Photochemical reactions of 13-demethyl visual pigment analogues at low temperatures

Measurements of the stabilities of isolated retinal chromophores

The barrier energies for isomerization and fragmentation were measured for a series of retinal chromophore derivatives using a tandem ion mobility spectrometry approach. These measurements allow us to quantify the effect of charge delocalization on the rigidity of chromophores. We find that the role of the methyl group on the C13 position is pivotal regarding the ground state dynamics of the chromophore. Additionally, a correlation between quasi-equilibrium isomer distribution and fragmentation pathways is observed.

Low-Temperature Trapping of Photointermediates of the Rhodopsin E181Q Mutant

Three active-site components in rhodopsin play a key role in the stability and function of the protein: 1) the counter-ion residues which stabilize the protonated Schiff base, 2) water molecules, and 3) the hydrogen-bonding network. The ionizable residue Glu-181, which is involved in an extended hydrogen-bonding network with Ser-186, Tyr-268, Tyr-192, and key water molecules within the active site of rhodopsin, has been shown to be involved in a complex counter-ion switch mechanism with Glu-113 during the photobleaching sequence of the protein. Herein, we examine the photobleaching sequence of the E181Q rhodopsin mutant by using cryogenic UV-visible spectroscopy to further elucidate the role of Glu-181 during photoactivation of the protein. We find that lower temperatures are required to trap the early photostationary states of the E181Q mutant compared to native rhodopsin. Additionally, a Blue Shifted Intermediate (BSI, λ_max = 498 nm, 100 K) is observed after the formation of E181Q Bathorhodopsin (Batho, λ_max = 556 nm, 10 K) but prior to formation of E181Q Lumirhodopsin (Lumi, λ_max = 506 nm, 220 K). A potential energy diagram of the observed photointermediates suggests the E181Q Batho intermediate has an enthalpy value 7.99 KJ/mol higher than E181Q BSI, whereas in rhodopsin, the BSI is 10.02 KJ/mol higher in enthalpy than Batho. Thus, the Batho to BSI transition is enthalpically driven in E181Q and entropically driven in native rhodopsin. We conclude that the substitution of Glu-181 with Gln-181 results in a significant perturbation of the hydrogen-bonding network within the active site of rhodopsin. In addition, the removal of a key electrostatic interaction between the chromophore and the protein destabilizes the protein in both the dark state and Batho intermediate conformations while having a stabilizing effect on the BSI conformation. The observed destabilization upon this substitution further supports that Glu-181 is negatively charged in the early intermediates of the photobleaching sequence of rhodopsin.

Constraints of opsin structure on the ligand-binding site: studies with ring-fused retinals

Ring-fused retinal analogs were designed to examine the hula-twist mode of the photoisomerization of the 9-cis retinylidene chromophore. Two 9-cis retinal analogs, the C11-C13 five-membered ring-fused and the C12-C14 five-membered ring-fused retinal derivatives, formed the pigments with opsin. The C11-C13 ring-fused analog was isomerized to a relaxed all-trans chromophore (lambda(max) > 400 nm) at even -269 degrees C and the Schiff base was kept protonated at 0 degrees C. The C12-C14 ring-fused analog was converted photochemically to a bathorhodopsin-like chromophore (lambda(max) = 583 nm) at -196 degrees C, which was further converted to the deprotonated Schiff base at 0 degrees C. The model-building study suggested that the analogs do not form pigments in the retinal-binding site of rhodopsin but form pigments with opsin structures, which have larger binding space generated by the movement of transmembrane helices. The molecular dynamics simulation of the isomerization of the analog chromophores provided a twisted C11-C12 double bond for the C12-C14 ring-fused analog and all relaxed double bonds with a highly twisted C10-C11 bond for the C11-C13 ring-fused analog. The structural model of the C11-C13 ring-fused analog chromophore showed a characteristic flip of the cyclohexenyl moiety toward transmembrane segments 3 and 4. The structural models suggested that hula twist is a primary process for the photoisomerization of the analog chromophores.

Steric hindrance between chromophore substituents as the driving force of rhodopsin photoisomerization: 10-methyl-13-demethyl retinal containing rhodopsin

A visual chromophore analogue, 10-methyl-13-demethyl (dm) retinal, was synthesized and reconstituted with bleached bovine rhodopsin to form a visual pigment derivative with absorbance maximum at 505 nm. The investigations with this new compound were stimulated from recent results using 13-dm retinal as a chromophore that revealed a remarkable loss in quantum efficiency (phi of 13-dm retinal-containing rhodopsin: 0.30, Ternieden and Gärtner, J. Photochem. Photobiol. B Biol, 33, 83-86, 1996). The quantum efficiency of the new pigment was determined as 0.59 by quantitative bleaching using reconstituted rhodopsin as a reference. The very similar quantum efficiencies of rhodopsin and the new pigment give experimental support for the recently presented hypothesis that a steric hindrance between the substituents at positions 10 and 13 in 11-cis-retinal is elevated during the photoisomerization and thus facilitates the rapid photoisomerization of the visual chromophore (Peteanu et al., Proc. Natl. Acad. Sci. USA 90, 11762-11766, 1993). Such steric hindrance is removed from the molecule by the elimination of the methyl group from position 13 and can be re-established via a rearrangement of the substitution pattern by introducing a methyl group at position 10 of 13-dm retinal.

Mechanisms of opsin activation

Rhodopsin is constrained in an inactive conformation by interactions with 11-cis-retinal including formation of a protonated Schiff base with Lys296. Upon photoisomerization, major structural rearrangements that involve protonation of the active site Glu113 and cytoplasmic acidic residues, including Glu134, lead to the formation of the active form of the receptor, metarhodopsin II b, which decays to opsin. However, an activated receptor may be generated without illumination by addition of all-trans-retinal or its analogues to opsin, as measured in this study by the increased phosphorylation of opsin by rhodopsin kinase. The potency of stimulation depended on the chemical and isomeric nature of the analogues and the length of the polyene chain with all-trans-C17 aldehyde and all-trans-retinal being the most active and trans-C12 aldehyde being the least active. Certain cis-isomers, 11-cis-13-demethyl-retinal and 9-cis-C17 aldehyde, were also active. Most of the retinal analogues tested did not regenerate a spectrally identifiable pigment, and many were incapable of Schiff base formation (ketone, stable oximes, and Schiff base-derivatives of retinal). Thus, receptor activation resulted from formation of non-covalent complexes with opsin. pH titrations suggested that an equilibrium exists between partially active (protonated) and inactive (deprotonated) forms of opsin. These findings are consistent with a model in which protonation of one or more cytoplasmic carboxyl groups of opsin is essential for activity. Upon addition of retinoids, the partially active conformation of opsin is converted to a more active intermediate similar to metarhodopsin II b. The model provides an understanding of the structural requirements for opsin activation and an interpretation of the observed activities of natural and experimental opsin mutants.

Photolysis of rhodopsin results in deprotonation of its retinal Schiff's base prior to formation of metarhodopsin II

Absorption changes following photolysis of bovine rhodopsin in mildly sonicated membrane suspensions are monitored at 25 degrees C. Difference spectra collected at 17 times between 1 microsecond and 75 ms following excitation are analyzed globally using singular value decomposition and non-linear least-squares fitting techniques. The results are not consistent with the simple scheme: Lumirhodopsin-->Metarhodopsin I<-->Metarhodopsin II, but indicate that an intermediate with a deprotonated Schiff's base is formed nearly simultaneously with metarhodopsin I upon the decay of Lumirhodopsin.

Photointermediates of visual pigments

Much progress has been made in recent years toward understanding the interactions between various proteins responsible for visual transduction which are initiated by an activated state of visual pigments. However, the changes which take place in the visual pigments themselves to convert them to the activated state are more poorly understood. Many spectroscopic techniques have been applied to this problem in recent years and considerable progress has been made. A major goal of these efforts is to understand at which stages protein change occurs and to characterize its structural features. In the visual system evidence is accumulating, for example, that chromophore independent protein change begins immediately prior to lumirhodopsin formation. Considerable insight has been gained recently into the early intermediates of visual transduction and the stage is set to achieve similar understanding of the later intermediates leading to rhodopsin's activated state.

Early photolysis intermediates of the artificial visual pigment 13-demethylrhodopsin

Nanosecond time-resolved absorption measurements are reported for the room temperature photolysis of a modified rhodopsin pigment, 13-demethylrhodopsin, which contains the chromophore 13-demethylretinal. The measurements are consistent with the formation of an equilibrium between a BA-THO-13-demethylrhodopsin species and a blue-shifted species (relative to the parent pigment), BSI-13-demethylrhodopsin. The results are compared to those acquired after photolysis of native bovine rhodopsin [Hug, S. J., Lewis, J. W., Einterz, C. M., Thorgeirsson, T. E., & Kliger, D. S. (1990) Biochemistry (preceding paper in this issue)] and to results obtained after photolysis of several modified isorhodopsin pigments in which the BSI species was first observed. It is concluded that in all of the pigments the results are consistent with the formation of an equilibrium between BATHO and BSI, which subsequently decays on a nanosecond time scale at room temperature to a lumirhodopsin intermediate.

Photolysis intermediates of the artificial visual pigment cis-5,6-dihydro-isorhodopsin

The photolysis intermediates of an artificial bovine rhodopsin pigment, cis-5,6-dihydro-isorhodopsin (cis-5,6,-diH-ISORHO, lambda max 461 nm), which contains a cis-5,6-dihydro-9-cis-retinal chromophore, are investigated by room temperature, nanosecond laser photolysis, and low temperature irradiation studies. The observations are discussed both in terms of low temperature experiments of Yoshizawa and co-workers on trans-5,6-diH-ISORHO (Yoshizawa, T., Y. Shichida, and S. Matuoka. 1984. Vision Res. 24: 1455-1463), and in relation to the photolysis intermediates of native bovine rhodopsin (RHO). It is suggested that in 5,6-diH-ISORHO, a primary bathorhodopsin intermediate analogous to the bathorhodopsin intermediate (BATHO) of the native pigment, rapidly converts to a blue-shifted intermediate (BSI, lambda max 430 nm) which is not observed after photolysis of native rhodopsin. The analogs from lumirhodopsin (LUMI) to meta-II rhodopsin (META-II) are generated subsequent to BSI, similar to their generation from BATHO in the native pigment. It is proposed that the retinal chromophore in the bathorhodopsin stage of 5,6-diH-ISORHO is relieved of strain induced by the primary cis to trans isomerization by undergoing a geometrical rearrangement of the retinal. Such a rearrangement, which leads to BSI, would not take place so rapidly in the native pigment due to ring-protein interactions. In the native pigment, the strain in BATHO would be relieved only on a longer time scale, via a process with a rate determined by protein relaxation.

Photochemical reaction of 7,8-dihydrorhodopsin at low temperatures

The photoreaction of 9-cis-7,8-dihydrorhodopsin was examined at liquid nitrogen temperatures (-180 degrees C) in order to elucidate the photochemical events in visual pigments. This rhodopsin analog was prepared by incubating 9-cis-7,8-dihydroretinal with bovine opsin in the dark. 9-cis-7,8-Dihydrorhodopsin (lambda max = 427 nm) was cooled to -180 degrees C, and then irradiated at -180 degrees C with a 390 nm light, resulting in formation of its bathochromic product (lambda max = 465 nm). This result indicates that the presence of four double-bonds adjacent to the Schiff base nitrogen is sufficient to allow formation of a bathochromic product. Thus, the mechanism of formation of bathorhodopsin (in bovine rhodopsin system) may be considered as some change of the interaction between the conjugated double-bond system from C-9 to the Schiff base nitrogen and its surrounding charges in opsin, caused by rotation of 11-12 double-bond.