Events in proton pumping by bacteriorhodopsin

Evidence that the photoelectric response of bacteriorhodopsin occurs in less than 5 picoseconds

The initial photoinduced charge separation in bacteriorhodopsin is shown to occur in <5 ps. This result is obtained by measuring the photovoltage rise time in an oriented film of bacteriorhodopsin (BR). A dye laser syncronously pumped by an Argon ion cw mode locked laser is used to produce 3-ps light pulses which, after passing through a dye amplifier chain, photoexcite the BR sample. The photovoltage transient is detected by an ultra-fast Josephson junction digital sampling oscilloscope with liquid-helium-cooled input circuitry.

Distributed kinetics of the charge movements in bacteriorhodopsin: evidence for conformational substates

The flash-induced charge movements during the photocycle of light-adapted bacteriorhodopsin in purple membranes attached to a black lipid membrane were investigated under voltage clamp and current clamp conditions. Signal registration ranged from 200 ns to 30 s after flash excitation using a logarithmic clock, allowing the equally weighted measurement of the electrical phenomena over eight decades of time. The active pumping signals were separated from the passive system discharge on the basis of an equivalent circuit analysis. Both measuring methods were shown to yield equivalent results, but the charge translocation could be accurately monitored over the whole time range only under current clamp conditions. To describe the time course of the photovoltage signals a model based on distributed kinetics was found to be more appropriate than discrete first order processes suggesting the existence of conformational substates with distributed activation energies. The time course of the active charge displacement is characterised by a continuous relaxation time spectrum with three broad peaks plus an unresolved fast transient (<0.3 mus) of opposite polarity. The time constants and relative amplitudes (in brackets) derived from the peak rate constants and relative areas of the three bands are: tau(1) = 32 mus (20%), tau(2) = 0.89 ms (15%) and tau(3) = 18 ms (65%) at 25 degrees C in 150 mM KCl at pH7. The Arrhenius plots of the peak rate constants were linear yielding activation energies of E(A1) = 57 kJ/mol, E(A2) = 52 kJ/mol, and E(A3) = 44 kJ/mol. The electrical signal at 890 mus has no counterpart in the photocycle of bacteriorhodopsin suspensions. Fits with a sum of exponentials required 5 to 6 components and were not reproducible. Analysis of photoelectrical signals with continuous relaxation time spectra gave equally good fits with fewer parameters and were well reproducible.

Reversal of the surface charge asymmetry in purple membrane due to single amino acid substitutions

Twenty-seven mutant bacteriorhodopsin's were screened to determine the PKa for reversal of the permanent electric dipole moment. The photoelectric response of an aqueous purple-membrane suspension was used to determine the direction of the purple-membrane dipole moment as a function of pH. The pK(a) for the dipole reversal of wild-type bacteriorhodopsin is 4.5. Six of the 27 mutant bacteriorhodopsin's were found to have a pK(a) for dipole reversal larger than that of wild-type bacteriorhodopsin. Two of these mutants, L93T and L93W, involve a neutral amino acid substitution in the interior of the protein. The direction of the purple-membrane permanent electric dipole moment is determined by the purple-membrane surface charge asymmetry. We conclude that these two substitutions, which do not involve charge replacement, alter the pK(a) for the reversal of the purple-membrane surface charge asymmetry. We suggest that these changes to the pK(a) are due to altered protein folding at the surface of the purple-membrane induced by single-site substitutions in the protein interior.

Bacteriorhodopsin expressed in Schizosaccharomyces pombe pumps protons through the plasma membrane

Bacterioopsin (bO) from Halobacterium salinarium ("Halobacterium halobium") has been functionally expressed in a heterologous system, the fission yeast Schizosaccharomyces pombe. Regeneration of bO to bacteriorhodopsin (bR) in S. pombe has been achieved in vivo by addition of the chromophore retinal to the culture medium, as shown for a retinal-negative mutant of H. salinarium (JW5). Western blot analysis revealed that bR is more stable than bO against proteolysis in fission yeast and also in JW5. The light-driven proton pump is expressed in the eukaryotic organism and incorporated into the plasma membrane. Illumination of intact yeast cells leads to acidification of the external medium due to the translocation of H+ from inside to outside of the cell, indicating the same orientation of bR in the yeast plasma membrane as in H. salinarium. The kinetics of proton release into the water phase was observed with the optical pH indicator pyranine. Time-resolved absorbance changes of isolated plasma membrane measured by flash spectroscopy showed rise and decay of the M intermediate during the photocycle similar to those in the homologous system. Photocurrents and photovoltages were recorded with yeast plasma membrane attached to a planar lipid membrane and to a polytetrafluoroethylene (Teflon) film, respectively. Stationary currents measured in the presence of a protonophore showed continuous pumping activity of bR. The action spectrum of the photocurrent and the kinetics of the photovoltage were analyzed and compared with signals obtained from purple membranes. From all these different investigations we conclude that the integral membrane protein bR is correctly folded in vivo into the cytoplasmic membrane of the fission yeast S. pombe.

Solving complex photocycle kinetics. Theory and direct method

A direct nonlinear least squares method is described that obtains the true kinetic rate constants and the temperature-independent spectra of n intermediates from spectroscopic data taken in the visible at three or more temperatures. A theoretical analysis, which is independent of implementation of the direct method, proves that well determined local solutions are not possible for fewer than three temperatures. This analysis also proves that measurements at more than n wavelengths are redundant, although the direct method indicates that convergence is faster if n + m wavelengths are measured, where m is of order one. This suggests that measurements should concentrate on high precision for a few measuring wavelengths, rather than lower precision for many wavelengths. Globally, false solutions occur, and the ability to reject these depends upon the precision of the data, as shown by explicit example. An optimized way to analyze vibrational spectroscopic data is also presented. Such data yield unique results, which are comparably accurate to those obtained from data taken in the visible with comparable noise. It is discussed how use of both kinds of data is advantageous if the data taken in the visible are significantly less noisy.

Development of membrane-based biosensors: measurement of current from photocycling bacteriorhodopsin on patch clamp electrodes

Our initial work toward developing membrane protein-based biosensors has involved use of bacteriorhodopsin (BR) as a model membrane protein. BR was incorporated into liposomes of a polymerizable lecithin, and was shown to pump protons in response to illumination both before and after polymerization of the lipids. In the work described in this paper, BR was first reincorporated in liposomes of asolectin by consonication with purple membrane. The liposomes, which sustained the function of the protein, were used to form a monolayer at the air-water interface. This monolayer was transferred as a bilayer onto patch electrode. When illuminated with a pulse of 514.5-nm light the lipid/protein patch produced a current spike into the pipette corresponding to events no later than the generation of the 412-nm intermediate, probably caused by pumping of protons across the patch membrane. The experiment demonstrates not only the extreme sensitivity of amperometric detection, but also a small tendency for membrane proteins to preferentially orient in this configuration.

Kinetic analysis of displacement photocurrents elicited in two types of bacteriorhodopsin model membranes

Fast displacement photocurrents have been reported in bacteriorhodopsin model membranes by several groups of investigators since 1977. A fast component (B1) is associated with positive charge displacement in the direction opposite to that of a physiological proton translocation. A slower component (B2) of opposite polarity is associated with positive charge displacement in the same direction as the proton translocation. Using two slightly different methods for model membrane formation, we observed photosignals with or without a significant B2 component under appropriate conditions. By means of the tunable voltage clamp method of measurement (Hong, F.T., and D. Mauzerall, 1974, Proc. Natl. Acad. Sci. USA, 71:1564-1568) we demonstrated that the time course of the B1 signal is completely predictable by an equivalent circuit containing a chemical capacitance. From the equivalent circuit analysis, we obtained a first-order relaxation time constant of 12.3 +/- 0.7 microseconds at room temperature. We also found a slight temperature dependence of the B1 relaxation with an activation energy of 2.54 +/- 0.24 kcal/mol. We found no pH dependence of the B1 component in the range of 0 to 11, whereas the B2 component is diminishing in a graded manner when the pH is varied from 0 to 10. These results are diametrically different from what reported previously (Drachev, L.A., A.D. Kaulen, L.V. Khitrina, and V.P. Skulachev, 1981, Eur. J. Biochem., 117:461-470). Our results support the interpretation that the B1 component is generated by an intramolecular charge displacement accompanying the light-induced reactions of bacteriorhodopsin and that the B2 component is generated by a process of proton uptake from the intracellular aqueous phase and subsequent release into the same aqueous phase. The impact of the present results on the conventional practice of identifying photointermediates of bacteriorhodopsin by spectroscopic means is discussed.

Temperature dependence of photovoltages generated by bacteriorhodopsin

The temperature dependence of the photovoltage developed by a model membrane containing bacteriorhodopsin (BR) is studied. The model membrane is formed by first coating a thin Teflon sheet with lipid and then fusing BR vesicles to it. The time course of the photoresponse is resolved down to 1 microsecond. The photoresponse is taken to be a sum of exponentials. Exponential time constants and amplitudes are determined by an analysis of the photoresponse with a photovoltage vs. log time plot, correlation filter, and nonlinear least-squares routine. The photovoltage is taken to be the sum of three exponentials but only two of the three time constants are resolved. Both are temperature dependent and indicate a thermally activated transport process. The corresponding activation energies are 55 kJ/mol and 62 kJ/mol. Since the photovoltage is proportional to charge times displacement the corresponding charge displacements are 11 and 34 A assuming a total displacement of 45 A. The remaining exponential term corresponds to a small negative transient in the photovoltage that has a rise time less than 1 microsecond even at -20 degrees C. The calculated charge displacement is estimated to be less than 2 A.

Simulations of frequency-dependent photoacoustic magnitude signals and their implications for bacteriorhodopsin photocycle energetics

The modulation frequency dependence of photoacoustic signals obtained from photoactive samples can provide information on the time-dependent enthalpy changes occurring during the light-induced process. The experimental requirements for this type of calorimetry, and the interpretation ot the results, are critically examined with reference to the light-driven proton pump bacteriorhodopsin. For a three-step unbranched model of the bacteriorhodopsin photocycle we derive an expression for the photoacoustic magnitude signal as a function of frequency. Simulations are performed for various values of the rate constants and energetic changes. It is shown that the net heat uptake during a low, final step postulated by some workers should be reflected in the photoacoustic magnitude frequency spectrum, giving rise to a characteristic maximum. However, this effect, which has been observed experimentally, may also be produced by a fast, strongly endothermic step occurring earlier. The precise chronology of an endothermic transition cannot be resolved unambiguously by magnitude measurements alone, although they are free from assumptions regarding difficult-to-measure phase relationships. Hence, the published photoacoustic observations showing the effect are consistent with a cyclic sequence of events in which the bacteriorhodopsin system first undergoes an increase of entropy, followed by a decrease on returning to the initial state, as well as the reverse. It is argued that the molecular disorder-order sequence is more probable.

Photoelectric signals generated by bovine rod outer segment disk membranes attached to a lecithin bilayer

Purified bovine rod outer segment disk membranes were attached to a lecithin bilayer membrane. After photoexcitation with a 500-nm flash delivered by a dye laser, a negative photovoltage was observed on the bilayer under normal ionic strengths (100 mM KCl), which had a rise phase of 1-3 ms at 20 degrees C. The photoresponse was obviously due to bleaching of rhodopsin as it decreased for successive flashes of light. It originated most probably during the metarhodopsin-I metarhodopsin-II (meta-I-II) transition of rhodopsin because it was pH dependent at 2 degrees C but not at 20 degrees C. At 10 mM KCl, i.e., under hypotonic conditions, a positive photovoltage with slower kinetics than at high salt was observed. As the disk membranes were merely attached to the bilayer membrane, the photovoltage was apparently due to a light-induced transmembrane potential change in the disk membranes. Possible electrogenic mechanisms underlying the photosignal will be discussed.