Circular dichroism, optical rotatory dispersion, and absorption studies on the conformation of bovine rhodopsin in situ and solubilized with detergent

Multistate Multiconfiguration Quantum Chemical Computation of the Two-Photon Absorption Spectra of Bovine Rhodopsin

Recently, progress in IR sources has led to the discovery that humans can detect infrared (IR) light. This is hypothesized to be due to the two-photon absorption (TPA) events promoting the retina dim-light rod photoreceptor rhodopsin to the same excited state populated via one-photon absorption (OPA). Here, we combine quantum mechanics/molecular mechanics and extended multiconfiguration quasi-degenerate perturbation theory calculations to simulate the TPA spectrum of bovine rhodopsin (Rh) as a model for the human photoreceptor. The results show that the TPA spectrum of Rh has an intense S₀ → S₁ band but shows also S₀ → S₂ and S₀ → S₃ transitions whose intensities, relative to the S₀ → S₁ band, are significantly increased when compared to the corresponding bands of the OPA spectrum. In conclusion, we show that IR light in the 950 nm region can be perceived by rod photoreceptors, thus supporting the two-photon origin of the IR perception. We also found that the same photoreceptor can perceive red (i.e., close to 680 nm) light provided that TPA induces population of S₂.

Characterization of Denatured States and Reversible Unfolding of Sensory Rhodopsin II

Our understanding on the folding of membrane proteins lags behind that of soluble proteins due to challenges posed by the exposure of hydrophobic regions during in vitro chemical denaturation and refolding experiments. While different folding models are accepted for soluble proteins, only the two-stage model and the long-range interactions model have been proposed so far for helical membrane proteins. To address our knowledge gap on how different membrane proteins traverse their folding pathways, we have systematically investigated the structural features of SDS-denatured states and the kinetics for reversible unfolding of sensory rhodopsin II (pSRII), a retinal-binding photophobic receptor from Natronomonas pharaonis. pSRII is difficult to denature, and only SDS can dislodge the retinal chromophore without rapid aggregation. Even in 30% SDS (0.998 Χ_SDS), pSRII retains the equivalent of six out of seven transmembrane helices, while the retinal-binding pocket is disrupted, with transmembrane residues becoming more solvent exposed. Folding of pSRII from an SDS-denatured state harboring a covalently bound retinal chromophore shows deviations from an apparent two-state behavior. SDS denaturation to form the sensory opsin apo-protein is reversible. We report pSRII as a new model protein which is suitable for membrane protein folding studies and has a unique folding mechanism that differs from those of bacteriorhodopsin and bovine rhodopsin.

Langmuir-Blodgett films of photosensitive proteins

The striking properties of monolayers and multilayers of photosensitive proteins obtained by using the Langmuir-Blodgett technique are described. The close packing of the protein molecules, which preserve most of the properties found in solution, seems to be the main cause for their thermal stability, which in some cases reached a temperature of 200 degrees C without the loss of the protein secondary structure. The review is focused on three of the most intensively studied photosensitive proteins, namely photosynthetic reaction centres, bacteriorhodopsin and bovine rhodopsin, and on their possible applications as molecular optical devices.

Differentiation between transmembrane helices and peripheral helices by the deconvolution of circular dichroism spectra of membrane proteins

The interpretation of the circular dichroism (CD) spectra of proteins to date requires additional secondary structural information of the proteins to be analyzed, such as X-ray or NMR data. Therefore, these methods are inappropriate for a CD database whose secondary structures are unknown, as in the case of the membrane proteins. The convex constraint analysis algorithm (Perczel, A., Hollósi, M., Tusnády, G., & Fasman, G. D., 1991, Protein Eng. 4, 669-679), on the other hand, operates only on a collection of spectral data to extract the common spectral components with their spectral weights. The linear combinations of these derived "pure" CD curves can reconstruct the original data set with great accuracy. For a membrane protein data set, the five-component spectra so obtained from the deconvolution consisted of two different types of alpha helices (the alpha helix in the soluble domain and the alpha T helix, for the transmembrane alpha helix), a beta-pleated sheet, a class C-like spectrum related to beta turns, and a spectrum correlated with the unordered conformation. The deconvoluted CD spectrum for the alpha T helix was characterized by a positive red-shifted band in the range 195-200 nm (+95,000 deg cm2 dmol-1), with the intensity of the negative band at 208 nm being slightly less negative than that of the 222-nm band (-50,000 and -60,000 deg cm2 dmol-1, respectively) in comparison with the regular alpha helix, with a positive band at 190 nm and two negative bands at 208 and 222 nm with magnitudes of +70,000, -30,000, and -30,000 deg cm2 dmol-1, respectively.

Multiple isoelectric forms of detergent-solubilized bovine rhodopsin. II. Spectral analysis

The absorption and circular dichroic spectra of three stable isoelectric forms of purified rhodopsin (in Emulphogene BC 720) with isoelectric points of 5.19, 5.58 and 6.14 were analysed over the accessible wavelength region of 190-800 nm. It was found from the spectral analysis of the 5.58 and 6.14 forms that the tertiary structure of these isoelectric forms was different, without any change being observed in the secondary structure of these proteins. The difference in structure was removed by denaturation with SDS. Specifically, the aromatic amino acid residues of the 6.14 isoelectric form were suggested to be in a more polar environment as compared to those for the 5.58 isoelectric form. The changes of structure in the microenvironment of retinal in these proteins were minor in comparison to the tertiary changes observed in the proteins. This indicated that the site of charge perturbation is not localized in the retinylidene microenvironment. Furthermore, the spectral features of these isoelectric forms were features common to the spectra of both purified unfocused rhodopsin and crude rhodopsin, suggesting that the isoelectric forms are not a consequence of purification. Spectral analysis of the 5.19 isoelectric form indicated that this form of rhodopsin had an increased tendency to aggregate after focusing. Based on the denaturation properties of these isoelectric forms, it was shown that the 5.19 and 5.58 isoelectric forms had different conformations. It was concluded from this study that preparations of 'purified' rhodopsin are a heterogeneous mixture of stable proteins with different tertiary structures and different isoelectric points.

Fourier transform infrared study of photoreceptor membrane. I. Group assignments based on rhodopsin delipidation and reconstitution

Fourier transform infrared spectroscopy has been used to study the structure of bovine photoreceptor membrane. Rhodopsin appears to contain an extensive alpha-helical structure which is arranged predominantly perpendicular to the membrane plane. Spectra of delipidated rhodopsin and rhodopsin membranes reconstituted from dioleyl-phosphatidylcholine were compared with native photoreceptor membrane from rod outer segments in order to facilitate peak assgnments. It is concluded that spectroscopic peaks characteristic of several protein and lipid groups can be assigned. We also find delipidation leads to alteration of the rhodopsin structure which is resorted upon reconstitution. Membranes both suspended in 2H2O and dehydrated were compared in order to detect possible conformational differences. Dehydration does not appear to grossly alter rhodopsin structure, although it may affect delipidated rhodopsin.

Investigations of the rhodopsin/Meta I and rhodopsin/Meta II transitions of bovine rod outer segments by means of kinetic infrared spectroscopy

We have applied our recently developed technique of flash induced kinetic infrared spectroscopy to the rhodopsin/Meta I and rhodopsin/Meta II transitions. Features of the infrared spectrum reflecting the C = C-vibration and the isomeric form of the chromophore are in agreement with resonant Raman experiments. Different results are obtained for the C = N-vibration of the Schiff base retinal opsin link. They are interpreted in terms of a Schiff base protonated via an hydrogen bond. A proton transfer in the excited state is suggested to explain the deviating results. In addition we have obtained spectral changes which cannot be attributed to molecular changes in the chromophore. We assume that these spectral features reflect molecular events in the protein part of rhodopsin.

A possible role of rhodopsin in maintaining bilayer structure in the photoreceptor membrane

31P-NMR measurements demonstrate that at 37 degrees C, independent of the photolytic state of the photopigment rhodopsin, the lipids in the photo-receptormembrane are almost exclusively organised in a bilayer. In strong contrast, the 31P-NMR spectra of the extracted lipids are characteristic for the hexagonal HII phase and an isotropic phase. The isotropic phase is characterised by freeze-fracture electron microscopy as particles and pits on smooth surfaces, possibly indicating inverted micelles. These results suggest a structural role for rhodopsin in maintaining the photoreceptor membrane lipids in a bilayer configuration.

Molecular mechanism for the initial process of visual excitation. III. Theoretical studies of optical spectra and conformations of chromophores in visual pigments, their analogues and intermdiates based on the torsion model

The torsion model with which we proposed to interpret the specific properties of the photoisomerization reaction of rhodopsin has been developed to apply to isorhodopsin I, isorhodopsin II and some intermediates. Based on this model, optical absorption wavelengths and oscillator strengths, as well as rotational strengths of visual pigments, analogues and intermediates at low temperatures are analyzed by varying twisted conformations of the chromophores. As a result, it was found that most of the optical data could be very well accounted for quantitatively by the torsion model. The twisting characters in the chromophore of rhodopsin are very similar to those of isorhodopsin. The obtained conformations of the chromophores are very similar in rhodopsin and its analogues, and in isorhodopsin and its analogues. Those of the chromophores of bathorhodopsin, lumirhodopsin and metarhodopsin I are similar to one another except that the conjugated chain of metarhodopsin I bends considerably when compared with the other intermediates.

The conformation of membrane-bound and detergent-solubilised bovine rhodopsin. A comparative hydrogen-isotope exchange study

The conformations of the intrinsic membrane protein, rhodopsin, in its membrane-bound and detergent-solubilised states have been compared by hydrogen isotope exchange measurements. The infrared peptide exchange data show that the highly hydrophobic nature of rhodopsin is conserved in the presence of the two detergents used: Cemulsol LA 90 and Ammonyx LO. Only about 50% of the peptide hydrogens exchange under conditions where about 80% would exchange in most soluble proteins. The conformational stability of rhodopsin in these two detergents is also demonstrated by the similarity of the tritium exchange-out kinetics and the infrared amide I band frequencies for both membrane-bound and detergent-solubilised rhodopsin. Upon illumination of rhodopsin (bleaching) in the presence of detergents, the hydrogen exchange rates are greatly increased and shifts in the amide I band frequencies are observed, indicative of a large conformation change. No such change occurs upon bleaching membrane-bound rhodopsin. We conclude that the conformation of rhodopsin is not altered by solubilisation in non-ionic detergents. However, in agreement with previously published results, bleached rhodopsin is stabilised by the membrane but does not retain a native conformation in these detergents.

Spectral studies on the conformation of rhodopsin

A summary is given of visible and ultraviolet spectral studies which deal with the conformation of rhodopsin in situ and solubilized with detergent. Emphasis is placed on studies which give specific information about the macromolecular structure and which set quantitative limits on the magnitude of light-induced conformational changes.