Regeneration of rhodopsin and bacteriorhodopsin. The role of retinal analogues as inhibitors

The role of carotenoids in proton-pumping rhodopsin as a primitive solar energy conversion system

Rhodopsin and carotenoids are two molecules that certain bacteria use to absorb and utilize light. Type I rhodopsin, the simplest active proton transporter, converts light energy into an electrochemical potential. Light produces a proton gradient, which is known as the proton motive force across the cell membrane. Some carotenoids are involved in light absorbance and transfer of absorbed energy to chlorophyll during photosynthesis. A previous study in Salinibacter ruber has shown that carotenoids act as antennae to harvest light and transfer energy to retinal in xanthorhodopsin (XR). Here, we describe the role of canthaxanthin (CAN), a carotenoid, as an antenna for Gloeobacter rhodopsin (GR). The non-covalent complex formed by the interaction between CAN and GR doubled the proton pumping speed and improved the pumping capacity by 1.5-fold. The complex also tripled the proton pumping speed and improved the pumping capacity by 5-fold in the presence of strong and weak light, respectively. Interestingly, when canthaxanthin was bound to Gloeobacter rhodopsin, it showed a 126-fold increase in heat resistance, and it survived better under drought conditions than Gloeobacter rhodopsin. The results suggest direct complementation of Gloeobacter rhodopsin with a carotenoid for primitive solar energy harvesting in cyanobacteria.

The role of the non-covalent β-ionone-ring binding site in rhodopsin: historical and physiological perspective

Bleached rhodopsin regenerates by way of the Schiff base formation between the 11-cis retinal and opsin. Recovery of human vision from light adapted states follows biphasic kinetics and each adaptive phase is assigned to two distinct classes of visual pigments in cones and rods, respectively, suggesting that the speed of Schiff base formation differs between iodopsin and rhodopsin. Matsumoto and Yoshizawa predicted the existence of a β-ionone ring-binding site in rhodopsin, which has been proven by structural studies. They postulated that rhodopsin regeneration starts with a non-covalent binding of the β-ionone ring moiety of 11-cis-retinal, followed by the Schiff base formation. Recent physiological investigation revealed that non-covalent occupation of the β-ionone ring binding site transiently activates the visual transduction cascade in the dark. In order to understand the role of non-covalent binding of 11-cis-retinal to opsin during regeneration, we studied the kinetics of rhodopsin regeneration from opsin and 11-cis-retinal and found that the Schiff base formation is accelerated ∼10(7) times compared to that between retinal and free amine. According to Cordes and Jencks, Schiff base formation in solution exhibits a bell-shaped pH dependence. However, we discovered that the rhodopsin formation is independent of pH over a wide pH range, suggesting that aqueous solvents do not have access to the Schiff base milieu during its formation. According to Hecht et al. the regeneration of iodopsin must be significantly faster than that of rhodopsin. Does this suggest that the Schiff base formation in iodopsin is favored due to its structural architecture? The iodopsin structure once solved would answer such a question as how molecular fine-tuning of retinal proteins realizes their dark adaptive functions. In contrast, bacteriorhodopsin does not require occupancy of a distinct β-ionone ring-binding site, enabling an aldehyde without the cyclohexene ring to form a pigment. Studies of regeneration reaction of other retinal proteins, which are scarcely available, would clarify the molecular structure-phenotype relationships and their physiological roles.

Vitamin A activates rhodopsin and sensitizes it to ultraviolet light

The visual pigment, rhodopsin, consists of opsin protein with 11-cis retinal chromophore, covalently bound. Light activates rhodopsin by isomerizing the chromophore to the all-trans conformation. The activated rhodopsin sets in motion a biochemical cascade that evokes an electrical response by the photoreceptor. All-trans retinal is eventually released from the opsin and reduced to vitamin A. Rod and cone photoreceptors contain vast amounts of rhodopsin, so after exposure to bright light, the concentration of vitamin A can reach relatively high levels within their outer segments. Since a retinal analog, β-ionone, is capable of activating some types of visual pigments, we tested whether vitamin A might produce a similar effect. In single-cell recordings from isolated dark-adapted salamander green-sensitive rods, exogenously applied vitamin A decreased circulating current and flash sensitivity and accelerated flash response kinetics. These changes resembled those produced by exposure of rods to steady light. Microspectrophotometric measurements showed that vitamin A accumulated in the outer segments and binding of vitamin A to rhodopsin was confirmed in in vitro assays. In addition, vitamin A improved the sensitivity of photoreceptors to ultraviolet (UV) light. Apparently, the energy of a UV photon absorbed by vitamin A transferred by a radiationless process to the 11-cis retinal chromophore of rhodopsin, which subsequently isomerized. Therefore, our results suggest that vitamin A binds to rhodopsin at an allosteric binding site distinct from the chromophore binding pocket for 11-cis retinal to activate the rhodopsin, and that it serves as a sensitizing chromophore for UV light.

Binding of more than one retinoid to visual opsins

Visual opsins bind 11-cis retinal at an orthosteric site to form rhodopsins but increasing evidence suggests that at least some are capable of binding an additional retinoid(s) at a separate, allosteric site(s). Microspectrophotometric measurements on isolated, dark-adapted, salamander photoreceptors indicated that the truncated retinal analog, β-ionone, partitioned into the membranes of green-sensitive rods; however, in blue-sensitive rod outer segments, there was an enhanced uptake of four or more β-ionones per rhodopsin. X-ray crystallography revealed binding of one β-ionone to bovine green-sensitive rod rhodopsin. Cocrystallization only succeeded with extremely high concentrations of β-ionone and binding did not alter the structure of rhodopsin from the inactive state. Salamander green-sensitive rod rhodopsin is also expected to bind β-ionone at sufficiently high concentrations because the binding site is present on its surface. Therefore, both blue- and green-sensitive rod rhodopsins have at least one allosteric binding site for retinoid, but β-ionone binds to the latter type of rhodopsin with low affinity and low efficacy.

{beta}-Apocarotenoids do not significantly activate retinoic acid receptors {alpha} or {beta}

beta-Carotene oxygenase 2 cleaves beta-carotene asymmetrically at non-central double bonds of the polyene chain, yielding apocarotenal molecules. The hypothesis tested was that apocarotenoids are able to stimulate transcription by activating retinoic acid receptors (RARs). The effects of long- and short-chain apocarotenals and apocarotenoic acids on the activation of RARalpha and RARbeta transfected into monkey kidney fibroblast cells (CV-1) were investigated. We synthesized or purified beta-apo-8'-carotenoic acid (apo-8'-CA), beta-apo-14'-carotenoic acid (apo-14'-CA), beta-cyclocitral (BCL), beta-cyclogernanic acid (BCA), beta-ionone (BI), beta-ionylideneacetaldehyde (BIA) beta-ionylideneacetic acid (BIAA) and a C13 ketone, beta-apo-13-carotenone (C13). None of the apocarotenoids tested showed significant transactivation activity for the RARs when compared with all-trans retinoic acid (RA). The results suggest that biological effects of these apocarotenoids are through mechanisms other than activation of RARalpha and beta.

Refinement of the geometry of the retinal binding pocket in dark-adapted bacteriorhodopsin by heteronuclear solid-state NMR distance measurements

The bacterial proton pump bacteriorhodopsin (BR) is a 26.5 kDa seven-transmembrane helical protein. Several structural models have been published at > or =1.55 A resolution. The initial cis-trans isomerization of the retinal moiety involves structural changes within <1 A. To understand the chromophore-protein interactions that are important for light-driven proton transport, very accurate measurements of the protein geometry are required. To reveal more structural details at the site of the retinal, we have, therefore, selectively labeled the tryptophan side chains of BR with (15)N and metabolically incorporated retinal, (13)C-labeled at position 14 or 15. Using these samples, heteronuclear distances were measured with high accuracy using SFAM REDOR magic angle spinning solid-state NMR spectroscopy in dark-adapted bacteriorhodopsin. This NMR technique is applied for the first time to a high-molecular mass protein. Two retinal conformers are distinguished by their different isotropic 14-(13)C chemical shifts. Whereas the C14 position of 13-cis-15-syn-retinal is 4.2 A from [indole-(15)N]Trp86, this distance is 3.9 A in the all-trans-15-anti conformer. This latter distance allows us to check on the details of the active center of BR in the various published models derived from X-ray and electron diffraction data. The experimental approach and the results reported in this paper enforce the notion that distances between residues of a membrane protein binding pocket and a bound ligand can be determined at subangstrom resolution.

Effect of 11-cis 13-demethylretinal on phototransduction in bleach-adapted rod and cone photoreceptors

We used 11-cis 13-demethylretinal to examine the physiological consequences of retinal's noncovalent interaction with opsin in intact rod and cone photoreceptors during visual pigment regeneration. 11-Cis 13-demethylretinal is an analog of 11-cis retinal in which the 13 position methyl group has been removed. Biochemical experiments have shown that it is capable of binding in the chromophore pocket of opsin, forming a Schiff-base linkage with the protein to produce a pigment, but at a much slower rate than the native 11-cis retinal (Nelson, R., J. Kim deReil, and A. Kropf. 1970. Proc. Nat. Acad. Sci. USA. 66:531-538). Experimentally, this slow rate of pigment formation should allow separate physiological examination of the effects of the initial binding of retinal in the pocket and the subsequent formation of the protonated Schiff-base linkage. Currents from solitary rods and cones from the tiger salamander were recorded in darkness before and after bleaching and then after exposure to 11-cis 13-demethylretinal. In bleach-adapted rods, 11-cis 13-demethylretinal caused transient activation of phototransduction, as evidenced by a decrease of the dark current and sensitivity, acceleration of the dim flash responses, and activation of cGMP phosphodiesterase and guanylyl cyclase. The steady state of phototransduction activity was still higher than that of the bleach-adapted rod. In contrast, exposure of bleach-adapted cones to 11-cis 13-demethylretinal resulted in an immediate deactivation of transduction as measured by the same parameters. These results extend the validity of a model for the effects of the noncovalent binding of a retinoid in the chromophore pockets of rod and cone opsins to analogs capable of forming a Schiff-base and imply that the noncovalent binding by itself may play a role for the dark adaptation of photoreceptors.

Conformational change in bacterio-opsin on binding to retinal

The detailed mechanism of retinal binding to bacterio-opsin is important to understanding retinal pigment formation as well as to the process of membrane protein folding. We have measured the temperature dependence of bacteriorhodopsin formation from bacterio-opsin and all-trans retinal. An Arrhenius plot of the apparent second-order rate constants gives an activation energy of 11.6 +/- 0.7 kcal/mol and an activation entropy of -4 +/- 2 cal/mol deg. Comparison of the activation entropy to model compound reactions suggests that chromophore formation in bacteriorhodopsin involves a substantial protein conformational change. Cleavage of the polypeptide chain between residues 71 and 72 has little effect on the activation energy or entropy, indicating that the connecting loop between helices B and C is not involved in this conformational change.

Occupancy of the chromophore binding site of opsin activates visual transduction in rod photoreceptors

The retinal analogue beta-ionone was used to investigate possible physiological effects of the noncovalent interaction between rod opsin and its chromophore 11-cis retinal. Isolated salamander rod photoreceptors were exposed to bright light that bleached a significant fraction of their pigment, were allowed to recover to a steady state, and then were exposed to beta-ionone. Our experiments show that in bleach-adapted rods beta-ionone causes a decrease in light sensitivity and dark current and an acceleration of the dim flash photoresponse and the rate constants of guanylyl cyclase and cGMP phosphodiesterase. Together, these observations indicate that in bleach-adapted rods beta-ionone activates phototransduction in the dark. Control experiments showed no effect of beta-ionone in either fully dark-adapted or background light-adapted cells, indicating direct interaction of beta-ionone with the free opsin produced by bleaching. We speculate that beta-ionone binds specifically in the chromophore pocket of opsin to produce a complex that is more catalytically potent than free opsin alone. We hypothesize that a similar reaction may occur in the intact retina during pigment regeneration. We propose a model of rod pigment regeneration in which binding of 11-cis retinal to opsin leads to activation of the complex accompanied by a decrease in light sensitivity. The subsequent covalent attachment of retinal to opsin completely inactivates opsin and leads to the recovery of sensitivity. Our findings resolve the conflict between biochemical and physiological data concerning the effect of the occupancy of the chromophore binding site on the catalytic potency of opsin. We show that binding of beta-ionone to rod opsin produces effects opposite to its previously described effects on cone opsin. We propose that this distinction is due to a fundamental difference in the interaction of rod and cone opsins with retinal, which may have implications for the different physiology of the two types of photoreceptors.

Enhancement of opsin activity by all-trans-retinal

The rod cell photoreceptor apoprotein, opsin, activates the G-protein, transducin, although at a much reduced level than light-activated rhodopsin. The ability of all-trans-retinal to enhance opsin apoprotein activity was investigated using a guanyl nucleotide exchange assay on transducin. All-trans-retinal enhanced opsin activity in a concentration-dependent manner. At high concentrations of all-trans-retinal, the activity of the all-trans-retinal-opsin complex was comparable to that from an equimolar amount of metarhodopsin(II). However, in contrast to metarhodopsin(II), the active all-trans-retinalopsin complex did not require a stable Schiff base linkage between opsin and all-trans-retinal. The lack of a stable Schiff base and differences in activity at high pH imply that opsin and all-trans-retinal form a complex that is distinct from metarhodopsin(II). The ability of all-trans-retinal to stimulate the transduction cascade may be a source of post-bleach noise in photoreceptors.

Binding pathway of retinal to bacterio-opsin: a prediction by molecular dynamics simulations

Formation of bacteriorhodopsin (bR) from apoprotein and retinal has been studied experimentally, but the actual pathway, including the point of entry, is little understood. Molecular dynamics simulations provide a surprisingly clear prediction. A window between bR helices E and F in the transmembrane part of the protein can be identified as an entry point for retinal. Steered molecular dynamics, performed by applying a series of external forces in the range of 200-1000 pN over a period of 0.2 ns to retinal, allows one to extract this chromophore from bR once the Schiff base bond to Lys216 is cleaved. Extraction proceeds until the retinal tail forms a hydrogen bond network with Ala144, Met145, and Ser183 side groups lining the exit/entry window. The manipulation induces a distortion with a fitted root mean square deviation of coordinates (ignoring retinal, water, and hydrogen atoms) of less than 1.9 A by the time the retinal carbonyl reaches the protein surface. The forces needed to extract retinal are due to friction and do not indicate significant potential barriers. The simulations therefore suggest a pathway for the binding of retinal. Water molecules are found to play a crucial role in the binding process.

Modulation of opsin apoprotein activity by retinal. Dark activity of rhodopsin formed at low temperature

The bovine opsin apoprotein activates transducin, although at a much reduced level than light-activated rhodopsin (Surya, A., Foster, K., and Knox, B. (1995) J. Biol. Chem. 270, 5024-5031). The ability of retinal to modulate opsin apoprotein activity was investigated using a guanyl nucleotide exchange assay on transducin. 11-cis-Retinal reacted with opsin at 22 degrees C to (a) reform pigment having maximal absorbance at 500 nm and (b) reduce opsin activity by >80%. Pigment formation also occurred at 0 degrees C with a t1/2 of 260 min. However, unlike rhodopsin formed at 22 degrees C (R22), the rhodopsin formed at 0 degrees C (R0) activated transducin with the same half-saturating concentration as opsin in an exhaustive binding assay. Thus, the formation of a protonated Schiff base associated with 500 nm absorbance does not by itself lead to the inactivation of opsin. The R0 conformation was partially inactivated by incubation at 22 degrees C (t1/2 = 61 +/- 9 min), suggesting that it may be an intermediate conformation in the regeneration of rhodopsin.

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.

Spectral tuning in bacteriorhodopsin in the absence of counterion and coplanarization effects

The basis for wavelength regulation in bacteriorhodopsin (BR) and retinylidene proteins in general has been studied for decades but is still only partially understood. Here we report the preparation and spectroscopic characterization of BR analogs aimed at investigating the existence of spectral tuning mechanisms other than the two widely accepted mechanisms, weakened counterion interactions and ring/chain coplanarization. We synthesized two novel retinal analogs containing a saturated 13-14 bond, which interrupts the interaction of the protein counterions with the chromophore conjugation system. Furthermore, one of the analogs has a planar polyene system so that the contribution to the red shift of BR by retinal ring/chain coplanarization is also absent. We incorporated these analogs into bacterioopsin and discovered a sizable amount of red shift, which can be accounted for by interactions between the polar or polarizable groups of the protein and the retinal polyene chain. Our results suggest that the wavelength regulation in BR is achieved by synergistic chromophore/protein interactions including ring/chain coplanarization, excited state stabilization by polar or polarizable protein side chains located along the polyene chain, and weakened counterion interactions near the Schiff base positive charge.

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.

Noncovalent occupancy of the retinal-binding pocket of opsin diminishes bleaching adaptation of retinal cones

Bright light bleaches visual pigment and leads to a persistent desensitization of isolated rod and cone photoreceptors called bleaching adaptation. Bleaching adaptation results from the combined effects of pigment depletion and adaptational modulation of certain cellular reactions in the visual transduction cascade. Here, we present evidence that in solitary cone photoreceptors isolated from the salamander retina, the latter effect is due to the presence of free opsin in the outer segment. Also, we demonstrate that this "opsin adaptation" can be reversed by treating the cells with synthetic retinoids similar to 11-cis retinal but having polyene chains too short to form protonated Schiff base attachments to opsin.

Artificial pigments of halorhodopsin and their chloride pumping activities

Halorhodopsin (HR), the light-driven chloride pump of Halobacterium halobium, was bleached with hydroxylamine and regenerated with all-trans-retinal under several different conditions. The largest recovery of the pigment was found with apoprotein obtained from detergent-free HR [HR(BB)]. To compare the chloride-pumping mechanism of HR with that of bacteriorhodopsin (BR; the light-driven proton pump of the same bacteria), HR pigment analogues were reconstituted with the bleached HR (BB) and retinal analogues. The corresponding BR pigment analogues have previously been shown to have little or no proton-pumping activity, except for retinal2 (3,4-dehydroretinal). Pigment analogues with 13-demethylretinal or retinal2 showed an "opsin shift" similar to that of the all-trans-retinal pigment of both HR and BR. Opsin shifts of the pigments of 9-12-phenylretinal and 3,7-dimethyl-2,4,6,8-decatetraenal and haloopsin are slightly different from those of the corresponding BR pigment analogues, presumably reflecting differences of the chromophoric structures in HR and BR. In addition to the spectral properties, the effect of chloride ion on deprotonation of the Schiff base was measured. These pigment analogues showed the "chloride effect" (a shift of the pK value for deprotonation of the Schiff base), but a smaller one than that seen in HR. For a measurement of the chloride-pumping activity, each retinal analogue was added to a culture of L07 cells (BOP-, HOP+, Ret-), and the activity was measured with the cell suspension. Only cultures with retinal or retinal2 showed chloride-pumping activity, as is true for proton pumping by BR. This suggests that a similar retinal-protein interaction is necessary for both ion pumps.

Quantum yield of CHAPSO-solubilized rhodopsin and 3-hydroxy retinal containing bovine opsin

The quantum yields of bleaching for two artificial pigments, bovine opsin combined with (3R)-3-hydroxy retinal or (3R,S)-3-methoxy retinal, were determined in comparison to the value for regenerated bovine rhodopsin. Regeneration of the visual pigments was performed by incubation of 3-[(3-Cholamidopropyl)-dimethylammonio]-2-hydroxy-1- propanesulfonate (CHAPSO)-solubilized opsin with the 11-cis isomers of retinal and the respective retinal derivatives. The extinction coefficients of the pigments in CHAPSO were determined to 35,000 M-1 cm-1 (native rhodopsin), 35,300 M-1 cm-1 (regenerated rhodopsin) and 34,500 M-1 cm-1 (3-OH retinal opsin). With respect to rhodopsin (lambda max: 500 nm), the pigments carrying the substituted chromophores exhibit blue shifted absorbance maxima (3-hydroxy and 3-methoxy retinal opsin: 488 nm). In parallel experiments under absolutely identical conditions we find related to the value of CHAPSO solubilized rhodopsin (identical to 1) a quantum efficiency of bleaching for the 3-hydroxy pigment of 1.2.

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.

Methoxyretinals in bacteriorhodopsin. Absorption maxima, cis-trans isomerization and retinal protein interaction

Analogue bacteriorhodopsins (BRs) were reconstituted from bacterioopsin and 9-, 11-, or 13-methoxyretinals or their demethyl derivatives, respectively. In organic solvents the retinals occur as cis isomers of the respective double bonds carrying the methoxy group. 9-Methoxyretinal, present as the 9-cis isomer, does not form an analogue BR with bacterioopsin in the dark. Upon illumination, a BR is produced with an absorbance maximum at 560 nm. This compound is thermally unstable, and converts back into the 9-cis-containing complex (lambda max = 410 nm) in the dark. Removal of the 13-methyl group from this compound (= 9-methoxy 13-demethyl retinal) does not change the 9-cis configuration of the free retinal, but allows the reconstitution of a thermally stable chromoprotein absorbing around 500 nm with a proton translocation rate of about 10% of the BR value, comparable to the 13-demethyl BR value [Gärtner, W., Towner, P., Hopf, H. & Oesterhelt, D. (1983) Biochemistry 22, 2637-2644]. 11-Methoxy BRs (13-demethyl and 9,13-didemethyl) absorb around 530 nm and are inactive. 13-Methoxy retinal (13-cis isomer) reconstitutes a chromoprotein with an absorbance maximum at 515 nm, which can be photoconverted to a thermostable 460-nm-absorbing complex. For the 515-nm-absorbing species of 13-methoxy BR a light-induced proton translocation was not detected in measurements with cell vesicles (detection of pH changes in the vesicle preparation). Only by photocurrent measurements in a bilayer experiment could a very diminished photocurrent be detected, about 1-2% of BR, [Fendler et al. (1987) Biochim. Biophys. Acta 893, 60-68]. The reconstitution rate of 13-methoxy BR from 13-methoxy retinal and bacterioopsin is slower by a factor of 40 compared to 13-ethyl BR, although both substituents are of similar size. The position 13 of retinal was found to be most sensitive for regulation of the absorption maximum and the formation and stability of the all-trans isomer, which is the active form for light-induced proton translocation. The results suggest that an electronic interaction with a charged residue of the binding site exists around position 13 of retinal, which is disturbed when a methoxy group replaces the methyl or ethyl group at that position. This electronic interaction is essential for maintaining the active all-trans configuration of retinal.

Activation of phosphodiesterase by rhodopsin and its analogues

Activation of guanosine 3',5'-cyclic monophosphate (cGMP) phosphodiesterase (EC 3.1.4.35.) in frog rod outer segment membrane by rhodopsin and its analogues was investigated. The Schiff-base linkage between opsin and retinal in rhodopsin was not always necessary for the phosphodiesterase activation. The binding of beta-ionone ring of retinal to a hydrophobic region of opsin was not enough to induce the enzyme activation. A striking photo-activation of the enzyme was induced by photo-isomerization of rhodopsin analogues from cis to trans form. It seems probable that an "expanded" conformation of opsin around the retinylidene chromophore induced by the cis to trans isomerization may be the trigger for the activation of phosphodiesterase. On the other hand, the phosphodiesterase in frog rod outer segment was activated by warming of bathorhodopsin to -12 degrees C and then incubating it at the same temperature. Thus, metarhodopsin II or an earlier intermediate than metarhodopsin II should be a direct intermediate for the enzyme activation.

Activation of phosphodiesterase in frog rod outer segment by rhodopsin analogues

Activation of guanosine 3',5'-cyclic monophosphate (cGMP) phosphodiesterase (EC 3.1.4.35) in frog rod outer segment membrane by rhodopsin analogues has been investigated. A rhodopsin analogue modified at the Schiff-base linkage (N-retinyl-opsin) or the beta-ionone ring (3-dehydro-rhodopsin) in the retinylidene chromophore of rhodopsin has some ability in activation of the enzyme. In consideration of our previous observation that opsin including a retinal-oxime can activate the enzyme, it seems likely that the Schiff-base linkage is not always necessary for the phosphodiesterase activation. On the other hand, a change in the length of the side chain of retinal (complex of opsin and beta-ionone, beta-ionylideneacetaldehyde or retinylideneacetaldehyde) or dissection of the conjugate double-bond system of the side chain (retro-gamma-rhodopsin) remarkably reduces the activation ability. However, 5,8-epoxy-rhodopsin having a similar dissected conjugate double-bond system induces some enzyme activation because of its rigid conformation around C7-C8-C9 single bonds. Consequently, it is suggested that the necessary portion of rhodopsin chromophore for the activation of the enzyme is the rigid conjugate double-bond system between the beta-ionone ring and the Schiff-base linkage in its all-trans form.

Inhibition of rhodopsin regeneration by cyclohexyl derivatives

Twenty two compounds, bearing some structural similarity either to the ring-end or aldehyde-end of 11-cis-retinal, have been screened for their effectiveness in decreasing the rate of regeneration of rhodopsin in vitro from 11-cis retinal and bovine opsin. The results of these experiments indicate that only those compounds containing a ring and having at least two methyl groups at the 1,5-ring positions are capable of decreasing the rate of regeneration of rhodopsin. Short-chain aldehydes resembling the aldehyde-end of retinal did not affect the rate of pigment formation. There was no additive effect of combining a dimethylcyclohexyl inhibitor and a short chain aldehyde. The methyl groups at the 1,5-positions may play a special role in anchoring the retinal into the protein binding site. This hypothesis is supported by the failure of the ring demethylated analogue, 11-cis, 1,1,5-desmethyl-5,6-dihydroretinal, to form pigment under the same conditions with which good pigment yields are obtained with 11-cis 5,6-dihydroretinal.