Occupancy of two primary chloride-binding sites in Natronobacterium pharaonis halorhodopsin is a necessary condition for active anion transport.

The existence of two primary chloride-binding sites was found on the basis of the study of halorhodopsin spectra at different chloride concentrations. SVD analysis of the spectra revealed two chloride-dependent components at low chloride concentration (0.1-10 mM). Global fitting of SVD components found K(D) values of 0.47 and 5.2 mM with unitity Hill coefficients. The second K(D) coincides with the apparent K(D) of the photovoltage response of halorhodopsin.

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.

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.

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.

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.

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.