct vibrational features attributable to BSI(RT), which suggest that the all-trans retinal in both BSI(RT) and lumiRT is strongly coupled to part of the retinal binding pocket. With regard to the energy storage/transduction mechanism in RhRT, these results support the hypothesis that during the formation of lumiRT, the majority of the photon energy absorbed by RhRT transfers to the apoprotein opsin.

A resonance Raman study of octopus bathorhodopsin with deuterium labeled retinal chromophores

The resonance Raman spectrum of octopus bathorhodopsin in the fingerprint region and in the ethylenic-Schiff base region have been obtained at 80 K using the "pump-probe" technique as have its deuterated chromophore analogues at the C7D; C8D; C8,C7D2; C10D; C11D; C11, C12D2; C14D; C15D; C14, C15D2; and N16D positions. While these data are not sufficient to make definitive band assignments, many tentative assignments can be made. Because of the close spectral similarity between the octopus bathorhodopsin spectrum and that of bovine bathorhodopsin, we conclude that the essential configuration of octopus bathorhodopsin's chromophore is all-trans like. The data suggest that the Schiff base, C = N, configuration is trans (anti). The observed conformationally sensitive fingerprint bands show pronounced isotope shifts upon chromophore deuteration. The size of the shifts differ, in certain cases, from those found for bovine bathorhodopsin. Thus, the internal mode composition of the fingerprint bands differs somewhat from bovine bathorhodopsin, suggesting a somewhat different in situ chromophore conformation. An analysis of the NH bend frequency, the Schiff base C = N stretch frequency, and its shift upon Schiff base deuteration suggests that the hydrogen bonding between the protonated Schiff base with its protein binding pocket is weaker in octopus bathorhodopsin than in bovine bathorhodopsin but stronger than that found in bacteriorhodopsin's bR568 pigment.

13C magic-angle spinning NMR studies of bathorhodopsin, the primary photoproduct of rhodopsin

Magic-angle spinning NMR spectra have been obtained of the bathorhodopsin photointermediate trapped at low temperature (less than 130 K) by using isorhodopsin samples regenerated with retinal specifically 13C-labeled at positions 8, 10, 11, 12, 13, 14, and 15. Comparison of the chemical shifts of the bathorhodopsin resonances with those of an all-trans-retinal protonated Schiff base (PSB) chloride salt show the largest difference (6.2 ppm) at position 13 of the protein-bound retinal. Small differences in chemical shift between bathorhodopsin and the all-trans PSB model compound are also observed at positions 10, 11, and 12. The effects are almost equal in magnitude to those previously observed in rhodopsin and isorhodopsin. Consequently, the energy stored in the primary photoproduct bathorhodopsin does not give rise to any substantial change in the average electron density at the labeled positions. The data indicate that the electronic and structural properties of the protein environment are similar to those in rhodopsin and isorhodopsin. In particular, a previously proposed perturbation near position 13 of the retinal appears not to change its position significantly with respect to the chromophore upon isomerization. The data effectively exclude charge separation between the chromophore and a protein residue as the main mechanism for energy storage in the primary photoproduct and argue that the light energy is stored in the form of distortions of the bathorhodopsin chromophore.