Rapid pH change due to bacteriorhodopsin measured with a tin-oxide electrode

K+-Dependent Photocycle and Photocurrent Reveal the Uptake of K+ in Light-Driven Sodium Pump

Engineering light-controlled K+ pumps from Na+-pumping rhodopsins (NaR) greatly expands the scope of optogenetic applications. However, the limited knowledge regarding the kinetic and selective mechanism of K+ uptake has significantly impeded the modification and design of light-controlled K+ pumps, as well as their practical applications in various fields, including neuroscience. In this study, we presented K+-dependent photocycle kinetics and photocurrent of a light-driven Na+ pump called Nonlabens dokdonensis rhodopsin 2 (NdR2). As the concentration of K+ increased, we observed the accelerated decay of M intermediate in the wild type (WT) through flash photolysis. In 100 mM KCl, the lifetime of the M decay was approximately 1.0 s, which shortened to around 0.6 s in 1 M KCl. Additionally, the K+-dependent M decay kinetics were also observed in the G263W/N61P mutant, which transports K+. In 100 mM KCl, the lifetime of the M decay was approximately 2.5 s, which shortened to around 0.2 s in 1 M KCl. According to the competitive model, in high KCl, K+ may be taken up from the cytoplasmic surface, competing with Na+ or H+ during M decay. This was further confirmed by the K+-dependent photocurrent of WT liposome. As the concentration of K+ increased to 500 mM, the amplitude of peak current significantly dropped to approximately ~60%. Titration experiments revealed that the ratio of the rate constant of H+ uptake (kH) to that of K+ uptake (kK) is >108. Compared to the WT, the G263W/N61P mutant exhibited a decrease of approximately 40-fold in kH/kK. Previous studies focused on transforming NaR into K+ pumps have primarily targeted the intracellular ion uptake region of Krokinobacter eikastus rhodopsin 2 (KR2) to enhance K+ uptake. However, our results demonstrate that the naturally occurring WT NdR2 is capable of intracellular K+ uptake without requiring structural modifications on the intracellular region. This discovery provides diverse options for future K+ pump designs. Furthermore, we propose a novel photocurrent-based approach to evaluate K+ uptake, which can serve as a reference for similar studies on other ion pumps. In conclusion, our research not only provides new insights into the mechanism of K+ uptake but also offers a valuable point of reference for the development of optogenetic tools and other applications in this field.

Biomimetic Membranes with Transmembrane Proteins: State-of-the-Art in Transmembrane Protein Applications

In biological cells, membrane proteins are the most crucial component for the maintenance of cell physiology and processes, including ion transportation, cell signaling, cell adhesion, and recognition of signal molecules. Therefore, researchers have proposed a number of membrane platforms to mimic the biological cell environment for transmembrane protein incorporation. The performance and selectivity of these transmembrane proteins based biomimetic platforms are far superior to those of traditional material platforms, but their lack of stability and scalability rule out their commercial presence. This review highlights the development of transmembrane protein-based biomimetic platforms for four major applications, which are biosensors, molecular interaction studies, energy harvesting, and water purification. We summarize the fundamental principles and recent progress in transmembrane protein biomimetic platforms for each application, discuss their limitations, and present future outlooks for industrial implementation.

Controllable stationary photocurrents generated from a bacteriorhodopsin/upconversion nanoparticle-based bionanosystem under NIR illumination

In the past few decades, tremendous effort has been dedicated to develop bacteriorhodopsin (bR)-based photo-electronic devices for generating a stationary photocurrent and further for use as a component of artificial retinas in constant illumination sensing. However, an IR-triggered stationary photocurrent with controllable amplitudes has never been realized to date. Herein, NaYF₄:Yb,Er and NaYF₄:Yb,Tm upconversion nanoparticles (UCNPs) with green and blue emissions, respectively, were synthesized and further incorporated with bR to build a bionanosystem. Under 980 nm NIR irradiation the UCNPs function as internal green and blue light sources to initiate the photocycle and speed up the transition of bR from M₄₁₀ to the ground state, consequently accelerating the bR photocycle for the generation of a stationary photocurrent. Moreover, the photocurrent profile could be tailored by changing the blue/green emission intensity ratio. The mechanism is analysed to explore the scientific insights. The system consisting of controllable blue and green light sources may not only hold great promise to construct new types of bR-based optical devices, but also offers a useful setup to investigate the fundamental science underlying the bR photoresponse.

Photocurrent generation in a light-harvesting system with multifunctional artificial nanochannels

Inspired by the biological light-driven proton pump, a light-harvesting system based on multifunctional artificial nanochannels is developed for photocurrent generation.

Integration of bacteriorhodopsin with upconversion nanoparticles for NIR-triggered photoelectrical response

Transient spikes from bacteriorhodopsin (bR) are triggered with NIR irradiation for the first time by integrating bR with upconversion nanoparticles.

Modeling of photocurrent kinetics upon pulsed photoexcitation of photosynthetic proteins: a case of bacteriorhodopsin

The proton pump of bacteriorhodopsin in an aqueous solution at varied pH upon pulsed excitation was monitored using a solution-based electrochemical module. The photocurrent action spectrum agreed with the absorption contour at 495-645 nm. Diminishing the photocurrent amplitude by adding a protonophore, carbonyl cyanide m-chlorophenyl hydrazone, revealed that protons were the charge carriers of the photocurrent. The evolution of the conventional proton pump is proposed to occur in three elementary steps consecutively: first, the proton relay from the protonated Schiff base to the purple membrane (PM) surface (k1), then the proton exchange between PM surface and bulk (k2), and finally, the proton uptake (k3). The fitted temporal profiles of the photocurrent agreed with observations in the pH range 5.8-9.5. At pH 7.3, k1, k2, and k3 were 2098 s(-1), 412 s(-1), and 44 s(-1), respectively. The rate coefficients at pH 9.5 were smaller than those at pH 6.3 by a factor of approximately 2, consistent with the differences in the intrinsic mobilities of the charge carriers proton and hydroxide ion. The combination of the electrochemical detection module and the concomitant model provides a promising tool for quantitative and qualitative characterization of the light-driven ion pumps.

Deposition of bacteriorhodopsin protein in a purple membrane form on nitrocellulose membranes for enhanced photoelectric response

Bacteriorhodopsin protein (bR)-based systems are one of the simplest known biological energy converters. The robust chemical, thermal and electrochemical properties of bR have made it an attractive material for photoelectric devices. This study demonstrates the photoelectric response of a dry bR layer deposited on a nitrocellulose membrane with indium tin oxide (ITO) electrodes. Light-induced electrical current as well as potential and impedance changes of dried bR film were recorded as the function of illumination. We have also tested bR in solution and found that the electrical properties are strongly dependent on light intensity changing locally proton concentration and thus pH of the solution. Experimental data support the assumption that bR protein on a positively charged nitrocellulose membrane (PNM) can be used as highly sensitive photo- and pH detector. Here the bR layer facilitates proton translocation and acts as an ultrafast optoelectric signal transducer. It is therefore useful in applications related to bioelectronics, biosensors, bio-optics devices and current carrying junction devices.

Melittin-regenerated purple membrane

We have investigated the character of melittin-regenerated purple membrane. Adding melittin to blue membrane causes the color transition and partial regeneration of the photocycle and the proton pump. The reconstitution of bacteriorhodopsin by melittin is proved to be charge-dependent. In studying the location of melittin binding on the blue membrane, we suggest that melittin anchors on the membrane through both hydrophobic and electrostatic interactions. The electrostatic interaction is dominant. The binding sites for the electrostatic interaction should be on the surface of the membrane.

Direct measurement of the photoelectric response time of bacteriorhodopsin via electro-optic sampling

The photovoltaic signal associated with the primary photochemical event in an oriented bacteriorhodopsin film is measured by directly probing the electric field in the bacteriorhodopsin film using an ultrafast electro-optic sampling technique. The inherent response time is limited only by the laser pulse width of 500 fs, and permits a measurement of the photovoltage with a bandwidth of better than 350 GHz. All previous published studies have been carried out with bandwidths of 50 GHz or lower. We observe a charge buildup with an exponential formation time of 1.68 +/- 0.05 ps and an initial decay time of 31.7 ps. Deconvolution with a 500-fs Gaussian excitation pulse reduces the exponential formation time to 1.61 +/- 0.04 ps. The photovoltaic signal continues to rise for 4.5 ps after excitation, and the voltage profile corresponds well with the population dynamics of the K state. The origin of the fast photovoltage is assigned to the partial isomerization of the chromophore and the coupled motion of the Arg-82 residue during the primary event.

High-performance photovoltaic behavior of oriented purple membrane polymer composite films

The photovoltaic behavior of films in which bacteriorhodopsin molecules are embedded in a polyvinyl alcohol matrix has been investigated by using both pulsed laser excitation and regular light illumination. Response times as short as milliseconds, photocurrents as great as 120 micro A/cm(2), and photovoltages as large as 3.8 V have been obtained. A theoretical model has been developed and used to extract several physical parameters and fit the experimental results. Some important intrinsic parameters have been obtained. Theoretical results indicate that the average displacement of the excited protons is on the order of several tens of microns. Other curve fits show that photocurrent and photovoltage increase linearly with external field, but increase exponentially with flash power. These theoretical models and results can be extended to other kinds of photoactive polymeric materials.

Proton transfer reactions in the F86D and F86E mutants of pharaonis phoborhodopsin (sensory rhodopsin II)

pharaonis phoborhodopsin (ppR, also called pharaonis sensory rhodopsin II, psRII), a negative phototaxis receptor of Natronobacterium pharaonis, can use light to pump a proton in the absence of its transducer protein. However, the pump activity is much lower than that of the light-driven proton-pump bacteriorhodopsin (BR). ppR's pump activity is known to be increased in a mutant protein, in which Phe86 is replaced with Asp (F86D). Phe86 is the amino acid residue corresponding to Asp96 in BR, and we expect that Asp86 plays an important role in the proton transfer at the highly hydrophobic cytoplasmic domain of the F86D mutant ppR. In this article, we studied protein structural changes and proton transfer reactions during the photocycles of the F86D and F86E mutants in ppR by means of Fourier transform infrared (FTIR) spectroscopy and photoelectrochemical measurements using a tin oxide (SnO2) electrode. FTIR spectra of the unphotolyzed state and the K and M intermediates are very similar among F86D, F86E, and the wild type. Asp86 or Glu86 is protonated in F86D or F86E, respectively, and the pK(a) > 9. During the photocycle, the pK(a) is lowered and deprotonation of Asp86 or Glu86 is observed. Detection of both deprotonation of Asp86 or Glu86 and concomitant reprotonation of the 13-cis chromophore implies the presence of a proton channel between position 86 and the Schiff base. However, the photoelectrochemical measurements revealed proton release presumably from Asp86 or Glu86 to the cytoplasmic aqueous phase in the M state. This indicates that the ppR mutants do not have the BR-like mechanism that conducts a proton uniquely from Asp86 or Glu86 (Asp96 in BR) to the Schiff base, which is possible in BR by stepwise protein structural changes at the cytoplasmic side. In ppR, there is a single open structure at the cytoplasmic side (the M-like structure), which is shown by the lack of the N-like protein structure even in F86D and F86E at alkaline pH. Therefore, it is likely that a proton can be conducted in either direction, the Schiff base or the bulk, in the open M-like structure of F86D and F86E.

Photoelectric response of purple membrane fragments adsorbed on a lipid monolayer supported by mercury and characterization of the resulting interphase

Purple membrane (PM) fragments were adsorbed on a dioleoylphosphatidylcholine (DOPC) monolayer supported by mercury to investigate the kinetics of light-driven proton transport by bacteriorhodopsin (bR). PM fragments were also adsorbed on a mercury-supported triethyleneoxythiol (TET) monolayer. On both monolayers, the light-on current exhibits a finite, potential dependent stationary component that decreases linearly with a positive shift in the applied potential. The light-on and light-off capacitive photocurrents were interpreted on the basis of a simple equivalent circuit, which accounts for the potential dependence of the stationary light-on current. The potential of zero stationary current is about equal to +0.010 V vs. saturated calomel electrode (SCE) on DOPC-coated mercury. The absolute potential difference across the PM fragments adsorbed at this applied potential was estimated on the basis of extrathermodynamic considerations and amounts to about +260 mV; it compares favorably with the value, +250 mV, of the transmembrane potential of zero stationary current across an oocyte plasma membrane incorporating bR [Biophys. J. 74 (1998) 403.]. The effect of the proton pumping activity of photoexcited PM fragments on the electroreduction kinetics of ubiquinone-10 incorporated in the DOPC monolayer underlying the PM fragments was investigated.

Effect of metal ion exchange on the photocurrent response from bacteriorhodopsin on tin oxide electrodes

The transient photocurrent response from bacteriorhodopsin (bR) on tin oxide electrodes was strongly influenced by metal ions bound to bR molecules. The photocurrent polarity reversal pH, which corresponded to the pH value for the reversal of the proton release/uptake sequence in the bR photocycle, of cation-substituted purple membrane (PM) was shifted to lower pH with the increase in the cation affinities to carboxyl groups and a close correlation was noted between the two values. This suggests that the metal ion present in the extracellular region of a bR molecule modulates the pK(a) of proton release groups of bR by stabilizing the ionized state of the proton-releasing glutamic acids. The behavior of photocurrents at light-off in alkaline media, reflecting the proton uptake by bR, was unchanged by binding monovalent (Na(+) and K(+)) or divalent cations (Mg(2+) and Ca(2+)), but was drastically changed by binding La(3+) ions. This can be explained by invoking a substantial slowing of the proton uptake process in the presence of La(3+).

Photo-induced proton transport of pharaonis phoborhodopsin (sensory rhodopsin II) is ceased by association with the transducer

Phoborhodopsin (pR; also sensory rhodopsin II, sRII) is a retinoid protein in Halobacterium salinarum and works as a receptor of negative phototaxis. Pharaonis phoborhodopsin (ppR; also pharaonis sensory rhodopsin II, psRII) is a corresponding protein of Natronobacterium pharaonis. In bacterial membrane, ppR forms a complex with its transducer pHtrII, and this complex transmits the light signal to the sensory system in the cytoplasm. We expressed pHtrII-free ppR or ppR-pHtrII complex in H. salinarum Pho81/wr(-) cells. Flash-photolysis experiments showed no essential changes between pHtrII-free ppR and the complex. Using SnO2 electrode, which works as a sensitive pH electrode, and envelope membrane vesicles, we showed the photo-induced outward proton transport. This membranous proton transport was also shown using membrane vesicles from Escherichia coli in which ppR was functionally expressed. On the other hand, the proton transport was ceased when ppR formed a complex with pHtrII. Using membrane sheet, it was shown that the complex undergoes first proton uptake and then release during the photocycle, the same as pHtrII-free ppR, although the net proton transport ceases. Taking into consideration that the complex of sRII (pR) and its transducer undergoes extracellular proton circulation (J. Sasaki and J. L., Biophys. J. 77:2145-2152), we inferred that association with pHtrII closes a cytoplasmic channel of ppR, which lead to the extracellular proton circulation.

Measurement of proton release and uptake by analogs of bacteriorhodopsin

Proton release and subsequent uptake by several forms of bacteriorhodopsin (bR), including 4-keto analogs of wild-type (WT) and D96N and D85N mutants as well as the 9-demethylretinal analog of WT and D96N mutants, have been measured using a highly sensitive electrochemical technique. Release and uptake of protons by bR in membrane patches on a tin oxide electrode produce a current transient whose amplitude is proportional to the rate of pH change at the electrode surface. Profiles of proton release by the analogs vs. pH are substantially different from the profiles of the native proteins.

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.

A new system for the measurement of electrogenicity produced by ion pumps using a thin polymer film: examination of wild type bacteriorhodopsin and the D96N mutant over a wide pH range

We developed a new assay system for the measurement of capacitive electric currents generated by ion pumps using the thin polymer film 'Lumirror' (Toray Co., Japan). This system enables us to examine the electrogenicity of ion pumps over a wide range of experimental conditions with high reproducibility due to the mechanical and chemical stability, the high electric resistance and the high electric capacitance of the thin polymer film. Using this method, we examined the photoelectric response of wild type bacteriorhodopsin and its D96N mutant over a wide pH range (2.8-10.0). The results were explained in terms of the affinities of the proton binding sites for translocated protons. A possibility that the direction of the proton transfer from the Schiff base was influenced by the protonation/deprotonation state of the surrounding proton binding sites was suggested. We also found that this film can be used as a substrate for atomic force microscopy (AFM) samples and hence the active purple membrane was observed with AFM.

Optical and electrical characterization of bacteriorhodopsin films

The lifetime of the M-state of bacteriorhodopsin (BR) is increased by genetic and chemical modifications and by solubilizing purple membranes (PMs) with detergent. Chemically modified D96E films as well as D96N films, possess close to 100% bleaching efficiency which makes them attractive for use as image storage media. The mutant S35C has spectral and kinetic properties identical to the WT, both in aqueous suspensions and in films. This indicates that substitution of Ser-35 with Cys has an insignificant effect on the photocycling activity of BR. This substitution provides an attachment site that does not interfere with the function of BR. The magnitude of photocurrent transients generated by mutant BR proteins is used to measure the efficiency of the ground-to-M-state transitions.

Titration of aspartate-85 in bacteriorhodopsin: what it says about chromophore isomerization and proton release

Titration of Asp-85, the proton acceptor and part of the counterion in bacteriorhodopsin, over a wide pH range (2-11) leads us to the following conclusions: 1) Asp-85 has a complex titration curve with two values of pKa; in addition to a main transition with pKa = 2.6 it shows a second inflection point at high pH (pKa = 9.7 in 150-mM KCl). This complex titration behavior of Asp-85 is explained by interaction of Asp-85 with an ionizable residue X'. As follows from the fit of the titration curve of Asp-85, deprotonation of X' increases the proton affinity of Asp-85 by shifting its pKa from 2.6 to 7.5. Conversely, protonation of Asp-85 decreases the pKa of X' by 4.9 units, from 9.7 to 4.8. The interaction between Asp-85 and X' has important implications for the mechanism of proton transfer. In the photocycle after the formation of M intermediate (and protonation of Asp-85) the group X' should release a proton. This deprotonated state of X' would stabilize the protonated state of Asp-85.2) Thermal isomerization of the chromophore (dark adaptation) occurs on transient protonation of Asp-85 and formation of the blue membrane. The latter conclusion is based on the observation that the rate constant of dark adaptation is directly proportional to the fraction of blue membrane (in which Asp-85 is protonated) between pH 2 and 11. The rate constant of isomerization is at least 10(4) times faster in the blue membrane than in the purple membrane. The protonated state of Asp-85 probably is important for the catalysis not only of all-trans <=> 13-cis thermal isomerization during dark adaptation but also of the reisomerization of the chromophore from 13-cis to all-trans configuration during N-->O-->bR transition in the photocycle. This would explain why Asp-85 stays protonated in the N and O intermediates.