Resonance raman spectroscopy of an ultraviolet-sensitive insect rhodopsin

Molecular basis for ultraviolet vision in invertebrates

Invertebrates are sensitive to a broad spectrum of light that ranges from UV to red. Color sensitivity in the UV plays an important role in foraging, navigation, and mate selection in both flying and terrestrial invertebrate animals. Here, we show that a single amino acid polymorphism is responsible for invertebrate UV vision. This residue (UV: lysine vs blue:asparagine or glutamate) corresponds to amino acid position glycine 90 (G90) in bovine rhodopsin, a site affected in autosomal dominant human congenital night blindness. Introduction of the positively charged lysine in invertebrates is likely to deprotonate the Schiff base chromophore and produce an UV visual pigment. This same position is responsible for regulating UV versus blue sensitivity in several bird species, suggesting that UV vision has arisen independently in invertebrate and vertebrate lineages by a similar molecular mechanism.

Vertebrate photoreceptors

The basis of the duplex theory of vision is examined in view of the dazzling array of data on visual pigment sequences and the pigments they form, on the microspectrophotometry measurements of single photoreceptor cells, on the kinds of photoreceptor cascade enzymes, and on the electrophysiological properties of photoreceptors. The implications of the existence of five distinct visual pigment families are explored, especially with regard to what pigments are in what types of photoreceptors, if there are different phototransduction enzymes associated with different types of photoreceptors, and if there are electrophysiological differences between different types of cones.

pKa of the protonated Schiff base of visual pigments

The pKa of bovine rhodopsin is greater than 15; that of the long-wave-length-sensitive gecko P521 pigment ranges from 8.4 to 10.5 depending on chloride concentration; and that of octopus, an invertebrate, is 10.5. These pKa values are much higher than are needed just to maintain the Schiff base in its protonated state in the photoreceptor cell. The high pKa of the Schiff base may be at least partially related to a low pKa of its counterion, which would lower the frequency of thermal isomerization of the chromophore and thus lower the dark noise in the photoreceptor cell. After light absorption, the high pKa of the protonated Schiff base of a vertebrate visual pigment must get lowered enough to allow it to deprotonate, a required step in vertebrate visual excitation. This deprotonation step is not required in invertebrate visual excitation.

Photons, femtoseconds and dipolar interactions: a molecular picture of the primary events in vision

The first 200 femtoseconds in the life of a photoexcited rhodopsin molecule are extremely important for the development of visual sensation. Immediately upon excitation, a dramatic change in the charge distribution of the cationic 11-cis-retinal protonated Schiff base chromophore occurs that is quantitated by the change in electronic dipole moment of approximately 15 Debye. The opsin protein tunes the absorption maximum of the pigment to the blue or to the red enabling colour vision by placing dipolar rather than charged residues in the chromophore binding site to differentially stabilize either the ground or the excited state charge distribution. Resonance Raman intensity analysis reveals that the 11-cis-retinal chromophore then distorts violently about the C11 = C12 double bond, reaching torsional angles of approximately 70 degrees in only 30 fs. This rapid torsional distortion is driven by the non-bonded interaction between the 13-methyl group and the 10-hydrogen that is unique to the 11-cis configuration of the chromophore. The excited state depopulates in approximately 50 fs through a rapid and vibrationally coherent transition to the ground electronic state manifold with relaxation to the formally trans photoproduct complete in only 200 fs. This unusually fast and efficient isomerization process establishes a new paradigm for condensed phase photochemical reactions.

The ecology of visual pigments

The visual systems of vertebrates have adapted to function in photic environments ranging from the broad spectrum of full sunlight to almost total darkness, including the restricted spectral ranges found in different coloured aquatic environments. Such adaptations are immediately obvious at the level of retinal photoreceptors. The basic vertebrate photoreceptor pattern consists of rods and four spectrally distinct classes of cone that span the spectrum from the near ultraviolet to the far red. This arrangement is found in many diurnal species including shallow-living teleosts, reptiles and birds, but is noticeably absent in mammals. In freshwater teleosts the visual pigments may be porphyropsins which have maximum sensitivities displaced to longer wavelengths than their equivalent rhodopsins. Water acts as a monochromator, so that with increasing depth the spectral range of the ambient light is restricted, primarily at long wavelengths. Therefore, at depth the down-welling daylight is not only attenuated in intensity, but is restricted to a narrow spectral band centred around 470 nm. Closely related species that live at increasing depths show a loss of long-wave-sensitive cones and a displacement of the maximum sensitivities of middle-wave-sensitive cones and rods to shorter wavelengths. Such species offer a natural model for determining specific amino acids in opsin responsible for the spectral tuning of these middle-wave-sensitive pigments.

Rhodopsin and phototransduction

Recent studies on rhodopsin structure and function are reviewed and the properties of vertebrate as well as invertebrate rhodopsin described. Open issues such as the 'red shift' of the absorbance spectra are emphasized in the light of the present model of the retinal-binding pocket. The processes that restore the rhodopsin content in photoreceptors are also presented with a comparison between vertebrate and invertebrate visual systems. The central role of rhodopsin in the phototransduction cascade becomes evident by examining the main reports on light-activated conformational changes of rhodopsin and its interaction with transducin. Shut-off mechanisms are considered by reporting the studies on the sites of rhodopsin phosphorylation and arrestin binding. Furthermore, recent findings on the energetics of phototransduction point out that the ATP needed for photoreception in vertebrates is synthesized in the outer segments where phototransduction events take place.

Sensitivity of cones from a cyprinid fish (Danio aequipinnatus) to ultraviolet and visible light

Photocurrents of cones in the retinas of a small fish, Danio aequipinnatus (Cyprinidae) were recorded with suction pipette electrodes. Spectral sensitivity was measured between 277 and 697 nm. Four spectral classes of cone were found, with lambdamax at 560, 480, 408, and 358 nm. For the latter, we provide the first complete characterization of spectral sensitivity of a vertebrate ultraviolet (UV) photoreceptor. All cones responded with similar kinetics, except for a subset of the 560-nm cones, which were distinctly faster. The alpha-bands of the three cones absorbing maximally in the visible have the same bandwidth when log sensitivity is plotted versus normalized frequency, and in this respect they are indistinguishable from primate cones ("Mansfield's rule"). An eighth-degree polynomial in lambdamax/lambda based on this combined data set (fish, primate) is presented as a template that is likely to have predictive value in describing cone spectra from other vertebrates. The alpha-band of the UV cone, however, is somewhat narrower than predicted by this function, is similar to other UV visual pigments, and an eighth-degree polynomial that describes its shape is also presented. These measurements also provide information on the beta-band (i.e. cis peak region), difficult to obtain by microspectrophotometry. The beta-band of cone pigments is found at longer wavelengths as the alpha-band shifts toward the red. A secondary rise in cone sensitivity around 280 nm indicates that photons absorbed by aromatic amino acids in the opsin (gamma-band) excite the transduction cascade, but the quantum efficiency is not as high as when absorption occurs in the retinal-protein chromophore.

An analysis of two spectral properties of vertebrate visual pigments

Spectral data are scrutinized on two properties: (1) the chromophore-induced wavelength shift, i.e. the variation in absorbance maximum (lambda max) upon exchanging the vitamin A1-based chromophore with one based on vitamin A2 in the same opsin; (2) the half-band width (HBW) variation as a function of the reciprocal of the peak absorbance (lambda max)-1. It is shown that in an extended spectral range that includes the UV and when data are plotted in wavenumbers, the chromophore-induced shifts can be approximated with a parabola, whose minimum occurs near 23,000 cm-1 (approximately 430 nm). Similarly, HBW variations can also be fitted with parabolas, however, these show maxima near 430 nm. The theoretical implications of the two phenomena concerning lambda max tuning in vertebrate visual pigments are discussed.

The lobster carapace carotenoprotein, alpha-crustacyanin. A possible role for tryptophan in the bathochromic spectral shift of protein-bound astaxanthin

Crustacyanin, cross-linked with dimethyl pimelimidate to stabilize the protein against denaturation, was used to test the effects of tryptophan modification with BNPS-skatole [3-bromo-3-methyl-2-(nitrophenylmercaptol)-3H-indole] on the ability of the apoprotein to recombine with astaxanthin. The cross-linked apoprotein re-forms alpha-crustacyanin with astaxanthin in reasonable yield following incubation of the protein under the conditions for tryptophan modification in the absence of BNPS-skatole. The BNPS-skatole-treated protein reconstitutes with astaxanthin to give a carotenoprotein with lambda max. at 472 nm, that of the carotenoid in hexane, in a yield similar to that of the BNPS-skatole-untreated control. The implied involvement of tryptophan residues at the sites of astaxanthin attachment in crustacyanin and their possible roles in the binding sites of vitamin A in vitamin A-proteins are discussed in relation to the bathochromic spectral shifts of the chromophores.

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.

Asp83, Glu113 and Glu134 are not specifically involved in Schiff base protonation or wavelength regulation in bovine rhodopsin

Site-specific mutagenesis was employed to investigate the proposed contribution of proton-donating residues (Glu, Asp) in the membrane domains of bovine rhodopsin to protonation of the Schiff base-linking protein and chromophore or to wavelength modulation of this visual pigment. Three point-mutations were introduced to replace the highly conserved residues Asp83 by Asn (D83N), Glu113 by Gln (E113 Q) or Glu134 by Asp (E134D), respectively. All 3 substitutions had only marginal effects on the spectral properties of the final pigment (less than or equal to 3 nm blue-shift relative to native rhodopsin). Hence, none of these residues by itself is specifically involved in Schiff base protonation or wavelength modulation of bovine rhodopsin.

Why are blue visual pigments blue? A resonance Raman microprobe study

A resonance Raman microscope has been developed to study the structure of the retinal prosthetic group in the visual pigments of individual photoreceptor cells. Raman vibrational spectra are obtained by focusing the probe laser on intact photoreceptors frozen on a 77 K cold stage. To elucidate the mechanism of wavelength regulation in blue visual pigments, we have used this apparatus to study the structure of the chromophore in the 440-nm absorbing pigment found in "green rods" of the toad (Bufo marinus). The 9-cis isorhodopsin form of the green rod pigment exhibits a 1662-cm-1 C = NH+ Schiff base stretching mode that shifts to 1636 cm-1 in deuterium-substituted H2O. This demonstrates that the Schiff base linkage to the protein is protonated. Protonation of the Schiff base is sufficient to explain the 440-nm absorption maximum of this pigment without invoking any additional protein-chromophore interactions. The absence of additional perturbations is supported by the observation that the ethylenic band and the perturbation-sensitive C-10-C-11 and C-14-C-15 stretching modes have the same frequency as those of the 9-cis protonated retinal Schiff base in solution. Our demonstration that a blue visual pigment contains an unperturbed protonated Schiff base provides experimental evidence that the protein charge perturbation responsible for the opsin shift in the 500-nm absorbing pigments is removed in the opsins of blue pigments, as suggested by the sequence data.