Photoelectrochemical cycle of bacteriorhodopsin.

The scheme of the bacteriorhodopsin photocycle associated with a transmembrane proton transfer and electrogenesis is considered. The role of conformational changes in the polypeptide chain during the proton transport is discussed.

Electrogenic processes and protein conformational changes accompanying the bacteriorhodopsin photocycle.

The possible mechanisms of electrogenic processes accompanying proton transport in bacteriorhodopsin are discussed on the basis of recent structural data of the protein. Apparent inconsistencies between experimental data and their interpretation are considered. Special emphasis is placed on the protein conformational changes accompanying the reprotonation of chromophore and proton uptake stage in the bacteriorhodopsin photocycle.

Formation of the M(N) (M(open)) intermediate in the wild-type bacteriorhodopsin photocycle is accompanied by an absorption spectrum shift to shorter wavelength, like that in the mutant D96N bacteriorhodopsin photocycle.

Maximum of the M intermediate difference spectrum in the wild-type Halobacterium salinarium purple membrane is localized at 405-406 nm under conditions favoring accumulation of the M(N) intermediate (6 M guanidine chloride, pH 9.6), whereas immediately after laser flash the maximum is localized at 412 nm. The maximum is also localized at 412 nm 0.1 msec after the flash in the absence of guanidine chloride at pH 11.3. Within several milliseconds the maximum is shifted to short-wavelength region by 5-6 nm. This shift is similar to that in the D96N mutant which accompanies the M(N) (M(open)) intermediate formation. The main two differences are: 1) the rate of the shift is slower in the wild-type bacteriorhodopsin, and is similar to the rate of the M to N intermediate transition (t1/2 approximately 2 msec); 2) the shift in the wild-type bacteriorhodopsin is observed at alkaline pH values which are higher than pK of the Schiff base (approximately 10.8 at 1 M NaCl) in the N intermediate with the deprotonated Asp-96. Thus, the M(N) (M(open)) intermediate with open water-permeable inward proton channel is observed only at high pH, when the Schiff base and Asp-96 are deprotonated. The data confirmed our earlier conclusion that the M intermediate observed at lower pH has the closed inward proton channel.

Membrane potential stabilizes the O intermediate in liposomes containing bacteriorhodopsin.

In the bacteriorhodopsin-containing proteoliposomes, a laser flash is found to induce formation of a bathointermediate decaying in several seconds, the difference spectrum being similar to the purple-blue transition. Different pH buffers do not affect the intermediate, whereas an uncoupler, gramicidin A, and lipophilic ions accelerate decay of the intermediate or inhibit its formation. In the liposomes containing E204Q bacteriorhodopsin mutant, formation of the intermediate is suppressed. In the wild-type bacteriorhodopsin liposomes, the bathointermediate formation is pH-independent within the pH 5-7 range. The efficiency of the long-lived O intermediate formation increases at a low pH. In the wild-type as well as in the E204Q mutant purple membrane, the O intermediate decay is slowed down at slightly higher pH values than that of the purple-blue transition. It is suggested that the membrane potential affects the equilibrium between the bacteriorhodopsin ground state (Glu-204 is protonated and Asp-85 is deprotonated) and the O intermediate (Asp-85 is protonated and Glu-204 is deprotonated), stabilizing the latter by changing the relative affinity of Asp-85 and Glu-204 to H(+). At a low pH, protonation of a proton-releasing group (possibly Glu-194) in the bacteriorhodopsin ground state seems to prevent deprotonation of the Glu-204 during the photocycle. Thus, all protonatable residues of the outward proton pathway should be protonated in the O intermediate. Under such conditions, membrane potential stabilization of the O intermediate in the liposomes can be attributed to the direct effect of the potential on the pK value of Asp-85.

Two forms of N intermediate (N(open) and N(closed)) in the bacteriorhodopsin photocycle.

Glutaraldehyde, aluminum ions and glycerol (that inhibit the M intermediate decay in the wild-type bacteriorhodopsin and azide-induced M decay in the D96N mutant by stabilization of the M(closed)) accelerate the N decay in the D96N mutant. The aluminum ions, the most potent activator of the N decay, induce a blue shift of the N difference spectrum by approximately 10 nm. Protonated azide as well as acetate and formate inhibit the N decay in both the D96N mutant and the wild-type protein. It is concluded that the N intermediate represents, in fact, an equilibrium mixture of the two ('open' and 'closed') forms. These two forms, like M(closed) and M(open), come to an equilibrium in the microseconds range. The absorption spectrum of the N(open) is slightly shifted to red in comparison to that of the N(closed). Again, this resembles the M forms. 13-cis-all-trans re-isomerization is assumed to occur in the N(closed) form only. Binding of 1-2 molecules of protonated azide stabilizes the N(open) form. Existence of the 'open' and 'closed' forms of the M and N intermediates provides the appropriate explanation of the cooperative phenomenon as well as some other effects on the bacteriorhodopsin photocycle. Summarizing the available data, we suggest that M(open) is identical to the M(N) form, whereas M1 and M2 are different substates of M(closed).

Resolution of electrogenic steps coupled to conversion of cytochrome c oxidase from the peroxy to the ferryl-oxo state.

Charge translocation across the membrane coupled to transfer of the third electron in the reaction cycle of bovine cytochrome c oxidase (COX) has been studied. Flash-induced reduction of the peroxy intermediate (P) to the ferryl-oxo state (F) by tris-bipyridyl complex of Ru(II) in liposome-reconstituted COX is coupled to several phases of membrane potential generation that have been time-resolved with the use of an electrometric technique applied earlier in the studies of the ferryl-oxo-to-oxidized (F --> O) transition of the enzyme [Zaslavsky, D., et al. (1993) FEBS Lett. 336, 389-393]. As in the case of the F --> O transition, the electric response associated with photoreduction of P to F includes a rapid KCN-insensitive electrogenic phase with a tau of 40-50 microseconds (reduction of heme a by CuA) and a multiphasic slower part; this part is cyanide-sensitive and is assigned to vectorial transfer of protons coupled to reduction of oxygen intermediate in the binuclear center. The net KCN-sensitive phase of the response is approximately 4-fold more electrogenic than the rapid phase, which is similar to the characteristics of the F --> O electrogenic transition and is consistent with net transmembrane translocation of two protons per electron, including vectorial movement of both "chemical" and "pumped" protons. The protonic part of the P --> F electric response is faster than in the F --> O transition and can be deconvoluted into three exponential phases with tau values varying for different samples in the range of 0.25-0.33, 1-1.5, and 6-7.5 ms at pH 8. Of these three phases, the 1-1.5 ms component is the major one contributing 50-60%. The P --> F conversion induced by single electron photoreduction of the peroxy state as studied in this work is several times slower than the P --> F transition resolved during oxidation of the fully reduced oxidase by molecular oxygen. The role of the CuB redox state in controlling the rate of P --> F conversion of heme a3 is discussed.

13-Demethylbacteriorhodopsin: formation and some properties.

Incorporation of 9-cis-13-demethylretinal into bacterioopsin was shown to form the covalent purple complex. This result was predicted by our hypothesis about the structure of the BR chromophore cavity (Mol. Biologiya 29:1398-1407 (1995) (in Russian)). It supports the hypothesis and eliminates the main objection known from the literature.

Photovoltage evidence that Glu-204 is the intermediate proton donor rather than the terminal proton release group in bacteriorhodopsin.

Electrogenic events in the E204Q bacteriorhodopsin mutant have been studied. A two-fold decrease in the magnitude of microsecond photovoltage generation coupled to M intermediate formation in the E204Q mutant is shown. This means that deprotonation of E204 is an electrogenic process and its electrogenicity is comparable to that of the proton transfer from the Schiff base to D85. pH dependence of the electrogenicity of M intermediate formation in the wild-type bacteriorhodopsin reveals only one component corresponding to the protonation of D85 in the bacteriorhodopsin ground state and transition of the purple neutral form into the blue acid form. Thus, the pK of E204 in the M state is close to the pK of D85 in the bacteriorhodopsin ground state (< 3) and far below the pK of the terminal proton release group (approximately 6). It is concluded that E204 functions as the intermediate proton donor rather than the terminal proton release group in the bacteriorhodopsin proton pump.

Photophosphorylation in alkalophilic halobacterial cells containing halorhodopsin: chloride-ion cycle?

Light-driven ATP synthesis is found in cells of the alkalophilic bacterium Natronobacterium pharaonis containing halorhodopsin but deficient in H+-pumping bacteriorhodopsin. Photophosphorylation occurs with cyanide-inhibited respiratory chain as well as without cyanide in conditions with low C1- concentration in the incubation medium. Increase in C1- concentration from 0.1 to 2.35 M in the incubation medium leads to inhibition of photophosphorylation. Continuous illumination increases membrane Delta Psi if respiration is inhibited by cyanide. This effect is stimulated by DCCD, an ATPase inhibitor. These data can be explained if one suggests that halorhodopsin pumps C1- into the cells whereas C1- release from the cells through C1--ATP-synthase is coupled with the ATP synthesis (chloride-ion cycle).

Flash-induced voltage changes in halorhodopsin from Natronobacterium pharaonis.

The flash-induced voltage response of halorhodopsin at high NaCl concentration comprises two main kinetic components. The first component with tau approximately 1 micros does not exceed 4% of the overall response amplitude and is probably associated with the formation of the L (hR520) intermediate. The second main component with tau approximately 1-2.5 ms which is independent of Cl- concentration can be ascribed to the transmembrane Cl- translocation during the L intermediate decay. The photoelectric response in the absence of Cl- has the opposite polarity and does not exceed 6% of the overall response amplitude at high NaCl concentration. A pH decrease results in substitution of the Cl(-)-dependent components by the photoresponse which is similar to that in the absence of Cl-. Thus, the difference between photoresponses of chloride-binding and chloride-free halorhodopsin forms resembles that of bacteriorhodopsin purple neutral and blue acid forms, respectively. The photovoltage data obtained can hardly be explained within the framework of the photocycle scheme suggested by Varo et al. [Biochemistry 34 (1995), 14490-14499]. We suppose that the O-type intermediate belongs to some form of halorhodopsin incapable of Cl- transport.

Inhibition of the M1-->M2 (M(closed) --> M(open)) transition in the D96N mutant photocycle and its relation to the corresponding transition in wild-type bacteriorhodopsin.

Glutaraldehyde, lutetium ions and glycerol inhibit the blue shift of the difference spectra maximum of the M intermediate in the D96N mutant. The M formed has a spectrum indistinguishable from the M intermediate in wild-type bacteriorhodopsin. It has been concluded that the M(open) form previously described by us is identical to the M2 and Mn intermediates postulated by Zimanyi et al. (Photochem. Photobiol. (1992) 56, 1049-1055) and Sasaki et al. (J. Biol. Chem. (1992) 267, 20782-20786), respectively. It is supposed that its formation is accompanied by the appearance of the cytoplasmic proton half-channel. M(open) in the wild-type protein is present in a very low amount due to the shift of the M(closed) <--> M(open) equilibrium towards the M(closed). The inhibitors used do not prevent the multiphase pattern of the M formation in either mutant or wild-type proteins.

Cl- -dependent photovoltage responses of bacteriorhodopsin: comparison of the D85T and D85S mutants and wild-type acid purple form.

Laser flash-induced photovoltage responses of the D85S and D85T mutants as well as of the wild-type acid blue form are similar and reflect intraprotein charge redistribution caused by retinal isomerization. The Cl- -induced transition of all of these blue forms into purple ones is accompanied by the appearance of electrogenic stages, which is probably associated with Cl- translocation in the cytoplasmic direction. Cl- translocation efficiency of these purple forms is much lower than that of the proton transport by the wild-type bacteriorhodopsin. The values of the efficiency do not exceed 15, 8 and 3% for the D85T, D85S and wild-type acid purple form, respectively. Cl- induces an additional electrogenic phase in the photovoltage responses of the D85S mutant and the wild-type acid purple form. This phase is supposed to be associated with the reversible Cl- movement in the extracellular direction. It is interesting that this component is absent in the photovoltage response of the D85T mutant which has, like halorhodopsin, a threonine residue at position 85.

Complicated character of the M decay pH dependence in the D96N mutant is due to the two pathways of the M conversion.

At high ionic strength, the pH dependence of the M intermediate decay in a photocycle of the D96N mutant bacteriorhodopsin shows a complicated behavior which is found to be due to the coexistence of two pathways of the M conversion. The M decay which dominates at pH < 5 is coupled to the proton uptake from the cytoplasmic surface and proceeds probably through the N intermediate. This pathway is inhibited by glutaraldehyde, the potent inhibitor of M decay in the wild-type bacteriorhodopsin and of the azide-facilitated M decay in the D96N mutant. Another pathway of the M decay is predominant at pH > 5. This pathway is insensitive to glutaraldehyde and some other similar inhibitors (lutetium ions, sucrose and glycerol). On the other hand, it is sensitive to the pK changes of the group X (Glu-204) in the outward proton pathway. Possibly, the M decay through this pathway represents a reverse H+ transport process (the proton uptake from the external surface) and proceeds via the L intermediate.

Two bacteriorhodopsin M intermediates differing in accesibility of the Schiff base for azide.

Glutaraldehyde treatment leads to the inhibition (i) of the M intermediate decay in wild-type bacteriorhodopsin (bR) and (ii) of the azide-facilitated M decay in the D96N mutant bR. LuCl3 is shown to be a more potent inhibitor of both processes. Glycerol and sucrose are also inhibitors. None of these agents change the linearity of the azide concentration dependency of the M decay in the D96N mutant but they do shift this dependency to higher azide concentrations. It is concluded that the two M forms are in equilibrium. These M forms differ in the accessibility of the Schiff base for azide and, probably, also for water molecules. The above-mentioned agents shift the equilibrium toward the less accessible M form. The data obtained are in line with the model of azide action as the penetrating proton donor and can hardly be realized within the framework of the model of Le Coutre et al. [(1995) Proc. Natl. Acad. Sci. USA 92, 4962-4966] which assumes that a bound anionic form of azide catalyzes proton transfer to the Schiff base.

delta psi-mediated signalling in the bacteriorhodopsin-dependent photoresponse.

It has been shown previously that the proton-pumping activity of bacteriorhodopsin from Halobacterium salinarium can transmit an attractant signal to the bacterial flagella upon an increase in light intensity over a wide range of wavelengths. Here, we studied the effect of blue light on phototactic responses by the mutant strain Pho8l-B4, which lacks both sensory rhodopsins but has the ability to synthesize bacteriorhodopsin. Under conditions in which bacteriorhodopsin was largely accumulated as the M412 bacteriorhodopsin photocycle intermediate, halobacterial cells responded to blue light as a repellent. This response was pronounced when the membrane electric potential level was high in the presence of arginine, active oxygen consumption, or high-background long-wavelength light intensity but was inhibited by an uncoupler of oxidative phosphorylation (carbonyl cyanide 3-chlorophenylhydrazone) and was inverted in a background of low long-wavelength light intensity. The response to changes in the intensity of blue light under high background light was asymmetric, since removal of blue light did not produce an expected suppression of reversals. Addition of ammonium acetate, which is known to reduce the pH gradient changes across the membrane, did not inhibit the repellent effect of blue light, while the discharge of the membrane electric potential by tetraphenylphosphonium ions inhibited this sensory reaction. We conclude that the primary signal from bacteriorhodopsin to the sensory pathway involves changes in membrane potential.

Cooperative phenomena in the photocycle of D96N mutant bacteriorhodopsin.

The M intermediate decay in the photocycle of D96N mutant bacteriorhodopsin does not depend on the light intensity of the exciting flash. Cooperative phenomena in the photocycle are revealed after addition of azide causing acceleration of the M decay and making it kinetically well separated from the N decay. Increase in the light intensity induces slight deceleration of the M decay and significant acceleration of the N decay. The data obtained directly confirm our recent model [Komrakov and Kaulen (1995) Biophys. Chem. 56, 113-119], according to which appearance of the Mslow intermediate in the photocycle of the wild type bR at high light intensity is due to destabilization of the N intermediate leading to the acceleration of the N-->M and N-->bR reactions.

M-decay in the bacteriorhodopsin photocycle: effect of cooperativity and pH.

The dependence of the bacteriorhodopsin (bR) photocycle on the intensity of the exciting flash was investigated in purple membranes. The dependence was most pronounced at slightly alkaline pH values. A comparison study of the kinetics of the photocycle and proton uptake at different intensities of the flash suggested that there exist two parallel photocycles in purple membranes at a high intensity of the flash. The photocycle of excited bR in a trimer with the two other bR molecules nonexcited is characterized by an almost irreversible M --> N transition. Excitation of two or three bR in a trimer induces the N --> M back reaction and accelerates the N --> bR transition. Based on the qualitative similarity of the pH dependencies of the photocycles of solubilized bR and excited dimers and trimers we proposed that the interaction of nonexcited bR in trimers alters the photocycle of the excited monomer as compared to solubilized bR and the changes in the photocycles in excited dimers and trimers are the result of decoupling of this interaction.

Rapid kinetics of membrane potential generation by cytochrome c oxidase with the photoactive Ru(II)-tris-bipyridyl derivative of cytochrome c as electron donor.

Yeast iso-1-cytochrome c covalently modified at cysteine-102 with (4-bromomethyl-4'-methylbipyridine)[bis(bipyridine)]Ru2+ (Ru-102-Cyt c) has been used as a photoactive electron donor to mitochondrial cytochrome c oxidase (COX) reconstituted into phospholipid vesicles. Rapid kinetics of membrane potential generation by the enzyme following flash-induced photoreduction of Ru-102-Cyt c heme has been measured and compared to photovoltaic responses observed with Ru(II)(bipyridyl)3 (RuBpy) as the photoreductant [D.L. Zaslavsky et al. (1993) FEBS Lett. 336, 389-393]. At low ionic strength, when Ru-102-Cyt c forms a tight electrostatic complex with COX, flash-activation results in a polyphasic electrogenic response corresponding to transfer of a negative charge to the interior of the vesicles. The initial rapid phase is virtually identical to the 50 microsecond transient observed in the presence of RuBpy as the photoactive electron donor which originates from electrogenic reduction of heme a by CuA. CuA reduction by Ru-102-Cyt c turns out to be not electrogenic in agreement with the peripheral location of visible copper in the enzyme. A millisecond phase (tau ca. 4 ms) following the 50 microsecond initial part of the response and associated with vectorial translocation of protons linked to oxygen intermediate interconversion in the binuclear centre, can be resolved both with RuBpy and Ru-102-Cyt c as electron donors; however, this phase is small in the absence of added H2O2. In addition to these two transients, the flash-induced electrogenic response in the presence of Ru-102-Cyt c reveals a large slow phase of delta psi generation not observed with RuBpy. This phase is completely quenched upon inclusion of 100 microM ferricyanide in the medium and originates from a second order reaction of COX with the excess Ru-102-Cyt c2+ generated by the flash in a solution.

On the two forms of bacteriorhodopsin.

In our previous work [(1993) FEBS Lett. 313, 248-250; (1993) Biochem. Int. 30, 461-469] M-intermediate formation of wild-type bacteriorhodopsin was shown to involve two components differing in time constants (tau 1 = 60-70 microseconds and tau 2 = 220-250 microseconds), which were suggested to reflect two independent pathways of M-intermediate formation. The contribution of the fast M was 4-times higher than the slow one. Our present research on M-intermediate formation in the D115N bacteriorhodopsin mutant revealed the same components but at a contribution ratio of 1:1. Upon lowering the pH, the slow phase of M-formation vanished at a pK of 6.2, and in the pH region 3.0-5.5 only the M-intermediate with a rise time of 60 microseconds was present. A 5-6 h incubation of D115N bacteriorhodopsin at pH 10.6 resulted in the irreversible transformation of 50% of the protein into a form with a difference absorbance maximum at 460 nm. This form was stable at pH 7.5 and had no photocycle, including M-intermediate formation. The remaining bacteriorhodopsin contained 100% fast M-intermediate. The disappearance of the 250-microseconds phase concomitant with bR460 formation indicates that at neutral pH bacteriorhodopsin exists as two spectroscopically indistinguishable forms.

Ion permeability induced in artificial membranes by the ATP/ADP antiporter.

The hypothesis on the additional function of the ATP/ADP antiporter (ANT) as uncoupling protein has been tested in proteoliposomes and planar bilayer phospholipid membranes (BLM). It is found that dissipation of the light-induced delta pH in the dark is very much faster in ANT-bacteriorhodopsin proteoliposomes than in proteoliposomes containing bacteriorhodopsin as the only protein. Mersalyl treatment of ANT-bacteriorhodopsin proteoliposomes causes further increase in the delta pH dissipation rate due to formation of a high conductance pore. The properties of this pore are studied on ANT incorporated to BLM. They proved to be similar to those of so-called multiple conductance channel or permeability transition pore of inner mitochondrial membrane. The conductance of the single channel is as high as 2.2 nS. The channel fails to discriminate between K+, Na+, H+ and Cl-. Thus the obtained data are consistent with the assumption that native and modified ANT might function as an H(+)-specific conductor and as a permeability transition pore, respectively.