Interaction between Asp-85 and the proton-releasing group in bacteriorhodopsin. A study of an O-like photocycle intermediate

The pH-dependence of photochemical intermediates of O and P in bacteriorhodopsin by continuous light

The pH-dependence of the O and P intermediates in the photocycle of bacteriorhodopsin (bR) on the intensity and duration of the exciting flash was investigated for bR glycerol suspensions and bR gelatin films. Green and red laser flashes (532 and 670 nm) were utilized to generate a photoequilibrium state of bR and O at ambient temperature, and UV-vis spectroscopy was used to determine the photoconversion for the bR suspensions and films. The maximal concentration of the O intermediate was observed to be pH-dependent and the dependency was most pronounced at a slightly alkaline pH values. The photochemical conversion from the O to P intermediate was investigated for both bR suspensions and films. The P intermediate was only found in bR gelatin film. These results indicate that bR gelatin film may be an attractive candidate for the information storage based on P intermediate. It is possible, with red light, to create photoproducts which are thermally stable at ambient temperature and that can be photochemically erased.

Crystal structures of acid blue and alkaline purple forms of bacteriorhodopsin

Bacteriorhodopsin, a light-driven proton pump found in the purple membrane of Halobacterium salinarum, exhibits purple at neutral pH but its color is sensitive to pH. Here, structures are reported for an acid blue form and an alkaline purple form of wild-type bacteriorhodopsin. When the P622 crystal prepared at pH 5.2 was acidified with sulfuric acid, its color turned to blue with a pKa of 3.5 and a Hill coefficient of 2. Diffraction data at pH 2-5 indicated that the purple-to-blue transition accompanies a large structural change in the proton release channel; i.e. the extracellular half of helix C moves towards helix G, narrowing the proton release channel and expelling a water molecule from a micro-cavity in the vicinity of the retinal Schiff base. In this respect, the acid-induced structural change resembles the structural change observed upon formation of the M intermediate. But, the acid blue form contains a sulfate ion in a site(s) near Arg82 that is created by re-orientations of the carboxyl groups of Glu194 and Glu204, residues comprising the proton release complex. This result suggests that proton uptake by the proton release complex evokes the anion binding, which in turn induces protonation of Asp85, a key residue regulating the absorption spectrum of the chromophore. Interestingly, a pronounced structural change in the proton release complex was also observed at high pH; i.e. re-orientation of Glu194 towards Tyr83 was found to take place at around pH 10. This alkaline transition is suggested to be accompanied by proton release from the proton release complex and responsible for rapid formation of the M intermediate at high pH.

Contribution of extracellular Glu residues to the structure and function of bacteriorhodopsin. Presence of specific cation-binding sites

Single and multiple mutants of extracellular Glu side chains of bacteriorhodopsin were analyzed by acid and calcium titration, differential scanning calorimetry, and thermal difference spectrophotometry. Acid titration spectra show that the second group protonating with Asp(85) is revealed in E204Q in the absence of Cl(-) but is not observed in the triple mutant E9Q/E194Q/E204Q or in the quadruple mutant E9Q/E74Q/E194Q/E204Q. The results point to Glu(9) as the second group protonating cooperatively with Asp(85). Comparison of the apparent pK(a) of Asp(85) protonation in water and in the deionized forms and results of calcium titration suggest that cation-binding sites are of low affinity in the multiple Glu mutants. Like for deionized wild type bacteriorhodopsin, differential scanning calorimetry reveals a lack of the pretransition in the multiple mutants, whereas in E9Q it appears at lower temperature and with lower cooperativity. Additionally, at neutral pH the band at 630 nm arising from cation release upon temperature increase is absent for the multiple mutants. Based on these results, we propose the presence of two cation-binding sites in the extracellular region of bacteriorhodopsin having as ligands Glu(9), Glu(194), Glu(204), and water molecules.

Photoinduced transformation of 14-F-bacteriorhodopsin gelatin films based on both wild type and D96N mutant

Spectral and kinetic transformations were studied in gelatin films made with 14-F wild type (WT) bacteriorhodopsin (BR) and 14-F D96N mutant BR. Unlike the recent study of water suspensions of the same pigments, where a red shifted species at 660 nm was shown to form under the light in 14-F WT only, there are no drastic differences in photoinduced behavior between gelatin films based on 14-F WT and 14-F D96N. It is not observed any photoinduced formation of red shifted species at 660 nm for both types of films as it is observed for corresponding pigments in water suspension. The observed results are explained in a terms of relationship between the rates of two photoinduced processes that occur in suspensions and films of corresponding pigments. Kinetic characteristics of the photoinduced processes for the films with chemical additives suggest that there are no advantages in using 14-F D96N films when compared to films based on 14-F WT.

On the protein residues that control the yield and kinetics of O(630) in the photocycle of bacteriorhodopsin

The effects of pH on the yield (phi(r)), and on the apparent rise and decay constants (k(r), k(d)), of the O(630) intermediate are important features of the bacteriorhodopsin (bR) photocycle. The effects are associated with three titration-like transitions: 1) A drop in k(r), k(d), and phi(r) at high pH [pK(a)(1) approximately 8]; 2) A rise in phi(r) at low pH [pK(a)(2) approximately 4.5]; and 3) A drop in k(r) and k(d) at low pH [pK(a)(3) approximately 4. 5]. (pK(a) values are for native bR in 100 mM NaCl). Clarification of these effects is approached by studying the pH dependence of phi(r), k(r), and k(d) in native and acetylated bR, and in its D96N and R82Q mutants. The D96N experiments were carried out in the presence of small amounts of the weak acids, azide, nitrite, and thiocyanate. Analysis of the mutant's data leads to the identification of the protein residue (R(1)) whose state of protonation controls the magnitude of phi(r), k(r), and k(d) at high pH, as Asp-96. Acetylation of bR modifies the Lys-129 residue, which is known to affect the pK(a) of the group (XH), which releases the proton to the membrane exterior during the photocycle. The effects of acetylation on the O(630) parameters reveal that the low-pH titrations should be ascribed to two additional protein residues R(2) and R(3). R(2) affects the rise of phi(r) at low pH, whereas the state of protonation of R(3) affects both k(r) and k(d). Our data confirm a previous suggestion that R(3) should be identified as the proton release moiety (XH). A clear identification of R(2), including its possible identity with R(3), remains open.

The titrations of Asp-85 and of the cation binding residues in bacteriorhodopsin are not coupled

An outstanding problem relating to the structure and function of bacteriorhodopsin (bR), which is the only protein in the purple membrane of the photosynthetic microorganism Halobacterium salinarium, is the relation between the titration of Asp-85 and the binding/unbinding of metal cations. An extensively accepted working hypothesis has been that the two titrations are coupled, namely, protonation of Asp-85 (located in the vicinity of the retinal chromophore) and cation unbinding occur concurrently. We have carried out a series of experiments in which the purple blue equilibrium and the binding of Mn2+ ions (monitored by electron spin resonance) were followed as a function of pH for several (1-4) R = [Mn2+]/[bR] molar ratios. Data were obtained for native bR, bR mutants, artificial bR and chemically modified bR. We find that in the native pigment the two titrations are separated by more than a pKa unit [delta pKa = pKa(P/B)-pKa(Mn2+) = (4.2-2.8) = 1.4]. In the non-native systems, delta pKa values as high as 5 units, as well as negative delta pKas, are observed. We conclude that the pH titration of cation binding residues in bR is not directly related to the titration of Asp-85. This conclusion is relevant to the nature of the high affinity cation sites in bR and to their role in the photosynthetic function of the pigment.

The structure and mechanism of the family of retinal proteins from halophilic archaea

Retinal proteins from halophilic archaea provide a unique opportunity to analyze vectorial ion translocation. Studies on its structure, conformational changes, proton conduction and electrogenic steps have helped to elucidate the catalytic cycle of bacteriorhodopsin in increasing detail. Experimental modulation of the vectoriality and ion specificity by altering the substrate availability, point mutations and light conditions for the different retinal proteins allows the proposal of a general model of ion transport for this protein family.

Phototransformation and proton pumping activity of the 14-fluoro bacteriorhodopsin derivatives

The photoinduced behavior and proton pumping characteristics of some bacteriorhodopsin (BR) analogs with fluorinated chromophores (all-trans 14-fluorinated [14-F] retinal and 13-cis 14-F retinal) derived from wild type (WT) and D96N mutant BR were investigated. These analogs were characterized using spectrophotometry and a highly sensitive electrochemical technique. Similar to the white membrane JW2N, the apomembranes WT ET 1000 and D96N form photoactive pigments with the 14-F chromophores. The resulting analogs have a major absorption band at 588 nm. Red-shifted pigment (lambdamax</=680 nm) has been previously observed as a minor component of the major 587-nm pigment in 14-F BR made with white membrane JW2N. A similar red-shifted pigment is formed under yellow light (lambda>500 nm) only in the 14-F analogs derived from WT ET 1000. The measurements of the photoinduced transformation in 14-F WT analogs show that the photocycle of the major pigment occurs simultaneously with the process in the red region and is partially masked by the formation of the red-shifted species. The 14-F D96N samples have a significantly slower and more complicated photoinduced behavior. Electrochemical measurements show that the photoinduced transformation of the red species is not accompanied by proton transport.