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PubMed1992

Mechanism of activation and inactivation of opsin: role of Glu113 and Lys296.

In previous studies, mutation of either Lys296 or Glu113 in bovine rhodopsin has been shown to result in constitutive activation of the apoprotein form, opsin [Robinson et al. (1992) Neuron 9, 719-725]. In this report, pH-rate profiles for the rhodopsin-catalyzed exchange of GTPgS for GDP on transducin are established for the constitutively active opsin mutants. All of the mutants, including the double-mutant E113Q,K296G, show a bell-shaped pH-rate profile. Therefore, it is evident that at least two ionizable groups in addition to Lys296 and Glu113 control the formation of the active opsin state. The sole effect of mutation at position 113 or 296 is to alter the ionization constant of the group with the higher pKa, called pka2. pKa2 decreases in the following order: rhodopsin/light (9.0) > K296E = K296G = E113Q,K296G (8.0) > E113Q (6.8) > K296H (6.6) >> wild-type opsin (< 5.0). These results are consistent with a model where activation of opsin involves (i) breaking of the salt bridge between Lys296 and Glu113, (ii) deprotonation of Lys296, and (iii) the net uptake of a proton from the solvent. Furthermore, exogenous addition of the chromophore all-trans-retinal shifts the wild-type and E113Q opsin equilibrium to favor the active state. In all these respects, the light-independent activation of the opsin mutants appears to proceed by a mechanism similar to that of light-activated rhodopsin.

Animals↗
PubMed1994

Unbleachable rhodopsin with an 11-cis-locked eight-membered ring retinal: the visual transduction process.

Visual transduction occurs through photorhodopsin, the primary photoproduct of rhodopsin, which relaxes to bathorhodopsin and a series of other intermediates until it reaches the metarhodopsin II stage, upon which the enzymatic cascade leading to vision is activated. Despite advances in areas related to visual transduction, the triggering process itself, a key problem in the chemistry of rhodopsin, has remained unsolved. In order to clarify the extent of involvement of the chromophoric excited state versus the 11-cis to trans isomerization, and as an extension of past studies with 11-cis-locked seven-membered ring rhodopsin (Rh7), 11-cis eight- and nine-membered ring retinal analogs, ret8 and ret9, respectively, have been synthesized. The bulkiness of the tetramethylene bridge in ret8 led to numerous unexpected obstacles in attempts to reconstitute a ret8-containing rhodopsin (Rh8) embedded in lipid bilayer membranes. These obstacles were solved by using methylated rhodopsin which gave MeRh8 containing 11-cis-ret8 as its chromophore. MeRh8 exhibited UV-vis and CD spectra very similar to those of native rhodopsin (Rh); furthermore, the quantum efficiency of photorhodopsin formation was comparable to that of Rh.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Bacteriorhodopsin can function without a covalent linkage between retinal and protein.

Light energy is transferred from retinal to the protein in bacteriorhodopsin after absorption of a photon resulting in changes of protein conformation. To examine whether the covalent bond, formed by the carbonyl group of retinal and the epsilon-amino group of lysine 216, is essential for this process, a mutant with lysine 216 replaced by alanine was expressed in Halobacterium salinarium L33 (BO-, retinal+). Reconstitution of the chromoprotein with varying retinylidene-n-alkylamines was possible in isolated membranes as well as in whole cells. When the protein in membranes with retinylidene Schiff bases of n-alkylamines of different lengths was reconstituted, the most stable chromoprotein was formed with retinylideneethylamine. The absorbance maximum was at 475 nm in alkaline solution and 620 nm in acidic solution. At neutral pH values both species equilibrate with a third one absorbing maximally at 568 nm. Reconstitution of whole cells with retinylideneethylamine led to a specific proton pump activity of 30 mol of protons per mol of BR per minute. This value indicates a lower limit of transport; no light saturation could be reached in these measurements in contrast to wild-type BR where transport activities of 162 mol of protons per mol of BR per minute under identical conditions can be achieved. Action spectra from flash photolysis experiments revealed that only the 568-nm form led to a M-intermediate with a half-time of decay of 17 ms. In summary, it could be shown that the covalent linkage between retinal and the protein is basically not required for the function of bacteriorhodopsin as a light-driven proton pump.

Bacteriorhodopsins↗
PubMed1994

Rat liver cytosolic retinal dehydrogenase: comparison of 13-cis-, 9-cis-, and all-trans-retinal as substrates and effects of cellular retinoid-binding proteins and retinoic acid on activity.

A basic pI retinal dehydrogenase has been purified recently that accounts for approximately 90% of the all-trans-retinal dehydrogenase activity of rat liver cytosol. In this work, we show that this enzyme also accounts for approximately 90% of the 9-cis-retinal dehydrogenase activity of rat liver cytosol. The partially-purified enzyme displayed allosteric kinetics for 9-cis-retinal [K0.5 = 5.2 microM, Hill coefficient = 1.4, Vmax = 7.85 nmol min-1 (mg of protein)-1] with the ratio Vmax/K0.5 = 1.5. The latter is similar to that of 2.1 for all-trans-retinal [K0.5 = 1.6 microM, Hill coefficient = 1.4, Vmax = 3.4 nmol min-1 (mg of protein)-1]. Competition between all-trans- and 9-cis-retinal occurred only when micromolar concentrations of both were present, indicating that the dehydrogenase could catalyze both all-trans- and 9-cis-retinoic acid syntheses simultaneously at the nanomolar amounts of the retinals that are likely to occur physiologically. Although reactions of all-trans- and 9-cis-retinoids were catalyzed with similar efficiencies, 13-cis-retinal was not an efficient substrate. This retinal dehydrogenase was not feedback-inhibited by all-trans- or 9-cis-retinoic acid, nor by holocellular retinoic acid-binding protein, but was stimulated modestly by apocellular retinoic acid-binding protein, an effect not observed in the presence of cellular retinol-binding protein. These data indicate that products, via feedback inhibition, do not regulate retinoic acid synthesis by this dehydrogenase. This dehydrogenase may serve as a common enzyme in the conversion of all-trans- and 9-cis-retinal into their acids.

Aldehyde Oxidoreductases↗
PubMed1994

A covalent link between the chromophore and the protein backbone of bacteriorhodopsin is not required for forming a photochemically active pigment analogous to the wild type.

Bacteriorhodopsin pigments lacking the retinal-Lys-216 covalent bond were prepared by reconstituting the K216G mutant protein with retinal alkylamine Schiff bases. The procedure follows the approach of Zhukovsky et al. [Zhukovsky, E., Robinson, P., & Oprian, D. (1991) Science 251, 558-560] in the case of visual (rhodopsin) pigments. Reconstitution leads to a mixture of three pigments. One of them, bR(K216G)/566a, absorbs (pH = 6.9) at 566 nm. Its absorption is pH-dependent, exhibiting a purple to blue transition. The pigment's laser-induced photocycle patterns are similar to those of wild-type all-trans-bR. A second component, bR(K216G)/566b, exhibits an independent photocycle reminiscent of that of wild-type 13-cis-bR. A third pigment component, bR(K216G)/630, absorbs around 630 nm. Experiments in the presence of a pH dye indicator show that illumination of bR(K216G)/566 produces a detectable proton gradient. It is concluded that a covalent bond between the retinal chromophore and the protein backbone is not a prerequisite for the basic structure and photochemical features of bR or for its proton pump activity.

Bacteriorhodopsins↗
PubMed1994

Deuterium solid-state nuclear magnetic resonance studies of methyl group dynamics in bacteriorhodopsin and retinal model compounds: evidence for a 6-s-trans chromophore in the protein.

Solid-state deuterium NMR spectroscopy is used to examine the dynamic behavior of 18-CD3 methyl groups in microcrystalline 6-s-cis-retinoic acid (triclinic) and 6-s-trans-retinoic acid (monoclinic) model compounds, as well as in the membrane protein bacteriorhodopsin (bR), regenerated with CD3-labeled retinal. Temperature dependent quadrupolar echo line shapes and T1 anisotropy measurements were used to characterize activation energies for 3-fold hopping motion of the methyl groups. These data provide supporting evidence that the conformation of the retinal chromophore in bR is 6-s-trans. The 6-s-cis conformer is characterized by strong eclipsing interactions between the 8-C proton and the 18-C methyl group protons; the 18-CD3 group shows an activation energy barrier for methyl 3-fold hopping of 14.5 +/- 1 kJ/mol. In contrast, the 18-CD3 group in the 6-s-trans isomer shows a considerably lower activation energy barrier of 5 +/- 1 kJ/mol. In bR, it is possible to obtain an approximate activation energy of 9 kJ/mol. This data is inconsistent with a 6-s-cis conformer but is consistent with the existence of a 6-s-trans-retinal Schiff base in bR with some interaction with the protein matrix. These results suggest that methyl rotor motions can be used to probe the van der Waals contact between a ligand and a protein binding pocket. The 6-s-trans conformer of the [16,17-(CD3)2]retinal in frozen hexane exhibits a major kinetic component with an activation energy barrier of of 14 -/+ 2 kJ/mol.(ABSTRACT TRUNCATED AT 250 WORDS)

Bacteriorhodopsins↗
PubMed1994

Distorted structure of the retinal chromophore in bacteriorhodopsin resolved by 2H-NMR.

Structural details about the geometry of the retinal chromophore in the binding pocket of bacteriorhodopsin are revealed by measuring the orientations of its individual methyl groups. Solid-state 2H-NMR measurements were performed on macroscopically oriented samples of purple membrane patches, containing retinal specifically deuterium-labeled at one of the three methyl groups along the polyene chain (C18, C19, C20). The deuterium quadrupole splitting of each "zero-tilt" spectrum is used to calculate the orientation of the corresponding C-CD3 bond vector with respect to the membrane normal; however, two possible solutions may arise. These ambiguities in angle could be resolved by recording a tilt series of spectra at different sample inclinations to the magnetic field and analyzing the resulting complex line shapes with the aid of computer simulations. The angles for the C18, C19, and C20 group are found to be 37 +/- 1 degree, 40 +/- 1 degree, and 32 +/- 1 degree, respectively. These highly accurate values imply that the polyene chain of the retinal chromophore is not straight but rather has an in-plane curvature and possibly an out-of-plane twist. Together with the angles of the remaining methyl groups on the cyclohexene ring that have been measured previously, an overall picture has thus emerged of the intramolecular conformation and the three-dimensional orientation of retinal within bacteriorhodopsin. The deduced geometry confirms and refines the known structural information on the chromophore, suggesting that this 2H-NMR strategy may serve as a valuable tool for other membrane proteins.

Bacteriorhodopsins↗
PubMed2001

Salt dependence of the formation and stability of the signaling state in G protein-coupled receptors: evidence for the involvement of the Hofmeister effect.

We studied the salt dependence of both the stability and the equilibrium of the late photoproducts metarhodopsin I (MI) and II (MII) of the artificial visual pigment 9-demethyl rhodopsin (9dm-Rho). In the photoproducts of 9dm-Rho, all-trans-9-demethyl retinal acts only as a partial agonist, enabling us to study the photoproduct equilibrium of the pigment both in membranes and in detergent micelles. Chloride, bromide, and phosphate salts shift this equilibrium from the inactive MI to the active MII receptor conformation both in native membranes and even more with purified pigment in detergent micelles. In the presence of these salts, the induced MII state seems to be structurally intact, as judged by Fourier transform infrared (FTIR) and UV-vis spectroscopy. In the long term, however, we observe an increased instability of the photoproducts and a change in the decay pathways. Both MII enhancement and destabilization are particularly pronounced with the strong chaotropic salts KI and KSCN. The results fit into the framework of the Hofmeister effect and are assigned to an increased solvation of the peptide moiety of the solvent-exposed domains, their resulting partial disordering favoring MII over MI. In this picture, increased solvation also affects helix-helix interactions, thereby leading to a structural instability of the protein in the long term. The reported influences of salts on conformation and stability of this membrane protein are likely to be general and may therefore also apply to other transmembrane proteins and particularly to other G protein-coupled receptors.

Animals↗
PubMed1994

Fluorescence studies of the location and membrane accessibility of the palmitoylation sites of rhodopsin.

Fluorescent fatty acid labels have been incorporated into the palmitoylation sites of rhodopsin and used to probe the membrane accessibility and location of these sites. The fluorescence properties of anthroyloxy and pyrenyl fatty acids bound to rhodopsin were investigated in a reconstituted vesicle system. Collisional quenching of fluorescence by stearic acid (DSA) labeled with doxyls in the 16, 12, and 5 positions was used to determine the membrane accessibility and disposition of the modifying fatty acids. To properly determine the membrane concentration of these quenchers, the dependence of the Stern-Volmer parameters on both quencher and vesicle concentration was determined. An analysis of these dependences provided a correction for partitioning of the quencher between the aqueous phase and the membrane. After this correction, the relative effectiveness of doxyl quenchers was 16-DSA > 12-DSA > 5-DSA. Parallel studies on free anthroyloxy and pyrenyl fatty acids incorporated into the reconstituted system showed the same dependence on quencher position. These results indicate that the labels at the palmitoylation sites of rhodopsin are situated in the membrane much as a free fatty acid. This anchoring of the palmitates in the membrane results in the formation of a fourth cytoplasmic loop.

Acyl Coenzyme A↗
PubMed1994

Structure and function in rhodopsin. 7. Point mutations associated with autosomal dominant retinitis pigmentosa.

Autosomal dominant retinitis pigmentosa (ADRP) is a hereditary form of retinitis pigmentosa which accounts for about 15% of all types of the latter disease. Recently, close to 50 mutations, mostly point mutations, have been identified in the rhodopsin gene in ADRP patients. We have introduced these mutations in the synthetic bovine rhodopsin gene and herein report on the expression of the mutant genes in COS-1 cells and studies in vitro of the properties of the expressed opsins. The mutant phenotypes fall into three classes: Class I mutants are expressed in COS-1 cells at wild-type levels, form the normal rhodopsin chromophore with 11-cis-retinal, and are transported to the cell surface. However, on illumination, they activate transducin inefficiently. Class II mutants remain in the endoplasmic reticulum and do not bind 11-cis-retinal to form the chromophore. Class III mutants are expressed at low levels and form rhodopsin chromophore only poorly. They also remain in the endoplasmic reticulum and, as expected, show high mannose glycosylation. Nearly all of the mutants studied show abnormal sensitivity to light compared to the wild type, and they activate transducin less efficiently. We conclude that the majority of the ADRP mutants have folding defects.

Amino Acid Sequence↗
PubMed1994

Determination of internuclear distances and the orientation of functional groups by solid-state NMR: rotational resonance study of the conformation of retinal in bacteriorhodopsin.

We have used a new solid-state NMR technique--rotational resonance--to determine both internuclear distances and the relative orientations of chemical groups (dihedral angles) in retinal bound to bacteriorhodopsin (bR) and in retinoic acid model compounds. By matching the rotational resonance condition (delta = n omega r/2 pi, where delta is the difference in isotropic chemical shifts for two dipolar coupled spins, omega r/2 pi is the mechanical rotational frequency of the sample in the MAS experiment, and n is a small integer denoting the order of the resonance), we selectively reintroduce the dipolar coupling and enhance the rate of magnetization exchange. Spectroscopic data and theoretical simulations of the magnetization exchange trajectories for the 8,18-13C dipolar coupled pair in retinoic acid model compounds, crystallized in both the 6-s-cis and 6-s-trans forms, indicate that an accurate determination of the internuclear distance is possible. For the n = 1 resonance we find the distance determination to be reasonably independent of the relative orientation of the groups. In contrast, for the n = 2 resonance, there is a more pronounced dependence on the relative orientation of the groups which permits an estimate of the angle around the 6-s bond for the cis and trans forms to be 42 +/- 5 degrees and 90 +/- 10 degrees, respectively, in good agreement with crystallography. In bR we demonstrate that the 8-13C-18-13C distance is 4.1 A and the average 8-13C-16-13C/8-13C-17-13C distance is 3.3-3.5 A.(ABSTRACT TRUNCATED AT 250 WORDS)

Bacteriorhodopsins↗
PubMed1994

Effects of carboxyl-terminal truncation on the stability and G protein-coupling activity of bovine rhodopsin.

A number of studies have suggested that G protein-coupled receptors possess domains within the carboxyl terminus that are important for the catalytic activation of G proteins. To define these regions, truncation mutants were generated in the cDNA of bovine rhodopsin, the receptor responsible for visual signal transduction in the retinal rod cell. The mutants were expressed in HEK-293 cells and analyzed for their ability to bind the chromophore, 11-cis-retinal, and for activating Gt, the G protein of the rod cell regulated by rhodopsin. Removal of 38 carboxyl-terminal amino acids resulted in the production of a mutant (K311 stop) that does not bind 11-cis-retinal, has an abnormal pattern of glycosylation, and does not catalyze light-dependent binding of GTP gamma S to Gt, suggesting that it is unable to fold properly during biogenesis. However, a truncation mutant with only five additional amino acids (C316stop) coupled normally to Gt, using membranes from transfected cells, despite the fact that it lacked the "fourth cytoplasmic loop" formed by palmitoylation of cysteines-322 and -323. When C316stop is extracted from the membrane with detergent, only a fraction is able to bind 11-cis-retinal, but the fraction that binds retinal activates Gt normally. In contrast, detergent-solubilized wild-type rhodopsin and K325stop (a truncation mutant with the longest carboxyl terminus) both bind retinal and activate Gt normally. These data suggest that the proximal region of the carboxyl terminus is critical for the proper folding and stability of the rhodopsin molecule and that amino acids Cys316 to Ala348 are not necessary for the activation of Gt.

Amino Acid Sequence↗
PubMed1994

Solid state 13C and 15N NMR investigations of the N intermediate of bacteriorhodopsin.

Previous solid state 13C NMR studies of bacteriorhodopsin (bR) have inferred the C = N configuration and the protonation state of the retinal-lysine Schiff base (SB) linkage from the [13-13C]-retinal, [14-13C]retinal, and [epsilon-13C]lysine-216 chemical shifts in the bR555, bR568, and M412 states. Here we determine the C = N configuration and the protonation state of the N photointermediate that is cryotrapped along with the M photointermediate at high salt concentrations (0.1 M NaCl) and high pH (10.0). We obtained 13C and 15N SSN MR spectra of [epsilon-15N]lysine bR and [12-13C]- and [13-13C]retinal bR for samples illuminated under the above conditions. Two species are observed, both of which decay to bR568 upon warming. One species has chemical shifts identical to those obtained previously for M thermally trapped in guanidine.HCl at high pH (Smith et al., 1989a; Farrar et al., 1993). In the other species, the [epsilon-15N]lysine and 13-13C chemical shifts indicate that the SB is protonated, the 12-13C shift indicates a 13 = 14 cis configuration, and the previously published [14-13C]- and [epsilon-13C]lysine shifts indicate a C = N anti configuration. These results are consistent with other studies of the N photointermediate.

Bacteriorhodopsins↗
PubMed1994

Is chicken green-sensitive cone visual pigment a rhodopsin-like pigment? A comparative study of the molecular properties between chicken green and rhodopsin.

Chicken green is a visual pigment present in chicken green-sensitive cones and has an amino acid sequence more similar than any other cone visual pigments to the rod visual pigments, rhodopsins. Here we have investigated the molecular properties of chicken green and compared them with those of rhodopsin to elucidate whether or not chicken green is a rhodopsin-like pigment. While chicken green has a molecular extinction coefficient and a photosensitivity very similar to those of rhodopsin, it displays faster regeneration from 11-cis-retinal and opsin and faster formation and decay of the physiologically active meta II intermediate than rhodopsin. These differences correlate with the physiological difference between cones and rods. Thus in spite of the similarity in amino acid sequence, chicken green displays molecular properties required for a cone visual pigment that are clearly different from those of rhodopsin.

Animals↗
PubMed1994

Probing of the retinal binding site of bacteriorhodopsin by affinity labeling.

The position of the chromophore within bacteriorhodopsin has been identified by cross-linking a cysteine group, introduced by site-specific mutagenesis, with a chromophore suitably derivatized with an active leaving group. Since bacteriorhodopsin has no cysteines, a site-specific cysteine mutant will contain only one free sulfhydryl group capable of reacting with the retinal analog. Met118, Thr121, and Ser141 were selected to be mutated to cysteine. No pigment absorbing in the visible region was obtained for the Ser141Cys mutant. The Met118Cys and Thr121Cys mutants have similar absorption maxima, proton pumping efficiencies and photocycles to those of the wild-type pigment. 4-Bromoretinal, in which the reactive allylic halide readily undergoes nucleophilic displacement, was used as the reactive chromophore. Pigments were obtained on reaction of all-trans-4-bromoretinal with the apoproteins of Met118Cys, Thr121Cys, and wild-type bacteriorhodopsin (lambda max = 464-470 nm). Analysis of the denatured pigments on SDS-polyacrylamide gels showed incorporation of tritiated chromophore into the Met118Cys mutant but not into the wild-type or Th4121Cys pigments. Met118Cys apoprotein which was preincubated with the cysteine-specific reagent N-ethylmaleimide formed a pigment with 4-bromoretinal but no cross-linking was observed, providing evidence that the cross-linking of the chromophore is to the cysteine at 118. We conclude that Met118 is positioned in the chromophore binding pocket, proximal to the C-4 position of cyclohexyl ring of retinal.

Affinity Labels↗
PubMed1991

Design of a helix-bundle cross-link: NMR and UV-visible spectroscopic analyses and molecular modeling of ring-oxidized retinals.

In order to generate potential chemical cross-links for studying the chromophore binding site of bacteriorhodopsin and related helix-bundle proteins, MnO2 was used to oxidize all-trans-retinal's ring moiety. The structures and solution conformations of three ring-oxidized retinal analogues have been determined by using UV-visible absorption and 1H and 13C NMR spectroscopies, primarily with regard to (i) the introduction of a functional group at the ring end of the chromophore, (ii) the retention of the all-trans geometry of the polyenal side chain, and (iii) the torsional angle of the ring-polyenal bond. Analyses of their UV-visible absorption spectral parameters (lambda max, epsilon max, and vibrational fine structure) and NMR spectral parameters (1H-1H coupling constants, 1H and 13C NMR chemical shifts, and 1H homonuclear Overhauser effects) indicated the 4-oxo and the 2,3-dehydro-4-oxo derivatives both possess the twisted 6-s-cis conformation adopted by most six-membered ring analogues of retinal in solution or crystal. However, the alpha-dioxocyclopentenyl analogue exists in solution predominantly (70-80%) as the planar 6-s-trans conformer, similar to violerythrine chromophore analogues. In order to identify the minor solution forms, molecular modeling and geometry optimizations using the semiempirical molecular orbital method AM1 defined two additional symmetry-related minima at +/- 30-40 degrees in its C6-C7 torsional energy profile. Because the chromophores of bacterio- and halorhodopsins and sensory rhodopsins are bound as the 6-s-trans conformer [Harbison, G.S., Smith, S.O., Pardoen, J.A., Courtin, J.M.L., Lugtenburg, J., Herzfeld, J., Mathies, R.A., & Griffin, R.G. (1985) Biochemistry 24, 6955-6962; Baselt, D.R., Fodor, S.P.A., van der Steen, R., Lugtenburg, J., Bogomolni, R.A., & Mathies, R.A. (1989) Biophys. J. 55, 193-196], we suggest that the cyclopentenyl analogue's alpha-diketo function may be favorably positioned within the binding pocket and sufficiently reactive toward nucleophilic attack to cross-link an arginine located in or near the ring end of the chromophore cavity: Arg134 according to the current model of bacteriorhodopsin's tertiary structure [Henderson, R., Baldwin, J.M., Ceska, T.A., Zemlin, F., Beckmann, E., & Downing, K.H. (1990) J. Mol. Biol. 213, 899-929] or Arg82 as postulated from an alternate model constructed primarily to accommodate the external point charge contribution to bacteriorhodopsin's opsin shift.

Bacteriorhodopsins↗
PubMed2001

All-trans-retinal forms a visible-absorbing pigment with human rod opsin.

Rhodopsin activation elicits transmembrane currents due to electrostatic events associated with conformational changes. We employed the sensitive rhodopsin early receptor current approach to reevaluate whether all-trans-retinal can form a visual pigment with rod opsin apoprotein. An opsin shift above 440 nm is induced in the action spectrum of charge motions caused by visible flashes in cells expressing human rod opsin and regenerated with all-trans-retinal, compared to cells without opsin. Near-ultraviolet stimulation of opsin regenerated with all-trans-retinal promotes charge motions similar to those arising from the meta-II signaling state while photochemically regenerating a pigment with ground state charge motion properties. These results indicate that all-trans-retinal can form a visual pigment with opsin, through both protonated and unprotonated Schiff base linkages and likely within the native ligand binding pocket at lysine-296. The agonist effects of all-trans-retinal may relate to its structural accommodation within the core of opsin, similar to other G-protein-coupled receptors.

Apoproteins↗
PubMed1991

Spectroscopic study of the batho-to-lumi transition during the photobleaching of rhodopsin using ring-modified retinal analogues.

Photochemical and subsequent thermal reactions of rhodopsin containing 9-cis-retinal [Rh(9)] or one of four analogues with 9-cis geometries formed from ring-modified retinals, alpha-retinal [alpha Rh(9)], acyclic retinal [AcRh(9)], acyclic alpha-retinal [Ac alpha Rh(9)], and 5-isopropyl-alpha-retinal [P alpha Rh(9)] were investigated by low-temperature spectrophotometry and nanosecond laser photolysis. Irradiation of each pigment at -180 degrees C produced a photosteady-state mixture containing the original 9-cis pigment, its 11-cis pigment, and a photoproduct, indicating that the primary process of each pigment is a photoisomerization of its chromophore. The photoproduct produced by the irradiation of AcRh(9) had an absorption spectrum red shifted from the original AcRh(9) and was identified as the batho intermediate of AcRh(9). It was converted to the lumi intermediate through a metastable species, the BL intermediate, which has never been detected in Rh(9) at low temperature and whose absorption maximum was at shorter wavelengths than that of the batho intermediate. In contrast, the absorption maxima of the photoproducts produced from the other analogue pigments were at shorter wavelengths than those of the original pigments. They were identified as BL intermediates on the basis of their absorption maxima and thermal stabilities. The formation time constant of the lumi intermediate at room temperature was found to be dependent on the extent of modification of the ring portion of the chromophore, decreasing with the complete truncation of the cyclohexenyl ring [Ac alpha Rh(9)] and increasing with the attachment of the isopropyl group to the ring [P alpha Rh(9)].(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗