Between the ground- and M-state of bacteriorhodopsin the retinal transition dipole moment tilts out of the plane of the membrane by only 3 degrees

Chromophore reorientation during the photocycle of bacteriorhodopsin: experimental methods and functional significance

Light-induced isomerization leads to orientational changes of the retinylidene chromophore of bacteriorhodopsin in its binding pocket. The chromophore reorientation has been characterized by the following methods: polarized absorption spectroscopy in the visible, UV and IR; polarized resonance Raman scattering; solid-state deuterium nuclear magnetic resonance; neutron and X-ray diffraction. Most of these experiments were performed at low temperatures with bacteriorhodopsin trapped in one or a mixture of intermediates. Time-resolved measurements at room temperature with bacteriorhodopsin in aqueous suspension can currently only be carried out with transient polarized absorption spectroscopy in the visible. The results obtained to date for the initial state and the K, L and M intermediates are presented and discussed. The most extensive data are available for the M intermediate, which plays an essential role in the function of bacteriorhodopsin. For this intermediate the various methods lead to a consistent picture: the curved all-trans polyene chain in the initial state straightens out in the M intermediate (13-cis) and the chain segment between C(5) and C(13) tilts upwards in the direction of the cytoplasmic surface. The kink at C(13) allows the positions of beta-ionone ring and Schiff base nitrogen to remain approximately fixed.

Chemical and physical evidence for multiple functional steps comprising the M state of the bacteriorhodopsin photocycle

In the photocycle of bacteriorhodopsin (bR), light-induced transfer of a proton from the Schiff base to an acceptor group located in the extracellular half of the protein, followed by reprotonation from the cytoplasmic side, are key steps in vectorial proton pumping. Between the deprotonation and reprotonation events, bR is in the M state. Diverse experiments undertaken to characterize the M state support a model in which the M state is not a static entity, but rather a progression of two or more functional substates. Structural changes occurring in the M state and in the entire photocycle of wild-type bR can be understood in the context of a model which reconciles the chloride ion-pumping phenotype of mutants D85S and D85T with the fact that bR creates a transmembrane proton-motive force.

The angles between the C(1)-, C(5)-, and C(9)-methyl bonds of the retinylidene chromophore and the membrane normal increase in the M intermediate of bacteriorhodopsin: direct determination with solid-state (2)H NMR

The orientations of three methyl bonds of the retinylidene chromophore of bacteriorhodopsin were investigated in the M photointermediate using deuterium solid-state NMR ((2)H NMR). In this key intermediate, the chromophore has a 13-cis, 15-anti conformation and a deprotonated Schiff base. Purple membranes containing wild-type or mutant D96A bacteriorhodopsin were regenerated with retinals specifically deuterated in the methyl groups of either carbon C(1) or C(5) of the beta-ionone ring or carbon C(9) of the polyene chain. Oriented hydrated films were formed by drying concentrated suspensions on glass plates at 86% relative humidity. The lifetime of the M state was increased in the wild-type samples by applying a guanidine hydrochloride solution at pH 9.5 and in the D96A sample by raising the pH. (2)H NMR experiments were performed on the dark-adapted ground state (a 2:1 mixture of 13-cis, 15-syn and all-trans, 15-anti chromophores), the cryotrapped light-adapted state (all-trans, 15-anti), and the cryotrapped M intermediate (13-cis, 15-anti) at -50 degrees C. Bacteriorhodopsin was first completely converted to M under steady illumination of the hydrated films at +5 degrees C and then rapidly cooled to -50 degrees C in the dark. From a tilt series of the oriented sample in the magnetic field and an analysis of the (2)H NMR line shapes, the angles between the individual C-CD(3) bonds and the membrane normal could be determined even in the presence of a substantial degree of orientational disorder. While only minor differences were detected between dark- and light-adapted states, all three angles increase in the M state. This is consistent with an upward movement of the C(5)-C(13) part of the polyene chain toward the cytoplasmic surface or with increased torsional strain. The C(9)-CD(3) bond shows the largest orientational change of 7 degrees in M. This reorientation of the chromophore in the binding pocket provides direct structural support for previous suggestions (based on spectroscopic evidence) for a steric interaction in M between the C(9)-methyl group and Trp 182 in helix F.

The projection structure of the low temperature K intermediate of the bacteriorhodopsin photocycle determined by electron diffraction

Bacteriorhodopsin (bR) is an integral membrane protein which absorbs visible light and pumps protons across the cell membrane of Halobacterium salinarium. bR is one of the few membrane-bound pumps whose structure is known at atomic resolution. Changes in the protein structure of bR are a crucial element in the mechanism of proton pumping and can be followed by a variety of spectroscopic, and diffraction methods. A number of intermediates in the photocycle have been identified spectroscopically and a number of laboratories have been successful in reporting the structural changes taking place in the later stages of the photocycle over the millisecond time-scale using diffraction techniques. These studies have revealed significant changes in the protein structure, possibly involving changes in flexibility and/or movement of helices. Earlier intermediates which arise and decay on the picosecond to microsecond time-scale have proven more difficult to trap. Here, we report for the first time the successful trapping and diffraction analysis of bR in a low temperature state resembling the very early intermediate, K. We have calculated a projection difference map to 3.5 A resolution. The map reveals no significant structural changes in the molecule, despite having a very low background noise level. This does not rule out the possibility of movements in a direction perpendicular to the plane of the membrane. However, the data are consistent with other evidence that significant structural changes do not occur in the protein itself.

Restricted motion of photoexcited bacteriorhodopsin in purple membrane containing ethanol

The molecular motion of retinal within the purple membrane was investigated by flash-induced absorption anisotropies with or without ethanol. In the absence of ethanol, the measured anisotropies at several wavelengths exhibited almost the same slow decay. This slow decay was attributed to only the rotation of purple membrane sheet itself in the aqueous suspension. In the presence of ethanol, however, we observed the wavelength-dependent anisotropies. The fluidity of the purple membrane, investigated with a fluorescence anisotropy method, was increased by the addition of ethanol. These facts indicated that the characteristic motion of bacteriorhodopsin is induced in perturbed purple membrane with ethanol. The data analysis was performed, taking account of the overlapping of absorption from ground-state bacteriorhodopsin and photointermediates. The results showed that the rotational motion of photointermediates within the membrane was more restricted than that of nonexcited bacteriorhodopsin. The addition of ethanol facilitated the rotation of nonexcited protein, whereas it did not significantly affect the motion of photointermediates. The restricted motion of photointermediates is probably caused by a conformational change in them, which may hinder the rotation of monomer protein and/or induce the interaction between photointermediate and neighboring proteins.

Cooperativity-induced optical anisotropy changes during the photocycle of bacteriorhodopsin

Previous photoselection measurements showed that if the excitation is weak no optical anisotropy changes appear in immobilized purple membranes during the photocycle. The present study demonstrates that surprisingly at stronger excitations the anisotropy changes versus time. At 412 nm the dichroic ratio decreases after a few milliseconds, while at 570 nm the similar decrease is followed by an increase. The phenomenon cannot be described by tiltings of the retinal chromophore. It is the consequence of the cooperative interaction among the photocycling bacteriorhodopsin molecules that regulates the yields of more than one (expectedly two main) parallel pathways existing in the millisecond time domain of the photocycle.

Chromophore reorientations in the early photolysis intermediates of bacteriorhodopsin

The photoselection-induced time-resolved linear dichroism of a bacteriorhodopsin suspension of purple membrane from 350 to 750 nm is measured by a new pseudo-null measurement technique. In combination with time-resolved absorption measurements, these linear dichroism measurements are used to determine the reorientation of the retinal chromophore of bacteriorhodopsin from 50 ns to 50 microseconds after photolysis. This time range covers the times when the K photointermediate decays to form L, as well as the early times during the formation of the M intermediate in the photocycle. An analysis of the photoselection-induced linear dichroism measured directly, along with the absorbance changes polarized parallel to the linearly polarized excitation, shows that the anisotropy is invariant over this time period, implying that the photolyzed chromophore rotates less than 8 degrees C with respect to unphotolyzed chromophores during this part of the photocycle.

Orientation of purple membrane in combined electric and magnetic fields

The orientation of purple membrane in gels for photoelectric measurements is relatively poor, when they are prepared with the standard technique of applying a DC electric field and rapid polymerization. We have improved it by adding a high magnetic field (17.5 T) and increasing the viscosity of the membrane suspension. This process has resulted so far in a 3-fold increase of the photoelectric signals obtained. The magnetic susceptibility of purple membrane was determined.

Introduction of a method for three-dimensional mapping of the charge motion in bacteriorhodopsin

Electric signals associated with the photocycle of bacteriorhodopsin carry valuable information about the proton transport process. Photocurrents measured by different experimental methods are interpreted in terms of intramolecular charge displacements. Permanent electrical asymmetry of the sample is considered to be a prerequisite for the detection of electric signals. The various photoelectric measuring techniques can be distinguished by the way of achievement of this asymmetry. A common feature of the available methods, however, is that the samples are cylindrically symmetric. Consequently, intramembraneous charge displacements can normally be monitored only along the axis of the membrane normal. We developed a novel method that allows also the detection of the in-plane components of the charge displacements. Samples containing oriented purple membrane fragments were used in the experiments, and the rotational symmetry was transiently broken via anisotropic excitation of the bR molecules by linearly polarized light. Kinetics of the normal and in-plane components were measured and interpreted as a result of spatial charge displacements associated with the proton transport process in bacteriorhodopsin.

Light-induced isomerization causes an increase in the chromophore tilt in the M intermediate of bacteriorhodopsin: a neutron diffraction study

Bacteriorhodopsin (BR) was regenerated with two selectively deuterated retinals, one with 11 deuterons in the beta-ionone ring (D11) and the other with 5 deuterons (D5) at the end of the polyene chain closest to the Schiff base at carbon atoms C-14, C-15, and C-20. Both label positions (centers of deuteration) were obtained from difference Fourier maps of projections onto the plane of the membrane by neutron diffraction at 90 K, both in the light-adapted ground-state BR568 and in the photocycle intermediate M412. To retard the decay of M412, purple membrane films were soaked in 0.1 M or 1 M guanidine hydrochloride at pH 9.6. M412 was produced by illuminating oriented membrane films at physiological temperature (278 K), followed by rapid cooling to 90 K in the absence of light. The results show that in the projected structure the ring position is unaltered during the transition from BR568 to M412, whereas the position of the D5 label shifts by 1.4 +/- 0.9 A toward the ring. The shortened interlabel distance in the projected structure for the M412 state implies that as a result of the all-trans/13-cis isomerization, the C-5 to C-13 part of the polyene chain tilts out of the plane of the membrane toward the cytoplasm by about 11 degrees +/- 6 degrees. Pairwise comparison of data sets with the same retinal for the two photocycle states M412 and BR568 leads to four difference-density maps for the protein, which are in agreement with previous work. They show changes in the protein density near helices G and F.

Reorientations in the bacteriorhodopsin photocycle

Reversible photoinduced reorientations of bacteriorhodopsin have been detected in suspensions of the purple membrane of Halobacterium salinarium. The anisotropy in bacteriorhodopsin during the nanosecond through millisecond stages of the photocycle was measured by time-resolved linear dichroism and transient absorption measurements. From these measurements the anisotropies of the K, L, M, and O intermediates were determined and related to the chromophore orientation with respect to the initially selected orientation. The anisotropies of the K and L states are 0.38 +/- 0.01 and 0.35 +/- 0.01, respectively. Further anisotropy decay after formation of the M intermediate in about 0.5 ms is evidence of orientational motion at this stage in the photocycle. A constant anisotropy with a value of 0.39 +/- 0.02 in the O intermediate demonstrates a recovery of the initial protein orientation with the formation of the O state. These results demonstrate that reorientations in BR are photoinduced and reversible. Similar measurements for L and M were carried out for purple membrane in polyacrylamide gels, where the anisotropies in the L and M states are 0.38 +/- 0.014 and 0.36 +/- 0.01, respectively. These results show that reorientations also occur in BR immobilized in gels. Anisotropy decay in the M state after formation of the M intermediate was not detected in the gels, in contrast to the M intermediate in suspensions. Orientational changes are observed for BR in purple membrane suspensions in the K state, during the K-->L step, in the M state possibly related to an M1-->M2 transition, and in the O state, where an almost complete return to the original orientation occurs.(ABSTRACT TRUNCATED AT 250 WORDS)

Proton translocation mechanism and energetics in the light-driven pump bacteriorhodopsin

In spite of many still unsolved problems, the mechanism and energetics of the light-driven proton transport are now basically understood. Energy captured during photoexcitation, and retained in the form of bond rotations and strains of the retinal, is transformed into directed changes in the pKa values of vectorially arranged proton transfer groups. The framework for the spatial and temporal organization of these changes is provided by the protein near the retinal Schiff base. The transport is completed by proton transfer among three essential groups in three domains lying roughly parallel with the membrane plane (Fig. 1): (a) the anionic D85 that is included in a complex of residues on the extracellular side containing also R82, D212, Y57 and bound water; (b) the protonated Schiff base; and (c) the protonated D96 that is included in a complex of residues on the cytoplasmic side containing also R227, T46, S226, and bound water. Other neighboring polar groups and water bound elsewhere which play a role in the transport do so either by further influencing the pKa values of the three protonable groups, or by providing passive pathways for proton transfer. The Schiff base proton, destabilized after photoexcitation, is transferred to the low pKa group D85 located on the extracellular side. The access of the deprotonated Schiff base then changes to the cytoplasmic side (the 'reprotonation switch') and its proton affinity increases. Finally, the proton of the high pKa group D96, with access to the cytoplasmic side, is destabilized by a protein conformational change through rearrangement of R227, T46, S226 and bound water, and becomes transferred to the Schiff base. As shown schematically in Fig. 3, these internal events are coupled to proton release and uptake at the two aqueous surfaces. The charge of the extracellular hydrogen-bonded complex is redistributed upon protonation of D85, and if the pH is above the pKa of the complex a proton is released to the bulk. After reprotonation of the Schiff base the pKa of the cytoplasmic hydrogen-bonded complex is raised well above the pH, and D96 regains a proton from the bulk. If the pH is lower than the pKa of the extracellular complex the proton release is delayed until the end of the photocycle. In either sequence there is net transfer of a proton from the cytoplasmic to the extracellular phase. The transfer of excess free energy from the chromophore to the protein, and finally to the transported proton, is described by a characteristic thermodynamic cycle.(ABSTRACT TRUNCATED AT 400 WORDS)

Light-induced reorientation in the purple membrane

Reorientation of bacteriorhodopsin in the native purple membrane was studied by time-resolved linear dichroism spectroscopy (TRLD) over the millisecond time regime. The time responses observed in TRLD are distinctly different from the isotropic transient absorption (TA) at wavelengths in the range 550-590 nm, where the bacteriorhodopsin ground state absorbs. In contrast, the TA and TRLD responses have nearly identical time dependence at 410 and 690 nm, where the intermediates M and O, respectively, principally contribute. These results demonstrate ground-state bacteriorhodopsin reorientation triggered by the photocycle. The TRLD and TA data are analyzed to test models for reorientational motion. Rotational diffusion of ground-state bacteriorhodopsin cannot account for the details of the data. Rather, the results are shown to be consistent with a reversible reorientation of "spectator" (nonexcited) members of the bacteriorhodopsin trimer in the purple membrane in response to the photocycling member of the trimer. This response may be associated with cooperativity in the trimer.

Interrelations of bioenergetic and sensory functions of the retinal proteins

Rhodopsins are intrinsic membrane retinal-containing proteins composed of 7 hydrophobic alpha-helical transmembrane columns and hydrophilic sequences of various length connecting the helices and localized at N- and C-ends of the polypeptide. The chromophore (retinal) forms a Schiff base with a lysine residue in the middle part of the last alpha-helix. Absorption of a photon results in isomerization of retinal which gives rise to a conformational change in the protein moiety. Rhodopsins can be involved in two entirely different types of activities, i.e. ion pumping and photosensing. Recent observations concerning the pumping and sensory mechanisms allowed both these events to be explained in terms of one and the same unitary concept, which postulates the formation of a hydrophilic cleft in the hydrophobic part of the protein molecule as a crucial step in energy conservation and photosensing.

The two consecutive M substates in the photocycle of bacteriorhodopsin are affected specifically by the D85N and D96N residue replacements

The photocycle of the proton pump bacteriorhodopsin contains two consecutive intermediates in which the retinal Schiff base is unprotonated; the reaction between these states, termed M1 and M2, was suggested to be the switch in the proton transport which reorients the Schiff base from D85 on the extracellular side to D96 on the cytoplasmic side (Váró and Lanyi, Biochemistry 30, 5016-5022, 1991). At pH 10 the absorption maxima of both M1 and M2 could be determined in the recombinant D96N protein. We find that M1 absorbs at 411 nm as do M1 and M2 in wild-type bacteriorhodopsin, but M2 absorbs at 404 nm. Thus, in M2 but not M1 the unprotonated Schiff base is affected by the D96N residue replacement. The connectivity of the Schiff base to D96 in the detected M2 state, but not in M1, is thereby established. On the other hand, the photostationary state which develops during illumination of D85N bacteriorhodopsin contains an M state corresponding to M1 with an absorption maximum shifted to 400 nm, suggesting that this species in turn is affected by D85. These results are consistent with the suggestion that M1 and M2 are pre-switch and post-switch states, respectively.