Evidence for a common BATHO-intermediate in the bleaching of rhodopsin and isorhodopsin

Nanosecond transient spectroscopic measurements of the BATHO products formed from photolysis of bovine rhodopsin (RHO) and isorhodopsin (ISO) are discussed. BATHO absorption spectra of RHO and ISO differ slightly, but both pigments exhibit wavelength maxima near 560 nm. An additional transient absorption at 440 nm is observed immediately following excitation of RHO but not ISO. The decay times and Arrhenius activation energies of the two 560 nm absorbing transients are the same. In addition to spectral differences in the photolysis of RHO and ISO, the bleaching yield as a function of 532 nm laser power is different, with the yield in RHO saturating at lower laser power than ISO. The bleaching yield of the two pigments has been modeled using the known extinction coefficients and quantum yields for the interconversion of RHO, ISO, and a single BATHO species. Agreement between experiment and the model is found if the effects of the laser polarization are considered. The data are consistent with a common BATHO in the photolysis of RHO and ISO.

Biological applications of picosecond spectroscopy

Technological advances in picosecond spectroscopy have permitted the mechanisms of various chemical, physical and biological processes to be elucidated and understood to a greater degree than ever before. By means of picosecond emission, absorption and Raman spectroscopy, one can probe and measure directly the transient intermediates and kinetics of primary events in complex biological processes. A description of two current types of laser systems--solid-state and synchronously pumped dye lasers--and their application to determining the primary events in the biological processes of dissociation of oxy- and carboxymyoglobin, excited-state relaxation of porphyrins and visual transduction, illustrate the power of picosecond spectroscopy.

Functional reassembly of membrane proteins in planar lipid bilayers

Recent progress in membrane biology has brought us to a stage where it is possible to associate complex biological processes to identifiable membrane proteins. Technical advances in the biochemical characterization and purification of membrane proteins have contributed a wealth of structural information. The reconstitution approach has proved to be valuable in our efforts to understand the molecular mechanisms of membrane transport and energy transduction.

Photochemical and functional properties of bacteriorhodopsins formed from 5,6-dihydro- and 5,6-dihydrodesmethylretinals

5,6-Dihydroretinal and 5,6-dihydro-1,1,5,9,13-desmethylretinal are synthesized, and their all-trans isomers are shown to form pigment analogues (lambda max at 475 and 460 nm, respectively) of bacteriorhodopsin (purple membrane protein). The shift of the absorption maximum od the pigment from that of the protonated Schiff base of the chromophore for 5,6-dihydrobacteriorhodopsin is small compared to that of the native pigment, suggesting that negative charges similar to those controlling the lambda max of visual pigment rhodopsin exist near the cyclohexyl ring. Both pigment analogues undergo reversible light-induced spectral shifts reflecting cyclic photoreactions of the pigments. These results indicate that the absence of the C-5--C-6 double bond and of the five methyl groups of retinal does not abolish the photochemistry of these pigment analogues and strongly suggest that these structural features are not directly required for the photoreactions of native bacteriorhodopsin. The apparent rates of the photochemical transformations of these artificial pigments are quite different from those of bacteriorhodopsin. A working hypothesis is proposed for the photocycle of the pigment analogues, which includes a slower light-induced cycling rate (for the light-adapted pigments) than that of native bacteriorhodopsin and an increased rate of dark adaptation. When incorporated into egg lecithin vesicles both pigment analogues show proton pumping ability, again indicating that the missing double bond and the methyl groups are not structurally required for the function of the pigments.

Light isomerizes the chromophore of bacteriorhodopsin

The primary photochemical event in the two light-transducing pigments whose chromophore is retinal, rhodopsin or bacteriorhodopsin, is a source of controversy. It was originally proposed that the primary photoevent in the bleaching of rhodopsin is the photoisomerization of the chromophore from 11-cis to all-trans retinal. Photochemical considerations suggested that a photoisomerization is the primary event in both rhodopsin and bacteriorhodopsin. However, this description of bacteriorhodopsin's photochemistry has been questioned. To elucidate this problem, we determined the isomeric conformation of retinal for two of the photolytic intermediates of bacteriorhodopsin, using a method that enables us to extract chromophores from the photocycle intermediates L and M at low temperatures (-74 degrees C), and have determined the isomeric conformation of the extracted retinals by HPLC. Here we provide direct evidence that isomerization of the chromophore has taken place in two of the early photocycle intermediates (L and M) of bacteriorhodopsin.

Photochemical studies of 7-cis-rhodopsin at low temperatures. Nature and properties of the bathointermediate

The photoreaction of 7-cis-rhodopsin derived from 7-cis-retinal and cattle opsin was studied by low-temperature spectrophotometry. Upon irradiation of 7-cis-rhodopsin at liquid nitrogen temperature (-190 degrees C) with blue light, its spectrum shifted to the longer wavelengths, indicating the formation of a bathoproduct. The bathoproduct thus formed was found to be identical with bathorhodopsin formed from rhodopsin in their spectroscopic, photochemical, and thermal properties. Therefore, we believe that the bathoproduct is, in fact, bathorhodopsin. The fact that 7-cis-rhodopsin can be readily converted to rhodopsin and to 9-cis-rhodopsin shows that the identical retinal binding site of opsin is involved in the three isomeric rhodopsins. These results appear to be consistent with the notion that the chromophore of bathorhodopsin is a twisted all-trans isomer, which is readily obtainable from the 7-cis, 9-cis, and 11-cis isomers.

Energy uptake in the first step of visual excitation

Perception of light by the retina starts with the absorption of a photon by 11-cis retinal, which is covalently incorporated into the membrane-bound protein, rhodopsin. The initial result of photon capture is the very rapid formation of a red-shifted species, bathorhodopsin (also known as prelumirhodopsin), which is (meta-)stable at liquid nitrogen temperature but which decomposes at higher temperatures, in the dark, through a series of intermediate stages, resulting in the release of all-trans retinal from the apoprotein, opsin. Bathorhodopsin formation is the only photochemical step in the overall reaction and, therefore, merits investigation. Several models for the process have been proposed, and have been critically reviewed, although no consensus yet exists as to the nature or mechanism of formation of the batho intermediate. I report here on the first direct measurement of photon energy uptake during bathorhodopsin formation from bovine rhodopsin, and on its possible significance.

Molecular mechanism for the initial process of visual excitation. IV. Energy surfaces of visual pigments and photoisomerization mechanism

Using the twisted conformations of the chromophores for visual pigments and intermediates which were theoretically determined in the previous paper, energy surfaces of the pigment at - 190 degrees C were obtained as functions of the torsional angles theta 9-10 and theta 11-12 or of the torsional angles theta 9-10 and theta 13-14. In these calculations, the existence of specific reaction paths between rhodopsin (R) and bathorhodopsin (B), between isorhodopsin I (I) and bathorhodopsin, and between isorhodopsin II (I') and bathorhodopsin were assumed. It was shown that the total energy surfaces of the excited states had minima C1 at theta 9-10 approximately -10 degrees and theta 11-12 approximately -80 degrees, C2 at theta 9-10 approximately -85 degrees and theta 11-12 approximately -5 degrees, and C3 at theta 9-10 approximately -0 degree and theta 13-14 approximately -90 degrees. These minima are considered to correspond to the thermally barrierless common states as denoted by Rosenfeld et al. Using the total energy surfaces in the ground and excited states, the molecular mechanism of the photoisomerization reaction was suggested. Quantum yields for the photoconversions among R, I, I' and B were related to the rates of vibrational relaxations, radiationless transitions and thermal excitations. Some discussion was made of the temperature effect on the quantum yield. Similar calculations of the energy surfaces were also made at other temperatures where lumirhodopsin or metarhodopsin I is stable. Relative energy levels of the pigments and the intermediates were discussed.

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.

On the light-stimulated coupling between rhodopsin and its disk membrane environment

Disks from bovine ROS undergo a rapid shrinkage when flash illuminated with green light (Uhl, R., et al. (1977) Biochim. Biophys. Acta 469. 113-122). This can be monitored as a light scattering transient, referred to as the P signal. In this paper the P signal is studied at various temperatures and pH. The temperature dependence of the kinetics reveals that "P" consists of two sequential reaction steps. Both appear to occur within the receptor molecule rhodopsin. The actually observed event, the shrinkage of the disk, is therefore not rate limiting under the tested conditions. Both steps of "P" take place while there is only one spectroscopically detectable reaction of the rhodopsin molecule, the metarhodopsin I-metarhodopsin II transition. This implies that there are intermediates of the rhodopsin photolytic cycle which are not evident as spectroscopically separate species. The amplitude of "P", i.e., the extent of the disk shrinkage, is independent of the state of the equilibrium between the two photoproducts absorbing at 478 and 380 nm respectively and called MI and MII. A scheme is suggested in which the irreversible decay of MI (478) triggers the disk shrinkage (and maybe transduction), and in which there is an equilibrium between MII (380) and a proposed isochromic photoproduct MI' (478).

Flash kinetic study of the last steps in the photoinduced reaction cycle of bacteriorhodopsin

The reaction cycle of light adapted bacteriorhodopsin (BR) in aqueous purple membrane suspensions was studied by laser flash photolysis at different temperatures (2--49 degrees C) and pH values (3--10). The activation energy for several reaction steps was determined at pH 7.6. The kinetics of O-bacteriorhodopsin (one of the last intermediates in the cycle) were analyzed in some detail and it was found that the simple consecutive reaction scheme M-BR leads to O-BR leads to BR may explain the kinetics of O-bacteriorhodopsin as measured at 680 nm. Since the pH change in neutral aqueous suspensions of purple membrane follows similar kinetics as O-bacteriorhodopsin it is suggested that protons are released during the reaction M-BR leads to O-BR and taken up again during the reacton O-BR leads to BR. Another long-lived intermediate, which absorbs to a greater extent than bacteriorhodopsin at 570 nm and less than bacteriorhodopsin at 420 nm, was identified with the strongly fluorescing species, pseudo- or P-bacteriorhodopsin. The decay of P-bacteriorhodopsin in bacteriohodopsin had an activation energy of only approx. 1.2 kcal/mol, which suggests that the last step of the photocycle is a relaxation around a single bond. At pH 9--10, the simple first-order kinetics of all the intermediates were changed into a kinetics consisting of two first-order decays. This change of kinetics was accompanied by a drastic decrease in the rotational diffusion relaxation time. To explain the results obtained in this work and those of others, a model involving proton uptake and release by the Schiff base nitrogen combined with an isomerization reaction is finally proposed.

Subpicosecond spectroscopy of bacteriorhodopsin

Subpicosecond pulses have been used to study the ultrafast dynamics of the photochemistry of bacteriorhodopsin. An optically induced absorption that appears in about 1.0 picosecond at physiological temperatures has been resolved in time. The data can be interpreted in terms of the photochemical formation of bathobacteriorhodopsin and provide support for an excitation mechanisms involving molecular rearrangement in the protein induced by electron redistribution in the chromophore.