Isomers of 3,7,11-trimethyldodeca-2,4,6,8,10-pentaenal (a linear analogue of retinal) and lower homologues in their interaction with bovine opsin and bacterioopsin

Schiff Base Proton Acceptor Assists Photoisomerization of Retinal Chromophores in Bacteriorhodopsin

In this study, we investigated the ultrafast dynamics of bacteriorhodopsins (BRs) from Haloquadratum walsbyi (HwBR) and Haloarcula marismortui (HmBRI and HmBRII). First, the ultrafast dynamics were studied for three HwBR samples: wild-type, D93N mutation, and D104N mutation. The residues of the D93 and D104 mutants correspond to the control by the Schiff base proton acceptor and donor of the proton translocation subchannels. Measurements indicated that the negative charge from the Schiff base proton acceptor residue D93 interacts with the ultrafast and substantial change of the electrostatic potential associated with chromophore isomerization. By contrast, the Schiff base proton donor assists the restructuring of the chromophore cavity hydrogen-bond network during the thermalization of the vibrational hot state. Second, the ultrafast dynamics of the wild-types of HwBR, HmBRI, and HmBRII were compared. Measurements demonstrated that the hydrogen-bond network in the extracellular region in HwBR and HmBRII slows the photoisomerization of retinal chromophores, and the negatively charged helices on the cytoplasmic side of HwBR and HmBRII accelerate the thermalization of the vibrational hot state of retinal chromophores. The similarity of the correlation spectra of the wild-type HmBRI and D104N mutant of HwBR indicates that inactivation of the Schiff base proton donor induces a positive charge on the helices of the cytoplasmic side.

Space and Time Evolution of the Electrostatic Potential During the Activation of a Visual Pigment

Animal and microbial retinal proteins employ the Schiff base of retinal as their chromophore. Here, the possible consequences of the charge translocation associated with the light-induced dynamics of the chromophore of a visual opsin are investigated along a representative semiclassical trajectory. We show that the evolution of the electrostatic potential projected by the chromophore onto the surrounding protein displays intense but topographically localized sudden variations in proximity of the decay region. pKa calculations carried out on selected snapshots used as probes, indicate that the only residue which may be sensitive to the electrostatic potential shift is Glu181. Accordingly, our results suggest that the frail Tyr191/268-Glu181-Wat2-Ser186 hydrogen bond network may be perturbed by the transient variations of the electrostatic potential.

Evidence from Chlamydomonas on the photoactivation of rhodopsins without isomerization of their chromophore

Attachment of retinal to opsin forms the chromophore N-retinylidene, which isomerizes during photoactivation of rhodopsins. To test whether isomerization is crucial, custom-tailored chromophores lacking the β-ionone ring and any isomerizable bonds were incorporated in vivo into the opsin of a blind mutant of the eukaryote Chlamydomonas reinhardtii. The analogs restored phototaxis with the anticipated action spectra, ruling out the need for isomerization in photoactivation. To further elucidate photoactivation, responses to chromophores formed from naphthalene aldehydes were studied. The resulting action spectral shifts suggest that charge separation within the excited chromophore leads to electric field-induced polarization of nearby amino acid residues and altered hydrogen bonding. This redistribution of charge facilitates the reported multiple bond rotations and protein rearrangements of rhodopsin activation. These results provide insight into the activation of rhodopsins and related GPCRs.

Development of Bacteriorhodopsin Analogues and Studies of Charge Separated Excited States in the Photoprocesses of Linear Polyenes†

Development of bacteriorhodopsin (bR) analogues employing chromophore substitution technique for the purpose of characterizing the binding site of bR and generating bR analogues with novel opto-electronic properties for applications as photoactive element in nanotechnical devices are described. Additionally, the photophysical and photochemical properties of variously substituted diarylpolyenes as models of photobiologically relevant linear polyenes are discussed. The role of charge separated dipolar excited states in the photoprocesses of linear polyenes is highlighted.

Diversion of the sign of phototaxis in a Chlamydomonas reinhardtii mutant incorporated with retinal and its analogs

The blind mutant FN68 of the unicellular flagellate green alga Chlamydomonas reinhardtii is negatively phototactic in the presence of the native chromophore all-trans retinal. In contrast, analog chromophores such as a ring-acyclic retinal and those in which trans/cis isomerization about the C11 = C12 double bond was blocked induced predominantly positive phototaxis in the same strain under the same experimental conditions. These observations can be interpreted by assuming that the negative and the positive phototaxis is mediated distinctively by two rhodopsin species which differ in their affinities with the exogenous chromophores. However, a more reasonable explanation, which requires fewer assumptions, is that the sign of phototaxis depends on a delay in intracellular photosignal transduction. This novel view was deduced directly from the widely accepted hypothesis [1980, Microbiol. Rev. 44, 572-630] on phototaxis mechanisms.

Sensitivity increase in the photophobic response of Halobacterium halobium reconstituted with retinal analogs: a novel interpretation for the fluence-response relationship and a kinetic modeling

Phoborhodopsin (also called sensory rhodopsin II) is a photoreceptor protein which mediates photophobic responses of Halobacterium halobium to blue-green light. Under conditions where the synthesis of the chromophore retinal is inhibited, the photophobic system is reconstituted in vivo by incorporation of all-trans retinal or retinal analogs into the apoprotein of phoborhodopsin. Retinal analogs which retard the cyclic photoreaction kinetics of phoborhodopsin increase significantly the sensitivity of the photophobic response. This supports the previously reported hypothesis that signal amplification occurs during the lifetime of intermediate states of the photocycle. The sensitivity increase caused by the chromophore substitution is observed in cells at several different growth stages, i.e. the naturally occurring chromophore (all-trans retinal) does not produce maximal sensitivity at any stage of the culture growth. These results are difficult to interpret in terms of the proposal by Marwan et al. (J. Mol. Biol. 199, 663-664, 1988) that only a single photon is sufficient to cause the photobehavioral response in cells containing native phoborhodopsin. A new interpretation for the fluence-response curves is described based in part on their Poisson statistical analysis. Further, a kinetic model which relates the receptor photochemical reaction cycle to the behavioral response is developed, which accounts for both the sensitivity increase and the shape of the fluence-response curves.

Mechanism of activation of sensory rhodopsin I: evidence for a steric trigger

Sensory rhodopsin I (SR-I) and bacteriorhodopsin (BR) from Halobacterium halobium show broad structural and spectroscopic similarities and yet perform distinct functions: photosensory reception and proton pumping, respectively. Probing the photoactive sites of SR-I and BR with 24 retinal analogs reveals differences in the protein environments near the retinal 13-methyl group and near the beta-ionone ring. 13-cis-Retinal does not form a retinylidene pigment with the SR-I apoprotein, although this isomer binds to the BR apoprotein even more rapidly than all-trans-retinal, the functional isomer of both pigments. The activation of both SR-I and BR requires all-trans/13-cis isomerization of retinal;however, a steric interaction between the retinal 13-methyl group and the protein is required for SR-I activation but not for that of BR. These results reveal a key difference between SR-I and BR that is likely to be the initial diverging point in their photoactivation pathways. We propose the 13-methyl group-protein interaction functions as a trigger for SR-I activation--i.e., converts photon absorption by the chromophore into protein conformational changes. A similar steric trigger is essential for activation of mammalian rhodopsin, indicating a common mechanism for receptor activation in archaebacterial and vertebrate retinylidene photosensors.

Spectral sensitivity, structure and activation of eukaryotic rhodopsins: activation spectroscopy of rhodopsin analogs in Chlamydomonas

Retinal normally binds opsin forming the chromophore of the visual pigment, rhodopsin. In this investigation synthetic analogs were bound by the opsin of living cells of the alga Chlamydomonas reinhardtii; the effect was assayed by phototaxis to give an activation spectrum for each rhodopsin analog. The results show the influence of different chromophores and the protein on the absorption of light. The maxima of the phototaxis action spectra shifted systematically with the number of double bonds conjugated with the imine (C = N+H) bond of the chromophore. Chromophores lacking a beta-ionone ring, methyl groups and all C = C double bonds photoactivated the rhodopsin of Chlamydomonas with normal efficiency. On the basis of a simple model involving one-electron transitions between occupied and virtual molecular orbitals, we estimate the charge distribution along the chromophore in the binding site. With this restraint we define a unique structural model for eukaryotic rhodopsins and explain the spectral clustering of pigments, the spectral differences between red and green rhodopsins and the molecular basis of color blindness. Our results are consistent with the triggering of the activation of rhodopsin by the light-mediated change in electric dipole moment rather than the steric cis-trans isomerization of the chromophore.

Color regulation in the archaebacterial phototaxis receptor phoborhodopsin (sensory rhodopsin II)

Phoborhodopsin, a repellent phototaxis receptor in Halobacterium halobium, exhibits vibrational fine structure, a feature that has not been identified for any other rhodopsin pigment at physiological temperatures. This conclusion follows form analysis of the absorption properties of the pigment in H. halobium membranes containing native retinal and an array of retinal analogues. The absorption spectrum of the native pigment has a maximum at 487 nm with a pronounced shoulder at 460 nm; however, the bandwidth is that expected for a single retinylidene species. Gaussian band-shape simulation with a spacing corresponding to the vibrational frequencies of polyene stretching modes reproduces the structured absorption spectra of native pigment as well as of analogue phoborhodopsin. Absorption shifts produced by a series of dihydroretinal and other retinal analogues strongly indicate that the dominant factor regulating the color of the pigment is planarization of the retinal ring with respect to the polyene chain.

Effects of modifications of the retinal beta-ionone ring on archaebacterial sensory rhodopsin I

Ring desmethyl and acyclic analogues of all-trans retinal were incorporated into the apoprotein of the phototaxis receptor sensory rhodopsin I (SR-I) in Halobacterium halobium membranes. All modified retinals generate SR-I analogue pigments which exhibit "opsin shifts," i.e., their absorption spectra are shifted to longer wavelengths compared with model protonated Schiff bases of the same analogues. Each SR-I pigment analogue exhibits cyclic photochemical reactions as monitored by flash spectroscopy, but the analogue photocycles differ from that of native SR-I by exhibiting pronounced biphasic recovery of flash-induced absorption changes and abnormal flash-induced absorption difference spectra. Despite perturbations in the photochemical properties, the SR-I pigment analogues are capable of both attractant (single photon) and repellent (two photon) phototaxis signaling in cells. Our interpretation is that the hydrophobic ring substituents interact with the binding pocket to maintain the correct configuration for native SR-I absorption and photochemistry, but these interactions are not essential for the physiological function of SR-I as a dual attractant/repellent phototaxis receptor. These results support the conclusion emerging from several studies that the photoactivation process that triggers the conformation changes of SR-I and the related proton pump bacteriorhodopsin is conserved despite the different biological functions of their photoactivation.