Sergei Balashov
University of California, Irvine, Physiology and Biophysics, Faculty Member
Рассмотрены свойства ретинального белка (ESR) психротрофной бактерии Exiguobacterium sibiricum, представляющего собой светозависимую протонную помпу. Уникальной структурной особенностью ESR является наличие остатка лизина в положении,... more
Рассмотрены свойства ретинального белка (ESR) психротрофной бактерии Exiguobacterium sibiricum, представляющего собой светозависимую протонную помпу. Уникальной структурной особенностью ESR является наличие остатка лизина в положении, соответствующем внутрибелковому донору протонов для основания Шиффа. Мы показали, что Lys96 успешно выполняет в молекуле ESR функцию донора, облегчая доставку протонов с цитоплазматической поверхности белка к основанию Шиффа. Поскольку поглощение протонов предшествует репротонированию основания Шиффа, можно предположить, что в исходном состоянии этот остаток не заряжен и приобретает протон в течение короткого промежутка времени после депротонирования основания Шиффа и образования интермедиата М. Это отличает ESR от родственных ретинальных белков - бактериородопсина (BR), протеородопсина (PR) и ксантородопсина (XR), в которых функцию донора выполняют остатки с карбоксильной группой, протонированные в исходном состоянии. Как и другие протонные помпы эубактерий (PR и XR), ESR содержит остаток гистидина, взаимодействущий с акцептором протонов Asp85. Это взаимодействие приводит к сдвигу рКа акцептора в более кислую область по сравнению с PR, обеспечивая его способность к функционированию в широком диапазоне рН. Наличие сильной водородной связи между остатками Asp85 и His57, структура вероятных протонпереносящих путей с цитоплазматической поверхности к основанию Шиффа и к наружной поверхности и другие особенности ESR продемонстрированы благодаря расшифровке его пространственной структуры, которая выявляет ряд отличий от известных структур BR и XR. Структура ESR, схема фотоцикла и реакций переноса протонов обсуждаются в сравнении с гомологичными ретинальными белками.
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Along with the inhibition illumination also causes the stimulation of the respiration of H. halobium R1 cells. When light is switched off photoinhibition of respiration (PIR) decays much faster (tau 1/2=12 sec) than photostimulation (PSR)... more
Along with the inhibition illumination also causes the stimulation of the respiration of H. halobium R1 cells. When light is switched off photoinhibition of respiration (PIR) decays much faster (tau 1/2=12 sec) than photostimulation (PSR) (tau 1/2=60 sec). This allows the evaluation of the contribution of the each phase into the total change of the respiration rate. PIR prevails in neutral and alkaline media (at pH 6.8 the amplitude ratio of PSR/PIR=0.3). At the same conditions light induced alkalization of the medium is observed, which at high light intensities is followed by acidification. The half rise time of PIR is 0.5 divided by 0.8 sec under excitation with short light flashes at 18C and pH 6.8. When pH of the medium is reduced the rate of dark respiration decreases, PSR amplitude increases, PIR is almost not changed and light-induced alkalinization of the medium decreases. At pH 5.5 PSR prevails: at light of 10(5) erg/(cm(2).s) the ratio PSR/PIR=2. The maximum value of PIR and PSR at 18C reaches 20-30 percent of the dark respiration level. Uncouplers (CCCP, DNF) and inhibitor (DCCD) of phosphorylation suppress PIR and light induced alkalinization of the medium and significantly (5-7 times at Ph 6.8) increase PSR and light induced acidification. The action spectra show that bacteriorhodopsin is responsible for all the observed light induced changes of O2 and H+ exchange; carotinoids do not participate in sensibilization. It is suggested that photophosphorylation is necessary for PIR and that PSR is caused by the rise of internal pH due to light induced efflux of protons mediated by bacteriorhodopsin.
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The conditions of preferential accumulation of intermediates of the photochemical reaction cycle of bacteriorhodopsin (BR) P550 and P419 at low temperature are found. Upon illumination P550 and P419 undergo photochemical conversions into... more
The conditions of preferential accumulation of intermediates of the photochemical reaction cycle of bacteriorhodopsin (BR) P550 and P419 at low temperature are found. Upon illumination P550 and P419 undergo photochemical conversions into the light-adapted form of BR (P570), forming during this conversions a number of new intermediates: P550 leads to P560-- -- -- leads to P570; P419 leads to P421-- -- -- leads to P565-- -- -- leads to P585-- -- -- leads to P570; P419 leads to P470-- -- -- leads to P570. All intermediates are photoactive. All light reactions are photoreversible and give formation to the products with absorption maximum shifted to the red as compared to the initial state. The absorption spectra of intermediates are complex and include several bands which are more pronounced in the spectrum of P419 (maxima at 442, 419, 398 nm, a shoulder at 375 nm) and P421, less in the spectrum of P570 (maximum at 578 nm, shoulders at 540 and 608 nm) and others.
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Research Interests: Biochemistry, Chemistry, Biology, Cyanobacteria, Carotenoids, and 15 moreMedicine, Circular Dichroism, Fluorescence polarization, Molecular Conformation, Amino Acid Substitution Rates, Recombinant Proteins, Protein Binding, Hydroxylamine, Light Harvesting, Rhodopsin, Glycosides, Biochemistry and cell biology, binding sites, Medical biochemistry and metabolomics, and bacteroidetes
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Retinal proteins function as photoreceptors and ion pumps. Xanthorhodopsin of Salinibacter ruber is a recent addition to this diverse family. Its novel and distinctive feature is a second chromophore, a caro- tenoid, which serves as... more
Retinal proteins function as photoreceptors and ion pumps. Xanthorhodopsin of Salinibacter ruber is a recent addition to this diverse family. Its novel and distinctive feature is a second chromophore, a caro- tenoid, which serves as light-harvesting antenna. Here we discuss the properties of this carotenoid/retinal complex most relevant to its function (such as the specific binding site controlled by the retinal) and its relationship to other retinal proteins (bacteriorho- dopsin, archaerhodopsin, proteorhodopsin and retinal photoreceptors of archaea and eukaryotes). Antenna addition to a retinal protein has not been observed among the archaea and emerged in bacteria appa- rently in response to environmental conditions where light-harvesting becomes a limiting factor in retinal protein functioning.
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Retinal proteins function as photoreceptors and ion pumps. Xanthorhodopsin of Salinibacter ruber is a recent addition to this diverse family. Its novel and distinctive feature is a second chromophore, a caro- tenoid, which serves as... more
Retinal proteins function as photoreceptors and ion pumps. Xanthorhodopsin of Salinibacter ruber is a recent addition to this diverse family. Its novel and distinctive feature is a second chromophore, a caro- tenoid, which serves as light-harvesting antenna. Here we discuss the properties of this carotenoid/retinal complex most relevant to its function (such as the specific binding site controlled by the retinal) and its relationship to other retinal proteins (bacteriorho- dopsin, archaerhodopsin, proteorhodopsin and retinal photoreceptors of archaea and eukaryotes). Antenna addition to a retinal protein has not been observed among the archaea and emerged in bacteria appa- rently in response to environmental conditions where light-harvesting becomes a limiting factor in retinal protein functioning.
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The conditions of preferential accumulation of intermediates of the photochemical reaction cycle of bacteriorhodopsin (BR) P550 and P419 at low temperature are found. Upon illumination P550 and P419 undergo photochemical conversions into... more
The conditions of preferential accumulation of intermediates of the photochemical reaction cycle of bacteriorhodopsin (BR) P550 and P419 at low temperature are found. Upon illumination P550 and P419 undergo photochemical conversions into the light-adapted form of BR (P570), forming during this conversions a number of new intermediates: P550 leads to P560-- -- -- leads to P570; P419 leads to P421-- -- -- leads to P565-- -- -- leads to P585-- -- -- leads to P570; P419 leads to P470-- -- -- leads to P570. All intermediates are photoactive. All light reactions are photoreversible and give formation to the products with absorption maximum shifted to the red as compared to the initial state. The absorption spectra of intermediates are complex and include several bands which are more pronounced in the spectrum of P419 (maxima at 442, 419, 398 nm, a shoulder at 375 nm) and P421, less in the spectrum of P570 (maximum at 578 nm, shoulders at 540 and 608 nm) and others.
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The maximum photosteady state fraction of K, xKmax, and the ratio of the quantum yields of the forward and back light reactions, trans‐bacteriorhodopsin (bR) hArr; K, φbR/φK, were obtained by measuring the absorption changes produced by... more
The maximum photosteady state fraction of K, xKmax, and the ratio of the quantum yields of the forward and back light reactions, trans‐bacteriorhodopsin (bR) hArr; K, φbR/φK, were obtained by measuring the absorption changes produced by illumination of frozen water‐glycerol (1:2) suspensions of light‐adapted purple membrane at different wavelengths at ‐165°C. An independent method based on the second derivative of the absorption spectrum in the region of the β‐bands was also used. It was found that
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Xanthorhodopsin (XR), the light-driven proton pump of the halophilic eubacterium Salinibacter ruber, exhibits substantial homology to bacteriorhodopsin (BR) of archaea and proteorhodopsin (PR) of marine bacteria, but unlike them contains... more
Xanthorhodopsin (XR), the light-driven proton pump of the halophilic eubacterium Salinibacter ruber, exhibits substantial homology to bacteriorhodopsin (BR) of archaea and proteorhodopsin (PR) of marine bacteria, but unlike them contains a light-harvesting carotenoid antenna, salinixanthin, as well as retinal. We report here the pH-dependent properties of XR. The pK, of the retinal Schiff base is as high as in BR, i.e. 212.4. Deprotonation of the Schiff base and the ensuing alkaline denaturation cause large changes in the absorption bands of the carotenoid antenna, which lose intensity and become broader, making the spectrum similar to that of salinixanthin not bound to XR. A small redshift of the retinal chromophore band and increase of its extinction, as well as the pH-dependent amplitude of the M intermediate indicate that in detergent-soluhilized XR the pK, of the Schiff base counterion and proton acceptor is about 6 (compared to 2.6 in BR, and 7.5 in PR). The protonation of the...
Fluorescence characteristics of all-trans-bacteriorhodopsin (BR570), 13-cis-BR (BR560), and the intermediate of the BR cycle (P585) are obtained by measuring variable fluorescence in the course of their phototransformations. The... more
Fluorescence characteristics of all-trans-bacteriorhodopsin (BR570), 13-cis-BR (BR560), and the intermediate of the BR cycle (P585) are obtained by measuring variable fluorescence in the course of their phototransformations. The fluorescence spectra (lambda greater than 580 nm) have a maximum at 720, 730 and 740 nm, their respective excitation spectra with a maximum of 578, 565 and 585 nm coincide with the absorption spectra of the three states. The emission quantum yield for P585 is approximately 10(-4) at -62 degrees C and 4 +/- 2 10(-4) for BR560 at -196 degrees C. The effect of sharp increase of BR570 fluorescence quantum yield (from less than or equal to 10(-4) to approximately 10(-3)) under action of light at -196 degrees C (photoactivation or induction of emission) is found which points to the existence of photoprocess parallel to the photoconversion of BR570 into P600 (formula: see text). The emission of BR560 is not photoactivated. The photoactivated state BR570 is not iden...
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Research Interests: Biophysics, Chemistry, Kinetics, Photochemistry, Biophysical Chemistry, and 15 moreMedicine, Biological Sciences, Darkness, Physical sciences, Bacteriorhodopsin, D-Aspartic Acid, CHEMICAL SCIENCES, Isomerization, Proton, Hydrogen-Ion Concentration, Protons, Halobacterium, Molecular Structure, protonation, and stereoisomerism
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Research Interests: Chemistry, Kinetics, Photochemistry, Biophysical Chemistry, Medicine, and 14 moreBiological Sciences, Physical sciences, Light, Bacteriorhodopsin, CHEMICAL SCIENCES, Time Factors, Arginine, Proton, Amino Acid Sequence, Recombinant Proteins, Hydrogen-Ion Concentration, Site-directed Mutagenesis, Halobacterium, and protonation
Research Interests: Biochemistry, Chemistry, Kinetics, Photochemistry, Medicine, and 15 moreEscherichia coli, Light, Bacteriorhodopsin, Liposomes, Arginine, Isomerization, Halobacterium Salinarum, Alanine, Base Sequence, Protein Denaturation, Biochemistry and cell biology, Dark Adaptation, Molecular Sequence Data, protonation, and Medical biochemistry and metabolomics
The factors determining the pH dependence of the formation and decay of the O photointermediate of the bacteriorhodopsin (bR) photocycle were investigated in the wild-type (WT) pigment and in the mutants of Glu-194 and Glu-204, key... more
The factors determining the pH dependence of the formation and decay of the O photointermediate of the bacteriorhodopsin (bR) photocycle were investigated in the wild-type (WT) pigment and in the mutants of Glu-194 and Glu-204, key residues of the proton release group (PRG) in bR. We have found that in the WT the rate constant of O --> bR transition decreases 30-fold upon decreasing the pH from 6 to 3 with a pKa of about 4.3. D2O slows the rise and decay of the O intermediate in the WT at pH 3.5 by a factor of 5.5. We suggest that the rate of the O --> bR transition (which reflects the rate of deprotonation of the primary proton acceptor Asp-85) at low pH is controlled by the deprotonation of the PRG. To test this hypothesis, we studied the E194D mutant. We show that the pKa of the PRG in the ground state of the E194D mutant, when Asp-85 is protonated, is increased by 1.2 pK units compared to that of the WT. We found a similar increase in the pKa of the rate constant of the O --> bR transition in E194D. This provides further evidence that the rate of the O --> bR transition is controlled by the PRG. In a further test, the E194Q mutation, which disables the PRG and slows proton release, almost completely eliminates the pH dependence of O decay at pHs below 6. A second phenomenon we investigated was that in the WT at neutral and alkaline pH the fraction of the O intermediate decreases with pKa 7.5. A similar pH dependence is observed in the mutants in which the PRG is disabled, E194Q and E204Q, suggesting that the decrease in the fraction of the O intermediate with pKa ca. 7.5 is not controlled by the PRG. We propose that the group with pKa 7.5 is Asp-96. The slowing of the reprotonation of Asp-96 at high pH is the cause of the decrease in the rate of the N --> O transition, leading to the decrease in the fraction of O.
Research Interests: Biochemistry, Chemistry, Catalysis, Photochemistry, Medicine, and 15 moreBacteriorhodopsin, Mathematical Computing, D-Aspartic Acid, Glutamine, Proton, Halobacterium Salinarum, Hydrogen-Ion Concentration, Protons, Site-directed Mutagenesis, Glutamic Acid, Biochemistry and cell biology, Titrimetry, protonation, Medical biochemistry and metabolomics, and Deuterium oxide
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Research Interests: Chemistry, Kinetics, Biophysical Chemistry, Carotenoids, Near Infrared, and 15 moreMedicine, Biological Sciences, Methanol, Physical sciences, Bacteriorhodopsin, Absorption, CHEMICAL SCIENCES, Femtosecond, Excited states, Spectrum analysis, Energy Transfer, Binding Site, Glycosides, binding sites, and Borohydrides
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Salinixanthin is carotenoid with complicated structure and contains a conjugated carbonyl group (Fig. S1) that is known to alter significantly excited-state properties in polar solvents (1-3). To test if salinixanthin excited-state... more
Salinixanthin is carotenoid with complicated structure and contains a conjugated carbonyl group (Fig. S1) that is known to alter significantly excited-state properties in polar solvents (1-3). To test if salinixanthin excited-state properties are affected by solvent polarity, both steady-state and transient absorption spectra were measured for purified salinixanthin dissolved in n-hexane and methanol. Absorption spectra of salinixanthin in both solvents are shown in Fig. S2. No significant changes were observed upon changing solvent polarity, though resolution of the vibrational bands is slightly less in methanol than in n-hexane. Yet, the magnitude of this effect, which is one of the typical markers of the polarity-dependent behavior (1-3), is significantly less than for other carbonyl carotenoids. Transient absorption spectra at 1 ps after 485-nm excitation (Fig. S3) further demonstrate negligible polarity-dependent changes. The transient absorption spectrum in methanol is broader...
A group of microbial retinal proteins most closely related to the proton pump xanthorhodopsin has a novel sequence motif and a novel function. Instead of, or in addition to, proton transport, they perform lightdriven sodium ion transport,... more
A group of microbial retinal proteins most closely related to the proton pump xanthorhodopsin has a novel sequence motif and a novel function. Instead of, or in addition to, proton transport, they perform lightdriven sodium ion transport, as reported for one representative of this group (KR2) from Krokinobacter. In this paper, we examine a similar protein, GLR from Gillisia limnaea, expressed in Escherichia coli, which shares some properties with KR2 but transports only Na. The absorption spectrum of GLR is insensitive to Na at concentrations of ≤3 M. However, very low concentrations of Na cause profound differences in the decay and rise time of photocycle intermediates, consistent with a switch from a “Na-independent” to a “Na-dependent” photocycle (or photocycle branch) at ∼60 μM Na. The rates of photocycle steps in the latter, but not the former, are linearly dependent on Na concentration. This suggests that a high-affinity Na binding site is created transiently after photoexcita...
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ESR, a light-driven proton pump from Exiguobacterium sibiricum, contains a lysine residue (Lys96) in the proton donor site. Substitution of Lys96 with a nonionizable residue greatly slows reprotonation of the retinal Schiff base. The... more
ESR, a light-driven proton pump from Exiguobacterium sibiricum, contains a lysine residue (Lys96) in the proton donor site. Substitution of Lys96 with a nonionizable residue greatly slows reprotonation of the retinal Schiff base. The recent study of electrogenicity of the K96A mutant revealed that overall efficiency of proton transport is decreased in the mutant due to back reactions (Siletsky et al., BBA, 2019). Similar to members of the proteorhodopsin and xanthorhodopsin families, in ESR the primary proton acceptor from the Schiff base, Asp85, closely interacts with His57. To examine the role of His57 in the efficiency of proton translocation by ESR, we studied the effects of H57N and H57N/K96A mutations on the pH dependence of light-induced pH changes in suspensions of Escherichia coli cells, kinetics of absorption changes and electrogenic proton transfer reactions during the photocycle. We found that at low pH (<5) the proton pumping efficiency of the H57N mutant in E. coli cells and its electrogenic efficiency in proteoliposomes is substantially higher than in the WT, suggesting that interaction of His57 with Asp85 sets the low pH limit for H+ pumping in ESR. The electrogenic components that correspond to proton uptake were strongly accelerated at low pH in the mutant indicating that Lys96 functions as a very efficient proton donor at low pH. In the H57N/K96A mutant, a higher H+ pumping efficiency compared with K96A was observed especially at high pH, apparently from eliminating back reactions between Asp85 and the Schiff base by the H57N mutation.
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Following light absorption, at neutral pH the bacteriorhodopsin mutant Y57N does not show Schiff base deprotonation (no M intermediate) or proton pumping activity. We reasoned that this might be due to improper delta pKa between the... more
Following light absorption, at neutral pH the bacteriorhodopsin mutant Y57N does not show Schiff base deprotonation (no M intermediate) or proton pumping activity. We reasoned that this might be due to improper delta pKa between the proton-donating Schiff base and the proton-accepting Asp-85 after light absorption. To test this, we reduced the intrinsic pKa of the protonated Schiff base in the pigment (and thus in the photointermediates) by replacing the retinal chromophore with an analogue, 14-F retinal. This substitution restores light-induced M formation, strongly suggesting that light-induced Schiff base deprotonation is accomplished by lowering its pKa during the photochemical cycle. Thus, while it is generally accepted that the Schiff base deprotonation during the photocycle takes place because of the light-induced reduction in its pKa, we provide here the first experimental evidence of this phenomenon.
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Abstract– At 90 K the photoproduct of the primary light reaction of (rani‐bacteriorhodopsin, the bathoproduct K1 consists of a mixture of at least three spectrally different species, K1I, K1II, and K1III having maxima in the difference... more
Abstract– At 90 K the photoproduct of the primary light reaction of (rani‐bacteriorhodopsin, the bathoproduct K1 consists of a mixture of at least three spectrally different species, K1I, K1II, and K1III having maxima in the difference absorption spectra at 645, 635 and 625 nm, respectively. The bathoproducts differ in their long wavelength absorption bands and in their rate constants for photo‐conversion to trans‐bacteriorhodopsin under far red light irradiation (λ > 720 nm). The bathoproducts are formed from different precursors–conformers of trans‐bacteriorhodopsin, which are stable at 90 K, but are in equilibrium with each other at 213 K. We suggest that the bathoproducts may initiate parallel conversion cycles of trans‐bacteriorhodopsin at low temperatures. The primary photoreaction of 13‐cis‐bacteriorhodopsin also yields three bathoproducts, KcI, KcII and KcIII having maxima in the difference absorption spectra at 615, 605 and 595 nm, respectively.
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Research Interests: Biochemistry, Chemistry, Kinetics, Medicine, Glutamate, and 15 moreLizards, Animals, Proton Transfer, Spectrophotometry, Schiff bases, Spectrum, Schiff Base, Surface Charge, Amino Acid Profile, Protons, Binding Site, Potassium Chloride, Biochemistry and cell biology, binding sites, and Medical biochemistry and metabolomics
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A retinal protein from Exiguobacterium sibiricum (ESR) functions as a light-driven proton pump. Unlike other proton pumps, it contains Lys96 instead of a usual carboxylic residue in the internal proton donor site. Nevertheless, the... more
A retinal protein from Exiguobacterium sibiricum (ESR) functions as a light-driven proton pump. Unlike other proton pumps, it contains Lys96 instead of a usual carboxylic residue in the internal proton donor site. Nevertheless, the reprotonation of the Schiff base occurs fast, indicating that Lys96 facilitates proton transfer from the bulk. In this study we examined kinetics of light-induced transmembrane electrical potential difference, ΔΨ, generated in proteoliposomes reconstituted with ESR. We show that total magnitude of ΔΨ is comparable to that produced by bacteriorhodopsin but its kinetic components and their pH dependence are substantially different. The results are in agreement with the earlier finding that proton uptake precedes reprotonation of the Schiff base in ESR, suggesting that Lys96 is unprotonated in the initial state and gains a proton transiently in the photocycle. The electrogenic phases and the photocycle transitions related to proton transfer from the bulk to the Schiff base are pH dependent. At neutral pH, they occur with τ 0.5ms and 4.5ms. At alkaline pH, the fast component ceases and Schiff base reprotonation slows. At pH8.4, a spectrally silent electrogenic component with τ 0.25ms is detected, which can be attributed to proton transfer from the bulk to Lys96. At pH5.1, the amplitude of ΔΨ decreases 10 fold, reflecting a decreased yield and rate of proton transfer, apparently from protonation of the acceptor (Asp85-His57 pair) in the initial state. The features of the photoelectric potential generation correlate with the ESR structure and proposed mechanism of proton transfer.
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Using FTIR spectroscopy, perturbations of several residues and internal water molecules have been detected when light transforms all-trans bacteriorhodopsin (BR) to its L intermediate having a 13-cis chromophore. Illumination of L at 80 K... more
Using FTIR spectroscopy, perturbations of several residues and internal water molecules have been detected when light transforms all-trans bacteriorhodopsin (BR) to its L intermediate having a 13-cis chromophore. Illumination of L at 80 K results in an intermediate L&#39; absorbing around 550 nm. L&#39; thermally converts to the original BR only at &gt;130 K. In this study, we used the light-induced transformation of L to L&#39; at 80 K to identify some amino acid residues and water molecules that closely interact with the chromophore and distinguish them from those residues not affected by the photoreaction. The L minus L&#39; FTIR difference spectrum shows that the chromophore in L&#39; is in the all-trans configuration. The perturbed states of Asp96 and Val49 and of the environment along the aliphatic part of the retinal and Lys216 seen in L are not affected by the L --&gt; L&#39; photoreaction. On the other hand, the environments of the Schiff base of the chromophore, of Asp115, and of water molecules close to Asp85 returned in L&#39; to their state in which they originally had existed in BR. The water molecules that are affected by the mutations of Thr46 and Asp96 also change to a different state in the L --&gt; L&#39; transition, as indicated by transformation of a water O-H vibrational band at 3497 cm-1 in L into an intense peak at 3549 cm-1 in L&#39;. Notably, this change of water bands in the L --&gt; L&#39; transition at 80 K is entirely different from the changes observed in the BR --&gt; K photoreaction at the same temperature, which does not show such intense bands. These results suggest that these water molecules move closer to the Schiff base as a hydrogen bonding cluster in L and L&#39;, presumably to stabilize its protonated state during the BR to L transition. They may contribute to the structural constraints that prevent L from returning to the initial BR upon illumination at 80 K.
Research Interests: Biochemistry, Water, Hydrogen, Photochemistry, Threonine, and 12 moreD-Aspartic Acid, Tryptophan, Schiff bases, Oxygen, Fourier transform infrared spectroscopy, Halobacterium Salinarum, Alanine, Amino Acid Substitution Rates, Asparagine, Biochemistry and cell biology, Valine, and Medical biochemistry and metabolomics
Retinal proteins function as photoreceptors and ion pumps. Xanthorhodopsin of Salinibacter ruber is a recent addition to this diverse family. Its novel and distinctive feature is a second chromophore, a carotenoid, which serves as... more
Retinal proteins function as photoreceptors and ion pumps. Xanthorhodopsin of Salinibacter ruber is a recent addition to this diverse family. Its novel and distinctive feature is a second chromophore, a carotenoid, which serves as light-harvesting antenna. Here we discuss the properties of this carotenoid/retinal complex most relevant to its function (such as the specific binding site controlled by the retinal) and its relationship to other retinal proteins (bacteriorhodopsin, archaerhodopsin, proteorhodopsin and retinal photoreceptors of archaea and eukaryotes). Antenna addition to a retinal protein has not been observed among the archaea and emerged in bacteria apparently in response to environmental conditions where light-harvesting becomes a limiting factor in retinal protein functioning.