Control of bacteriorhodopsin color by chloride at low pH. Significance for the proton pump mechanism

Lidocaine turns the surface charge of biological membranes more positive and changes the permeability of blood-brain barrier culture models

The surface charge of brain endothelial cells forming the blood-brain barrier (BBB) is highly negative due to phospholipids in the plasma membrane and the glycocalyx. This negative charge is an important element of the defense systems of the BBB. Lidocaine, a cationic and lipophilic molecule which has anaesthetic and antiarrhytmic properties, exerts its actions by interacting with lipid membranes. Lidocaine when administered intravenously acts on vascular endothelial cells, but its direct effect on brain endothelial cells has not yet been studied. Our aim was to measure the effect of lidocaine on the charge of biological membranes and the barrier function of brain endothelial cells. We used the simplified membrane model, the bacteriorhodopsin (bR) containing purple membrane of Halobacterium salinarum and culture models of the BBB. We found that lidocaine turns the negative surface charge of purple membrane more positive and restores the function of the proton pump bR. Lidocaine also changed the zeta potential of brain endothelial cells in the same way. Short-term lidocaine treatment at a 10 μM therapeutically relevant concentration did not cause major BBB barrier dysfunction, substantial change in cell morphology or P-glycoprotein efflux pump inhibition. Lidocaine treatment decreased the flux of a cationic lipophilic molecule across the cell layer, but had no effect on the penetration of hydrophilic neutral or negatively charged markers. Our observations help to understand the biophysical background of the effect of lidocaine on biological membranes and draws the attention to the interaction of cationic drug molecules at the level of the BBB.

Weakened coupling of conserved arginine to the proteorhodopsin chromophore and its counterion implies structural differences from bacteriorhodopsin

In wild-type proteorhodopsin (pR), titration of the chromophore's counterion Asp(97) occurs with a pK(a) of 8.2+/-0.1. R94C mutation reduces this slightly to 7.0+/-0.2, irrespective of treatment with ethylguanidinium. This contrasts with the homologous archaeal protein bacteriorhodopsin (bR), where R82C mutation was previously shown to elevate the pK(a) of Asp(85) by approximately 5 units, while reconstitution with ethylguanidinium restores it nearly to the wild-type value of 2.5. We conclude there is much weaker electrostatic coupling between Arg(94) and Asp(97) in the unphotolyzed state of pR, in comparison to Arg(82) and Asp(85) in bR. Therefore, while fast light-driven H(+) release may depend on these two residues in pR as in bR, no tightly conserved pre-photolysis configuration of them is required.

Analogies between halorhodopsin and bacteriorhodopsin

The light-activated proton-pumping bacteriorhodopsin and chloride ion-pumping halorhodopsin are compared. They belong to the family of retinal proteins, with 25% amino acid sequence homology. Both proteins have seven alpha helices across the membrane, surrounding the retinal binding pocket. Photoexcitation of all-trans retinal leads to ion transporting photocycles, which exhibit great similarities in the two proteins, despite the differences in the ion transported. The spectra of the K, L, N and O intermediates, calculated using time-resolved spectroscopic measurements, are very similar in both proteins. The absorption kinetic measurements reveal that the chloride ion transporting photocycle of halorhodopsin does not have intermediate M characteristic for deprotonated Schiff base, and intermediate L dominates the process. Energetically the photocycle of bacteriorhodopsin is driven mostly by the decrease of the entropic energy, while the photocycle of halorhodopsin is enthalpy-driven. The ion transporting steps were characterized by the electrogenicity of the intermediates, calculated from the photoinduced transient electric signal measurements. The function of both proteins could be described with the 'local access' model developed for bacteriorhodopsin. In the framework of this model it is easy to understand how bacteriorhodopsin can be converted into a chloride pump, and halorhodopsin into a proton pump, by changing the ion specificity with added ions or site-directed mutagenesis.

Photocurrent from oriented membrane films containing acid-blue and acid-purple bacteriorhodopsin and its mutants

This paper investigates the fast photocurrent components, B1 and B2, from oriented bacteriorhodopsin (BR) membrane films at low pH, under pulsed laser excitation. Adding chloride ion changes the acid-blue BR to its acid-purple form. In the presence of chloride, the acid-purple BR shows a positive B2 component in the same direction as that of BR at neutral pH, indicating a rapid intramolecular charge transfer. In the absence of chloride, the acid-blue BR shows only a negative B1 with multi-components, indicating a rapid charge separation process associated with retinal photoisomerization. The multi-components in B1 are possibly formed due to the heterogeneity of the acid-blue BR. In addition, BR mutants, D85N and D115N, at low pH and in the presence of chloride, generate the B2 component as well. The observation of chloride-dependent B2 component in various cases at low pH, is in favor of a possible transient chloride ion transfer, although the nature of the charge being transferred cannot be identified so far.

Two groups control light-induced Schiff base deprotonation and the proton affinity of Asp85 in the Arg82 his mutant of bacteriorhodopsin

Arg(82) is one of the four buried charged residues in the retinal binding pocket of bacteriorhodopsin (bR). Previous studies show that Arg(82) controls the pK(a)s of Asp(85) and the proton release group and is essential for fast light-induced proton release. To further investigate the role of Arg(82) in light-induced proton pumping, we replaced Arg(82) with histidine and studied the resulting pigment and its photochemical properties. The main pK(a) of the purple-to-blue transition (pK(a) of Asp(85)) is unusually low in R82H: 1.0 versus 2.6 in wild type (WT). At pH 3, the pigment is purple and shows light and dark adaptation, but almost no light-induced Schiff base deprotonation (formation of the M intermediate) is observed. As the pH is increased from 3 to 7 the M yield increases with pK(a) 4.5 to a value approximately 40% of that in the WT. A transition with a similar pK(a) is observed in the pH dependence of the rate constant of dark adaptation, k(da). These data can be explained, assuming that some group deprotonates with pK(a) 4.5, causing an increase in the pK(a) of Asp(85) and thus affecting k(da) and the yield of M. As the pH is increased from 7 to 10.5 there is a further 2.5-fold increase in the yield of M and a decrease in its rise time from 200 micros to 75 micros with pK(a) 9. 4. The chromophore absorption band undergoes a 4-nm red shift with a similar pK(a). We assume that at high pH, the proton release group deprotonates in the unphotolyzed pigment, causing a transformation of the pigment into a red-shifted "alkaline" form which has a faster rate of light-induced Schiff base deprotonation. The pH dependence of proton release shows that coupling between Asp(85) and the proton release group is weakened in R82H. The pK(a) of the proton release group in M is 7.2 (versus 5.8 in the WT). At pH < 7, most of the proton release occurs during O --> bR transition with tau approximately 45 ms. This transition is slowed in R82H, indicating that Arg(82) is important for the proton transfer from Asp(85) to the proton release group. A model describing the interaction of Asp(85) with two ionizable residues is proposed to describe the pH dependence of light-induced Schiff base deprotonation and proton release.

Chloride ion binding to bacteriorhodopsin at low pH: an infrared spectroscopic study

Bacteriorhodopsin (bR) and halorhodopsin (hR) are light-induced ion pumps in the cell membrane of Halobacterium salinarium. Under normal conditions bR is an outward proton transporter, whereas hR is an inward Cl- transporter. There is strong evidence that at very low pH and in the presence of Cl-, bR transports Cl- ions into the cell, similarly to hR. The chloride pumping activity of bR is connected to the so-called acid purple state. To account for the observed effects in bR a tentative complex counterion was suggested for the protonated Schiff base of the retinal chromophore. It would consist of three charged residues: Asp-85, Asp-212, and Arg-82. This quadruplet (including the Schiff base) would also serve as a Cl- binding site at low pH. We used Fourier transform infrared difference spectroscopy to study the structural changes during the transitions between the normal, acid blue, and acid purple states. Asp-85 and Asp-212 were shown to participate in the transitions. During the normal-to-acid blue transition, Asp-85 protonates. When the pH is further lowered in the presence of Cl-, Cl- binds and Asp-212 also protonates. The binding of Cl- and the protonation of Asp-212 occur simultaneously, but take place only when Asp-85 is already protonated. It is suggested that HCl is taken up in undissociated form in exchange for a neutral water molecule.

Chloride- and pH-dependent proton transport by BR mutant D85N

Photocurrents from purple membrane suspensions of D85N BR mutant adsorbed to planar lipid membranes (BLM) were recorded under yellow (lambda > 515 nm), blue (360 nm < lambda < 420 nm) and white (lambda > 360 nm) light. The pH dependence of the transient and stationary currents was studied in the range from 4.5 to 10.5. The outwardly directed stationary currents in yellow and blue light indicate the presence of a proton pumping activity, dependent on the pH of the sample, in the same direction as in the wild-type. The inwardly directed currents in white light, due to an inverse proton translocation, in a two-photon process, show a pH dependence as well. The stationary currents in blue and white light are drastically increased in the presence of azide, but not in yellow light. The concentration dependence of the currents on azide indicates binding of azide to the protein. In the presence of 1 M sodium chloride, the stationary proton currents in yellow light show an increase by a factor of 25 at pH 5.5. On addition of 50 mM azide, the stationary current in yellow light decreases again, possibly by competition between azide and chloride for a common binding site. The observed transport modes are discussed in the framework of the recently published IST model for ion translocation by retinal proteins [U. Haupts et al., Biochemistry 36 (1997) 2-7].

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.

Guanidinium restores the chromophore but not rapid proton release in bacteriorhodopsin mutant R82Q

Replacement of the Arg residue at position 82 in bacteriorhodopsin by Gln or Ala was previously shown to slow the rate of proton release and raise the pK of Asp 85, indicating that R82 is involved both in the proton release reaction and in stabilizing the purple form of the chromophore. We now find that guanidinium chloride lowers the pK of D85, as monitored by the shift of the 587-nm absorbance maximum to 570 nm (blue to purple transition) and increased yield of photointermediate M. The absorbance shift follows a simple binding curve, with an apparent dissociation constant of 20 mM. When membrane surface charge is taken into account, an intrinsic dissociation constant of 0.3 M fits the data over a range of 0.2-1.0 M cation concentration (Na+ plus guanidinium) and pH 5.4-6.7. A chloride counterion is not involved in the observed spectral changes, as chloride up to 0.2 M has little effect on the R82Q chromophore at pH 6, whereas guanidinium sulfate has a similar effect to guanidinium chloride. Furthermore, guanidinium does not affect the chromophore of the double mutant R82Q/D85N. Taken together, these observations suggest that guanidinium binds to a specific site near D85 and restores the purple chromophore. Surprisingly, guanidinium does not restore rapid proton release in the photocycle of R82Q. This result suggests either that guanidinium dissociates during the pump cycle or that it binds with a different hydrogen-bonding geometry than the Arg side chain of the wild type.

Correlation between surfactant/micelle structure and the stability of bacteriorhodopsin in solution

The rate of solubilization and isothermal bleaching of bacteriorhodopsin (bR) in a series of nine alkylammonium surfactants is studied by using time-resolved optical spectroscopy. The surfactant series RN(+)R'(3) covers a range in tail length (R = C(12)H(25), C(14)H(29), or C(16)H(33)) and headgroup size and hydrophobicity (R' = CH(3); C(2)H(5), or C(3)H(7)). The rate of bleaching increases initially with increasing surfactant concentration but decreases at higher concentrations. Possible explanations for this behavior are discussed. The kinetic data are consistent with the penetration of the surfactant into the protein interior. Interaction of the surfactants with the protein is a complicated, multistep process, and the rate curves are a function of at least four variables: 1) the micellar environment, 2) the length of the surfactant tail, 3) the size of the headgroup, and 4) the hydrophobicity of the headgroup. Our data provide new insights into the molecular characteristics that help define the performance of surfactants in the solubilization and denaturation of membrane-bound proteins.

Photovoltage kinetics of the acid-blue and acid-purple forms of bacteriorhodopsin: evidence for no net charge transfer

Time-resolved photovoltage measurements were performed with the acid-blue (bR605A) and acid-purple (bR565A) forms of bacteriorhodopsin (bR) in the time range from 25 ns to 100 s. The bR605A and bR565A pigments were formed by titration with H2SO4 in the absence and presence of 150 mM KCI, respectively. Qualitatively the kinetics of the charge displacement in these two states are similar and consist of two fast phases in one direction (100 ns bandwidth limited and approximately 1 microsecond) followed by a decay in the opposite direction via one component for bR605A (4.4 +/- 0.6 ms) or two components for bR565A (33 +/- 8 microseconds and 3.6 +/- 0.5 ms). The transient photovoltage signal returns exactly to the initial value after several milliseconds, well before the passive discharge of the electrical measuring system at 2 s. We conclude that no net charge transfer occurs in either bR605A or bR565A. The direction of the fast components is opposite that of net proton translocation in bR at pH 7. So, if the charge that moves back and forth is due to a proton, it moves first in the direction of the cytoplasmic side of the membrane (< 1 microsecond) and returns to its initial position via the 4.4 ms (bR605A) or the 33 microseconds and 3.6 ms (bR565A) decay components. The amplitude of the charge motion in both low pH forms is too large to be due to isomerization alone and is comparable to one of the major components in bR at pH 7.2

Intramolecular charge transfer in the bacteriorhodopsin mutants Asp85-->Asn and Asp212-->Asn: effects of pH and anions

The photovoltage kinetics of the bacteriorhodopsin mutants Asp212-->Asn and Asp85-->Asn after excitation at 580 nm have been investigated in the pH range from 0 to 11. With the mutant Asp85-->Asn (D85N) at pH 7 no net charge translocation is observed and the signal is the same, both in the presence of Cl- (150 mM) and in its absence (75 mM SO4(2-)). Under both conditions the color of the pigment is blue (lambda max = 615 nm). The time course of the photovoltage kinetics is similar to that of the acid-blue form of wild-type, except that an additional transient charge motion occurs with time constants of 60 microseconds and 1.3 ms, indicating the transient deprotonation and reprotonation of an unknown group to and from the extracellular side of the membrane. It is suggested that this is the group XH, which is responsible for proton release in wild-type. At pH 1, the photovoltage signal of D85N changes upon the addition of Cl- from that characteristic for the acid-blue state of wild-type to that characteristic for the acid-purple state. Therefore, the protonation of the group at position at 85 is necessary, but not sufficient for the chloride-binding. At pH 11, well above the pKa of the Schiff base, there is a mixture of "M-like" and "N-like" states. Net proton transport in the same direction as in wild-type is restored in D85N from this N-like state. With the mutant Asp212-->Asn (D212N), time-resolved photovoltage measurements show that in the absence of halide ions the signal is similar to that of the acid-blue form of wild-type and that no net charge translocation occurs in the entire pH range from 0 to 11. Upon addition of Cl- in the pH range from 3.8 to 7.2 the color of the pigment returns to purple and the photovoltage experiments indicate that net proton pumping is restored. However, this Cl(-)-induced activation of net charge-transport in D212N is only partial. Outside this pH range, no net charge transport is observed even in the presence of chloride, and the photovoltage shows the same chloride-dependent features as those accompanying the acid-blue to acid-purple transition of the wild-type.

Differences between the photocycles of halorhodopsin and the acid purple form of bacteriorhodopsin analyzed with millisecond time-resolved FTIR spectroscopy

At pH 1, bacteriorhodopsin (bR) is thought to function as a halide ion pump, in contrast to its biological function as a proton pump at neutral pH. Despite the apparent similarity in function between this 'acid purple' form of bR and the native form of halorhodopsin (hR), their FTIR difference spectra measured ca. 5 ms after photolysis are significantly different. The most striking difference is the appearance of a positive band at 1753 cm-1 and a negative band at 1732 cm-1 in the bRacid purple difference spectrum. These and other spectral features are similar, but not identical, to those of the bR-->O difference spectrum measured at neutral pH. The structure of the bRacid purple longest-lived product therefore corresponds more closely to the O photoproduct of the bR proton-pumping photocycle, rather than the hL photoproduct seen on a similar time scale in the hR photocycle. The 1753- and 1732-cm-1 bands are largely unaffected by the D212N mutation, but both appear to lose a portion of their intensities with either the D85N or D96N mutation. Thus Asp-85 and -96 likely undergo substantial changes in hydrogen-bonding environment during the halide-pumping cycle of bRacid purple. Our FTIR results deepen the distinctions between the hR and bR photocycles. The mechanism of chloride pumping in hR has been thought not to involve protonation or hydrogen bonding changes of carboxylic acid groups. In bRacid purple, however, it seems likely that at least one carboxylic acid might play an important role in the mechanism of chloride pumping, leading to an increase in thermodynamic or kinetic stabilization of the O intermediate.

Long-range effects on the retinal chromophore of bacteriorhodopsin caused by surface carboxyl group modification

Carboxyl groups of bacteriorhodopsin (bR) that are modified by 1-ethyl-3-[3-(trimethylamino)-propyl]carbodiimide (ETC) have been identified. Reaction of deionized purple membrane with a 400-fold molar excess of ETC or [14C]ETC for 1 h at 0 degree C incorporates about 3.5 mol of ETC/mol of bR. Proteinase K cleavage of ETC-modified bacterioopsin (bO) produced small 14C-labeled peptides. Amino acid sequence analysis showed three major ETC-modified residues: Glu 234, Asp 38, and Glu 74. Proteolysis of purple membrane with papain removes the ETC site at Glu 234. Treatment of ETC-modified, papain-cleaved purple membrane with hydroxylamine removes half of the remaining ETC label. Subsequent cleavage with chymotrypsin, followed by amino acid sequence analysis, revealed that most of the remaining label was at Glu 74. bR modified by ETC primarily at Glu 74 displays two alterations in the retinal chromophore, located in the membrane interior at a distance more than 2 nm away from the modified carboxyl group. (1) The acid-induced purple-to-blue transition undergoes a shift in apparent pK from 3.2 to 2.3. (2) The second-order rate constant for chromophore regeneration from bO and retinal is diminished from 3600 to 1700 M-1 s-1 in membrane sheets. Most of the shift in the pK of the purple-to-blue transition can be explained by the quaternary ammonium ion of ETC attached to Glu 74 overlapping the postulated location of the guanidinium group of Arg 82.(ABSTRACT TRUNCATED AT 250 WORDS)

Proton transfer and energy coupling in the bacteriorhodopsin photocycle

A description of the rate constants and the energetics of the elementary reaction steps of the photocycle of bacteriorhodopsin has been helpful in understanding the mechanism of proton transport in this light-driven pump. The evidence suggests a single unbranched reaction sequence, BR-hv----K in equilibrium with L in equilibrium with M1----M2 in equilibrium with N in equilibrium with O----BR, where coupling to the proton-motive force is at the energetically and mechanistically important M1----M2 step. The consequences of site-specific mutations expressed homologously in Halobacterium halobium have revealed characteristics of the Schiff base deprotonation in the L----M1 reaction, the reorientation of the Schiff base from the extracellular to the cytoplasmic side in the M1----M2 reaction, and the reprotonation of the Schiff base in the M2----N reaction.

Protection by opioid ligands against modification of the opioid receptor by a carbodiimide

Opioid receptors in membranes prepared from guinea-pig cerebellum were modified irreversibly by treatment with a water soluble carbodiimide, 1-ethyl,3-(3-dimethylaminoethyl)carbodiimide (EDAC). This decreased the number of [3H]bremazocine binding sites (Bmax reduced from 140 to 100 fmol/mg by 1 mM EDAC) without changing their affinity. When the EDAC concentration used was sufficient (500 mM) to inactivate almost all of the opioid receptors, the modification was partly prevented by inclusion of high concentrations (100 microM) of opioid agonists ([D-Ala2, MePhe4, Glyol5]-enkephalin, [D-Ala2, D-Leu5]-enkephalin,(+)-trans-N-methyl-N-[2-(1-pyrrolidinyl)- cyclohexyl]benzo(b)thiophene-4-acetamide hydrochloride), although they exhibited equal efficacy irrespective of their mu, delta or kappa type selectivity. However, almost all of the opioid binding sites were protected when a guanine nucleotide analogue (GppNHP, 100 microM) was also included with the agonists during carbodiimide treatment.