Rhodopsin regeneration is accelerated via noncovalent 11-cis retinal-opsin complex--a role of retinal binding pocket of opsin

The regeneration of bovine rhodopsin from its apoprotein opsin and the prosthetic group 11-cis retinal involves the formation of a retinylidene Schiff base with the epsilon-amino group of the active lysine residue of opsin. The pH dependence of a Schiff base formation in solution follows a typical bell-shaped profile because of the pH dependence of the formation and the following dehydration of a 1-aminoethanol intermediate. Unexpectedly, however, we find that the formation of rhodopsin from 11-cis retinal and opsin does not depend on pH over a wide pH range. These results are interpreted by the Matsumoto and Yoshizawa (Nature 258 [1975] 523) model of rhodopsin regeneration in which the 11-cis retinal chromophore binds first to opsin through the beta-ionone ring, followed by the slow formation of the retinylidene Schiff base in a restricted space. We find the second-order rate constant of the rhodopsin formation is 6100+/-300 mol(-1) s(-1) at 25 degrees C over the pH range 5-10. The second-order rate constant is much greater than that of a model Schiff base in solution by a factor of more than 10(7). A previous report by Pajares and Rando (J Biol Chem 264 [1989] 6804) suggests that the lysyl epsilon-NH(2) group of opsin is protonated when the beta-ionone ring binding site is unoccupied. The acceleration of the Schiff base formation in rhodopsin is explained by stabilization of the deprotonated form of the lysyl epsilon-NH(2) group which might be induced when the beta-ionone ring binding site is occupied through the noncovalent binding of 11-cis retinal to opsin at the initial stage of rhodopsin regeneration, followed by the proximity and orientation effect rendered by the formation of noncovalent 11-cis retinal-opsin complex.     

Functional Expression of Gloeobacter Rhodopsin in Synechocystis sp. PCC6803

Proteorhodopsins are light‐driven proton pumps that occur widespread in Nature, where they function predominantly in environments with high incident irradiance. Their maximal absorbance is usually in the blue range, but can be extended into the (far)red range of the electromagnetic spectrum. Because they can be expressed heterologously, they may be exploited in studies aimed at increasing the efficiency of photosynthesis. Here we report further studies toward this goal, by comparing the expression of two different bacterial rhodopsins (Proteorhodopsin and Gloeobacter rhodopsin) in the model cyanobacterium Synechocystis sp. PCC6803. In particular, we investigated the pigments bound by the respective apo‐opsins, and the oligomeric state of the corresponding holo‐rhodopsins, both in Escherichia coli and in the cyanobacterial membranes. We conclude that the two proton‐pumping rhodopsins are predominantly present in an oligomeric state (hexamers for Proteorhodopsin and trimers for Gloeobacter rhodopsin). Furthermore, Gloeobacter rhodopsin is able to bind an antenna carotenoid (in addition to retinal) and has the highest pumping rate at given light intensity. However, its lower expression level will decrease its physiological effectiveness. It remains to be established which of these two bacterial rhodopsins is best in stimulating the growth rate of its cyanobacterial host.     

Excitation of Biomolecules by Coherent vs. Incoherent Light: Model Rhodopsin Photoisomerization

Light-induced processes in biological molecules, which occur naturally in continuous incoherent light, are often studied using pulsed coherent light sources. With a focus on timescales, the relationship between excitation due to these two types of light sources is examined through a uniform minimal model of the photoisomerization of retinal in rhodopsin, induced by either coherent laser light or low level incoherent light (e.g. moonlight). Realistic timescales for both processes are obtained and a kinetic scheme involving rates for both coherent and incoherent light excitation is introduced, placing all timescales into a uniform framework. The rate limiting step in the natural light-absorption process is shown to be the low incoherent photon flux.     

Electromagnetic field of extremely low frequency decreased adenylate kinase activity in retinal rod outer segment membranes

Adenylate kinase activity in rod outer segment membranes of bovine retina decreased of about 55% when exposed to an extremely low frequency electromagnetic field of 75 Hz and 250 microT. The effect was independent of the time of permanence in the field. Negligible effects of the field were found on the enzymatic activity of a soluble isoform of adenylate kinase or of rod outer segment membranes solubilized with Triton, suggesting the importance of the membrane in determining the conditions of the enzyme inactivation.     

About the intrinsic photochemical properties of the 11-cis retinal chromophore: computational clues for a trap state and a lever effect in Rhodopsin catalysis

CASPT2//CASSCF/6-31G* computations are used on the singlet S ₁ and S ₂ states to map the photoisomerization process of the 11-cis retinal protonated Schiff base in vacuo and to characterize its optical properties. It is shown that the spectroscopic observations recorded in Rhodopsin are reproduced quite well, calling for a substantially neutral effect of the protein. Furthermore, a rationale is proposed for the unreactive population recently observed in Rhodopsin, which is here addressed to the accessible S ₂ state, behaving as a trap. The experimental transient absorption and (absorption-wavelength dependent) emission are discussed and interpreted under the light of this novel model. Finally, a planarization of the β-ionone ring is observed on S ₁, which may cause a steric lever effect into the protein pocket, thus assisting photoisomerization catalysis. The reported results constitute a solid reference for further studies aimed to rationalize the effect of the environment on the photochemical reactivity of retinal chromophores. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Contribution to the Fernando Bernardi Memorial Issue.     

Catalytic phosphorylation of polyfluoroalkanols 13. Some ammonium and phosphonium salts as phosphorylation catalysts

It was shown that it is possible to use a series of ammonium and phosphonium salts as effective catalysts for the preparative synthesis of tris(polyfluoroalkyl) phosphates. The effect of the structure of substituted ammonium salts on their catalytic activity was studied in the phosphorylation of 1,1-dihydroperfluorobutanol by phosphorus oxychloride. It was determined that the reaction rate is affected not only by the solubility of the salt, but also by the nature of the ion pair and the steric access of the onium center for solvation by phosphoryl compounds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2789–2792, December, 1989.     

Catalytic phosphorylation of polyfluoroalkanols. 11. α-Polyfluoroalkylbenzyldichlorophosphates as phosphorylating agents in the catalytic phosphorylation of primary polyfluoroalkanols

Conclusions -Polyfluoroalkylbenzyldichlorophosphates react, in the presence of catalytic quantities of certain metals or their salts, with polyfluorinated primary alcohols with the formation of phosphorylation products [bis(polyfluoroalkyl)(-polyfluoroalkylbenzyl)phosphates] exclusively.For previous communication, see [1].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1660–1664, July, 1989.     

Catalytic phosphorylation of polyfluoroalkanols. 15. Catalytic phosphorylation of α-polyfluoroalkylbenzyl alcohols by bis(polyfluoroalkyl) chlorophosphates

The catalytic phosphorylation of -polyfluoroalkylbenzyl alcohols by bis(polyfluoroalkyl) chlorophosphates is a convenient method for the synthesis of various bis(polyfluoroalkyl)(-polyfluoroalkylbenzyl) phosphates, including those having donor substituents in the benzene ring. It was shown that in the series of p-methyl--trifluoromethylbenzyl phosphates p-CH₃C₆H₄CH-(CF₃)OP(O)XY (where X, Y=Cl, CF₃CH₂O), the ability to act as O-alkylating agents with respect to polyfluorinated alcohols is a common characteristic, and the alkylating ability increases symbatically with increase in the overall electron-acceptor strength of the X and Y substituents at the phosphorus atom.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 878–886, April, 1990.     

Glucose oxidase-immobilized glass disks for imaging of D-glucose in acute brain slices.

A biotinylated glucose oxidase (bGOD)-immobilized glass disk was prepared for visualizing D-glucose fluxes in acute brain slices. A mouse hippocampal slice was placed on the bGOD disk and stimulated with a stimulant solution containing horseradish peroxidase (HRP) and a substrate DA-64, followed by capturing digital images of Bindschedler's Green (BG), an oxidized form of DA-64, with a CCD camera. The bGOD membranes responded proportionally to D-glucose, ranging from 2.0 to 5.0 mM. Sucrose, GABA, L-glutamic acid, L-aspartic acid, glycine, acetylcholine and L-ascorbic acid at 10 mM did not cause any responses. The D-glucose fluxes in mouse hippocampal slices stimulated by a hypoxia solution were neuronal region-dependent, i.e., dentate gyrus (DG), cornu ammonis 1 (CA1) and cornu ammonis 3 (CA3), while those stimulated by KCl was independent of the neuronal regions. The response of bGOD disks is discussed in terms of the principle, concentration dependence and selectivity.     

Catalytic phosphorylation of polyfluoroalkanols Communication 10. Catalytic phosphorylation of α-polyfluoroalkylbenzyl alcohols by diaryl chlorophosphates

Conclusions Irrespective of the structure of the alcohol and the reaction conditions, catalytic phosphorylation of -polyfluoroalkylbenzyl alcohols by diaryl chlorophosphates leads exclusively to -polyfluoroalkylbenzyl diaryl phosphates.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 878–882, April, 1985.     

Flow rates of polymer solutions through porous disks as a function of solute. I. method

A method for studying the thickness of adsorbed polymer films, based on the reduction in effective cross section for liquid flow caused by the deposition of such a film on the interior of a capillary, is described. A sintered disk is used instead of a single capillary to raise the flow rate to an easily measured level, to avoid the difficulties of obtaining or fabricating the extremely fine capillaries which would be required, and to eliminate the extreme precautions needed to prevent plugging of a single capillary. Techniques for characterizing the sintered disks are presented. When applied to benzene solutions of stearic acid and of polystyrene, film thicknesses of 30 and 312 A., respectively, were obtained. This is in good agreement with present knowledge of fatty acid adsorption, and suggests that the polymer is adsorbed in a highly extended configuration. The application of this method to solutions of poly(vinyl acetate), poly(methyl methacrylate), and polystyrene in several solvents at various temperatures is described in a following paper.     

Rapid high voltage isoelectric focusing of proteins in rod gels.

A rapid procedure of isoelectric focusing (IEF) of proteins in polyacrylamide rod gels (i.d., 1.1 mm; length, 7.5 cm) is described. The time required for IEF can be reduced to 0.5 h by using high voltages up to 3000 V in the presence or absence of urea in the gels. When used as the first dimension of a two-dimensional technique for IEF sodium dodecyl sulphate electrophoresis, high voltage IEF gives smaller protein spots on the second dimension gel, associated with an increase in resolution. The method has been tested by a two-dimensional separation of an eye sample of the goodeid fish Xenotoca eiseni.     

pH‐dependent Interaction of Rhodopsin with Cyanidin‐3‐glucoside. 1. Structural Aspects†

Anthocyanins are a class of natural compounds common in flowers and vegetables. Because of the increasing preference of consumers for food containing natural colorants and the demonstrated beneficial effects of anthocyanins on human health, it is important to decipher the molecular mechanisms of their action. Previous studies indicated that the anthocyanin cyanidin‐3‐glucoside (C3G) modulates the function of the photoreceptor rhodopsin. In this paper, we show using selective excitation ¹H NMR spectroscopy that C3G binds to rhodopsin. Ligand resonances broaden upon rhodopsin addition and rhodopsin resonances exhibit chemical shift changes as well as broadening effects in specific resonances, in an activation state‐dependent manner. Furthermore, dark‐adapted and light‐activated states of rhodopsin show preferences for different C3G species. Molecular docking studies of the flavylium cation, quinoidal base, carbinol pseudobase and chalcone forms of C3G to models of the dark, light‐activated and opsin structures of rhodopsin also support this conclusion. The results provide new insights into anthocyanin–protein interactions and may have relevance for the enhancement of night vision by this class of compounds. This work is also the first report of the study of ligand binding to a full‐length membrane receptor in detergent micelles by ¹H NMR spectroscopy. Such studies were previously hampered by the presence of detergent micelle resonances, a problem overcome by the selective excitation approach.     

pH‐dependent Interaction of Rhodopsin with Cyanidin‐3‐glucoside. 2. Functional Aspects†

Anthocyanins are a class of phytochemicals that confer color to flowers, fruits, vegetables and leaves. They are part of our regular diet and serve as dietary supplements because of numerous health benefits, including improved vision. Recent studies have shown that the anthocyanin cyanidin‐3‐O‐glucoside (C3G) increased regeneration of the dim‐light photoreceptor rhodopsin (Matsumoto et al. [2003] J. Agric. Food Chem., 51 , 3560–3563). In an accompanying study (Yanamala et al. [2009] Photochem. Photobiol.), we show that C3G directly binds to rhodopsin in a pH‐dependent manner. In this study, we investigated the functional consequences of C3G binding to rhodopsin. As observed previously in rod outer segments, regeneration of purified rhodopsin in detergent micelles is also accelerated in the presence of C3G. Thermal denaturation and stability studies using circular dichroism, fluorescence and UV/visible absorbance spectroscopy show that C3G exerts a destabilizing effect on rhodopsin structure while it only modestly alters G‐protein activation and the rates at which the light‐activated Metarhodopsin II state decays to opsin and free retinal. These results indicate that the mechanism of C3G‐enhanced regeneration may be based on changes in opsin structure promoting access to the retinal binding pocket.     

Enantioselective binding of an 11-cis-locked cyclopropyl retinal. The conformation of retinal in bovine rhodopsin

[formula: see text] The conformation of the retinal chromophore in rhodopsin is central for understanding the visual transduction process. The absolute twist around the 12-s bond of the chromophore in rhodopsin has been determined by studies with 11-cis-locked 11,12-cyclopropylretinal analogues (11S,12R)-2 and (11R,12S)-3, enantioselectively synthesized with the aid of an enzyme. The finding that enantiomer 2 binds to opsin while the other 3 does not defines the absolute sense of twist around the 12-s bond.