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PubMed1984

Two distinct quinone-modulated modes of antimycin-sensitive cytochrome b reduction in the cytochrome bc1 complex.

Reduction of cytochrome b-560 (analogous to cyt b-562 of mitochondria) via an antimycin-sensitive route has been revealed in chromatophores of the photosynthetic bacterium, Rhodopseudomonas sphaeroides Ga. Indeed, the results suggest that two reductive mechanisms can be operative. One is consistent with the idea that the quinol generated at the reaction center QB site enters the Q pool and, via the Qc site, equilibrates with cytochrome b-560. The other reductive mode circumvents redox equilibrium with the pool; we consider that this could result from a direct encounter of the reaction center with the bc1 complex perhaps involving a direct QB-Qc site interaction. This latter reaction is suppressed by occupancy of the Qc site, not only by antimycin but by ubiquinol and ubiquinone.

Antimycin A↗

PubMed1982

Roles of 5-hydroxytryptamine and dopamine as neurotransmitters eliciting release of erythrophorotropic hormones in the fiddler crab, UCA pugilator.

This article reviews the endocrinological, pharmacological and biochemical evidence ascribing neurotransmitter roles for 5-hydroxytryptamine (5-HT, serotonin) in eliciting the release of red pigment-dispersing hormone (RPDH) and for dopamine (DA) in stimulating the release of red pigment-concentrating hormone (RPCH) in the fiddler crab, Uca pugilator. 5-HT produces red pigment dispersion in intact crabs, but only indirectly. Likewise, DA evokes red pigment concentration in vivo but it has no effect on red chromatophores (erythrophores) in isolated legs. The data obtained with 5-HT and DA agonists and antagonists on red pigment translocation in vivo and in vitro, are consistent with their neurotransmitter candidacies in evoking the release of these erythrophorotropic hormones.

Animals↗

PubMedDate not supplied

Toxic exposure to ethylene dibromide and mercuric chloride: effects on laboratory-reared octopuses.

The effects of acute and chronic exposure to either ethylene dibromide (EDB) or mercuric chloride (MC) were studied in laboratory-reared Octopus joubini, O. maya and O. bimaculoides. The advantages of using octopuses were that the responses were immediate, highly visible and sensitive. All species demonstrated signs of toxicity to acute and chronic exposure to EDB and to MC. A dosage-sensitive relationship for the loss and subsequent recovery of locomotor response and of chromatophore expansion was found for each species after acute exposure. For each species the LC50 for chronic exposure occurred within 12 hr at 100 mg/l for EDB and within 3 hr at 1,000 mg/l for MC. This study demonstrated the potential usefulness of laboratory-reared octopuses in evaluating the toxicity of marine environmental pollutants.

Animals↗

PubMed2005

Proton slip in the ATP synthase of Rhodobacter capsulatus: induction, proton conduction, and nucleotide dependence.

FOF1-ATP synthase converts two energetic "currencies" of the cell (ATP and protonmotive force, pmf) by coupling two rotary motors/generators. Their coupling efficiency is usually very high. Uncoupled proton leakage (slip) has only been observed in chloroplast enzyme at unphysiologically low nucleotide concentration. We investigated the properties of proton slip in chromatophores (sub-bacterial vesicles) from Rhodobacter capsulatus in the single-enzyme-per-vesicle mode. The membrane was energized by excitation with flashing light and the relaxation of the transmembrane voltage and pH difference was photometrically detected. We found that: (1) Proton slip occurred only at low nucleotide concentration (<1 microM) and after pre-illumination over several seconds. (2) Slip induction by pmf was accompanied by the release of approximately 0.25 mol ADP per mole of enzyme. There was no detectable detachment of F1 from FO. (3) The transmembrane voltage and the pH difference were both efficient in slip induction. Once induced, slip persisted for hours, and was only partially reverted by the addition of ADP or ATP (>1 microM). (4) There was no pmf threshold for the proton transfer through the slipping enzyme; slip could be driven both by voltage and pH difference. (5) The conduction was ohmic and weakly pH-dependent in the range from 5.5 to 9.5. The rate constant of proton transfer under slip conditions was 185 s(-1) at pH 8. Proton slip probably presents the free-wheeling of the central rotary shaft, subunit gamma, in an open structure of the (alphabeta)3 hexagon with no nucleotides in the catalytic sites.

Bacterial Chromatophores↗

PubMed2006

Modulation of proton pumping efficiency in bacterial ATP synthases.

The ATP synthase in chromatophores of Rhodobacter caspulatus can effectively generate a transmembrane pH difference coupled to the hydrolysis of ATP. The rate of hydrolysis was rather insensitive to the depletion of ADP in the assay medium by an ATP regenerating system (phospho-enol-pyruvate (PEP) and pyruvate kinase (PK)). The steady state values of DeltapH were however drastically reduced as a consequence of ADP depletion. The clamped concentrations of ADP obtained using different PK activities in the assay medium could be calculated and an apparent Kd approximately 0.5 microM was estimated. The extent of proton uptake was also strongly dependent on the addition of phosphate to the assay medium. The Kd for this effect was about 70 microM. Analogous experiments were performed in membrane fragment from Escherichia coli. In this case, however, the hydrolysis rate was strongly inhibited by Pi, added up to 3 mM. Inhibition by Pi was nearly completely suppressed following depletion of ADP. The Kd's for the ADP and Pi were in the micromolar range and submillimolar range, respectively, and were mutually dependent from the concentration of the other ligand. Contrary to hydrolysis, the pumping of protons was rather insensitive to changes in the concentrations of the two ligands. At intermediate concentrations, proton pumping was actually stimulated, while the hydrolysis was inhibited. It is concluded that, in these two bacterial organisms, ADP and phosphate induce a functional state of the ATP synthase competent for a tightly coupled proton pumping, while the depletion of either one of these two ligands favors an inefficient (slipping) functional state. The switch between these states can probably be related to a structural change in the C-terminal alpha-helical hairpin of the epsilon-subunit, from an extended conformation, in which ATP hydrolysis is tightly coupled to proton pumping, to a retracted one, in which ATP hydrolysis and proton pumping are loosely coupled.

Adenosine Diphosphate↗

PubMed2005

Architecture of the native photosynthetic apparatus of Phaeospirillum molischianum.

The ubiquity and importance of photosynthetic organisms in nature has made the molecular mechanisms of photosynthesis a widely studied subject at both structural and functional levels. A current challenge is to understand the supramolecular assembly of the proteins involved in photosynthesis in native membranes. We have used atomic force microscopy to study the architecture of the photosynthetic apparatus and analyze the structure of single molecules in chromatophores of Phaeospirillum molischianum. Core complexes are formed by the reaction center enclosed by an elliptical light harvesting complex 1. LH2 are octameric rings, assembled either with cores or in hexagonally packed LH2 antenna domains. The symmetry mismatch caused by octameric LH2 packing in a hexagonal lattice, that could be avoided in a square lattice, suggests lipophobic effects rather than specific inter-molecular interactions drive protein organization. The core and LH2 complexes are organized to form a supramolecular assembly reminiscent to that found in Rhodospirillum photometricum, and very different from that observed in Rhodobacter sphaeroides, Rb. blasticus, and Blastochloris viridis.

Bacterial Chromatophores↗

PubMed2006

Ultrastructure of the skin melanophores and iridophores in paddlefish, Polyodon spathula.

The ultrastructure of melanophores and iridophores of Polyodon spathula has been examined by transmission electron microscopy. In the skin, two types of chromatophores, melanophores and iridophores were founded. Melanophores were localized both in epidermis and dermis. Epidermal melanophores were present on the dorsal region of the trunk, sides, outer surface of the operculum and rostrum. Iridophores were founded in the dermis from ventral skin. The cytoplasm of iridophores is filled with reflecting platelets with variable orientation. The length of the long axis of the platelets varies from 1 to 2.10 microm.

Animals↗

PubMed2000

Escape probability and trapping mechanism in purple bacteria: revisited.

Despite intensive research for decades, the trapping mechanism in the core complex of purple bacteria is still under discussion. In this article, it is attempted to derive a conceptionally simple model that is consistent with all basic experimental observations and that allows definite conclusions on the trapping mechanism. Some experimental data reported in the literature are conflicting or incomplete. Therefore we repeated two already published experiments like the time-resolved fluorescence decay in LH1-only purple bacteria Rhodospirillum rubrum and Rhodopseudomonas viridis chromatophores with open and closed (Q(A)(-)) reaction centers. Furthermore, we measured fluorescence excitation spectra for both species under the two redox-conditions. These data, all measured at room temperature, were analyzed by a target analysis based on a three-state model (antenna, primary donor, and radical pair). All states were allowed to react reversibly and their decay channels were taken into consideration. This leads to seven rate constants to be determined. It turns out that a unique set of numerical values of these rate constants can be found, when further experimental constraints are met simultaneously, i.e. the ratio of the fluorescence yields in the open and closed (Q(A)(-)) states F(m)/F(o) approximately 2 and the P(+)H(-)-recombination kinetics of 3-6 ns. The model allows to define and to quantify escape probabilities and the transfer equilibrium. We conclude that trapping in LH1-only purple bacteria is largely transfer-to-the-trap-limited. Furthermore, the model predicts properties of the reaction center (RC) in its native LH1-environment. Within the framework of our model, the predicted P(+)H(-)-recombination kinetics are nearly indistinguishable for a hypothetically isolated RC and an antenna-RC complex, which is in contrast to published experimental data for physically isolated RCs. Therefore RC preparations may display modified kinetic properties.

Bacterial Chromatophores↗

PubMed2004

Proton transfer dynamics at the membrane/water interface: dependence on the fixed and mobile pH buffers, on the size and form of membrane particles, and on the interfacial potential barrier.

Crossing the membrane/water interface is an indispensable step in the transmembrane proton transfer. Elsewhere we have shown that the low dielectric permittivity of the surface water gives rise to a potential barrier for ions, so that the surface pH can deviate from that in the bulk water at steady operation of proton pumps. Here we addressed the retardation in the pulsed proton transfer across the interface as observed when light-triggered membrane proton pumps ejected or captured protons. By solving the system of diffusion equations we analyzed how the proton relaxation depends on the concentration of mobile pH buffers, on the surface buffer capacity, on the form and size of membrane particles, and on the height of the potential barrier. The fit of experimental data on proton relaxation in chromatophore vesicles from phototropic bacteria and in bacteriorhodopsin-containing membranes yielded estimates for the interfacial potential barrier for H(+)/OH(-) ions of approximately 120 meV. We analyzed published data on the acceleration of proton equilibration by anionic pH buffers and found that the height of the interfacial barrier correlated with their electric charge ranging from 90 to 120 meV for the singly charged species to >360 meV for the tetra-charged pyranine.

Bacterial Chromatophores↗

PubMed1994

Energy transfer in the inhomogeneously broadened core antenna of purple bacteria: a simultaneous fit of low-intensity picosecond absorption and fluorescence kinetics.

The excited state decay kinetics of chromatophores of the purple photosynthetic bacterium Rhodospirillum rubrum have been recorded at 77 K using picosecond absorption difference spectroscopy under strict annihilation free conditions. The kinetics are shown to be strongly detection wavelength dependent. A simultaneous kinetic modeling of these experiments together with earlier fluorescence kinetics by numerical integration of the appropriate master equation is performed. This model, which accounts for the spectral inhomogeneity of the core light-harvesting antenna of photosynthetic purple bacteria, reveals three qualitatively distinct stages of excitation transfer with different time scales. At first a fast transfer to a local energy minimum takes place (approximately 1 ps). This is followed by a much slower transfer between different energy minima (10-30 ps). The third component corresponds to the excitation transfer to the reaction center, which depends on its state (60 and 200 ps for open and closed, respectively) and seems also to be the bottleneck in the overall trapping time. An acceptable correspondence between theoretical and experimental decay kinetics is achieved at 77 K and at room temperature by assuming that the width of the inhomogeneous broadening is 10-15 nm and the mean residence time of the excitation in the antenna lattice site is 2-3 ps.

Bacterial Chromatophores↗

PubMed2001

Electrogenic proton transfer in Rhodobacter sphaeroides reaction centers: effect of coenzyme Q(10) substitution by decylubiquinone in the Q(B) binding site.

An electrometric technique was used to investigate the effect of coenzyme Q(10) (UQ), substitution by decylubiquinone (dQ) at the Q(B) binding site of reaction centers (UQ-RC and dQ-RC, respectively) on the electrogenic proton transfer kinetics upon Q(B) reduction in Rhodobacter sphaeroides chromatophores. Unlike dQ-RC, the kinetics of the second flash-induced proton uptake in UQ-RC clearly deviated from the mono-exponential one. The activation energy (about 30 kJ/mol) and the pH profile of the kinetics in dQ-RC were similar to those in UQ-RC, with the power law approximation used in the latter case. The interpretation of the data presumed the quinone translocation between the two binding positions within the Q(B) site. It is proposed that the native isoprenyl side chain (in contrast to decyl chain) favors the equilibrium binding of neutral quinone at the redox-active 'proximal' position, but causes a higher barrier for the hydroquinone movement from 'proximal' to 'distal' position.

Bacterial Chromatophores↗

PubMed2003

Photoreduction of the quinone pool in the bacterial photosynthetic membrane: identification of infrared marker bands for quinol formation.

The photoreduction of the quinone (Q) pool in the photosynthetic membrane of the purple bacterium Rhodobacter sphaeroides was investigated by steady-state and time-resolved Fourier transform infrared difference spectroscopy. The results are consistent with the existence of a homogeneous Q pool inside the chromatophore membrane, with a size of around 20 Q molecules per reaction center. IR marker bands for the quinone/quinol (Q/QH(2)) redox couple were recognized. QH(2) bands are identified at 1491, 1470, 1433 and 1388-1375 cm(-1). The 1491 cm(-1) band, which is sensitive to (1)H/(2)H exchange, is assigned to a C-C ring mode coupled to a C-OH mode. A feature at approximately 1743/1720 cm(-1) is tentatively related to a perturbation of the carbonyl modes of phospholipid head groups induced by QH(2) formation. Complex conformational changes of the protein in the amide I and II spectral ranges are also apparent during reduction and reoxidation of the Q pool.

Bacterial Chromatophores↗

PubMed1997

Temperature dependence of the electrogenic reaction in the QB site of the Rhodobacter sphaeroides photosynthetic reaction center: the QA-QB --> QAQB- transition.

The temperature dependencies for the kinetics and relative amplitudes of electrogenic reaction(s) coupled with the first reduction of the secondary quinone acceptor QB were measured with dark-adapted chromatophores of Rhodobacter sphaeroides. The kinetics, while acceptably fitted by a single exponent at room temperature, clearly split into two components below 15 degrees C (rise times, 25 micros and 300 micros at pH 7.0 and 10 degrees C) with the slow phase ousting the fast one at pH > 9.0. The activation energies of the fast and slow phases were estimated at pH 7.0 as < 10 kJ/mol and 60-70 kJ/mol, respectively. To explain the kinetic heterogeneity of the QB --> QB- transition, we suggest two possible conformations for the neutral oxidized ubiquinone at the QB site: one with a hydrogen bond between the side chain carboxyl of Glu-L212 and the methoxy oxygen at C3 of the QB ring (QB-H-Glu centers) and the other one, without this bond (QB:Glu- centers). The fast phase is attributed to QA- QB-H-Glu --> QA QB-H-Glu transition, whereas the slow one to the QA- QB:Glu- --> QA- QB-H-Glu --> QA QB(-)-H-Glu transition.

Bacterial Chromatophores↗

PubMed1989

Regeneration, neural crest derivatives and retinoids: a new synthesis.

Consideration of recent data from diverse fields of biology permits the presentation of a general theory to explain the underlying mechanism and phylogenetic distribution of vertebrate regenerative capacity. It is suggested that dermal xanthophores, which are neural crest derivatives that contain carotenoid pigments, serve as storage reservoirs for proretinoids. At trauma, carotenoids are released and are converted to retinoids. The spatial distribution of xanthophores at the amputation site determines the amount of carotenoids released, which in turn determines the number of cells which will participate in regeneration and their degree of dedifferentiation. It also influences the proliferative and morphogenetic potential of the blastema. The theory is based on several factors. (1) The pluripotency of neural crest derivatives in general and that of chromatophores in particular; (2) The storage metabolism of carotenoids, especially their convertability to retinoids; (3) The known roles of retinoids in regeneration; (4) Evidence suggesting a relationship between carotenoids and regeneration in invertebrates; and (5) Dynamic characteristics of regenerating systems. The theory is experimentally testable with currently available technology. Specific review of data concerning urodele lens regeneration illustrates the theory. Evidence from amphibian limb regeneration is also presented. Methods of evaluation of other regeneration systems are outlined.

Amphibians↗

PubMed2002

Choline acetyltransferase immunoreactive sympathetic ganglion cells in a teleost, Stephanolepis cirrhifer.

The present study showed neurons immunoreactive for choline acetyltransferase (ChAT) in the cranial sympathetic ganglia lying close to the trigeminal-facial nerve complex of the filefish. In these ganglia, less than 1% of ganglion cells were positive for choline acetyltransferase. Choline acetyltransferase-positive neurons were significantly larger than the randomly sampled neurons in this ganglion. The majority of choline acetyltransferase-positive neurons were negative for tyrosine hydroxylase, but many of them were positive for galanin (GAL). Some neurons were positive for both choline acetyltransferase and tyrosine hydroxylase, but these neurons were rarely immunoreactive for dopamine beta hydroxylase, suggesting that they are not adrenergic. In the cranial sympathetic ganglia and the celiac ganglia, many nerve fibers immunoreactive for galanin were seen, and varicose terminals were in contact selectively with neurons negative for both choline acetyltransferase and tyrosine hydroxylase, but not with those positive for choline acetyltransferase or tyrosine hydroxylase. Nerve fibers immunoreactive for choline acetyltransferase were found to be present in contact with the deep layer of chromatophores, which was observed only in the labial region. These results suggest that cholinergic postganglionic neurons are present in the filefish cranial sympathetic ganglia, and that they also contain galanin. As few cholinergic sympathetic neurons express tyrosine hydroxylase and none express dopamine beta hydroxylase, they are unlikely to synthesize noradrenaline or adrenaline.

Acetylcholine↗

PubMed2006

Scanning electrochemical microscopy of the photosynthetic reaction center of Rhodobacter sphaeroides in different environmental systems.

The present work uses a scanning electrochemical microscopy technique to study systems containing the membrane-bound reaction center protein (RC) from the purple photosynthetic bacteria Rhodobacter spheroides to chromatophores (spherical reorganization of cell membrane following its mechanical rupture) and liposomes (reconstituted membrane systems at lower degree of complexity). Scanning electrochemical microscopy is a useful tool to investigate redox processes involving a RC, because the effective heterogeneous rate constants for the redox reaction with different mediators can be measured. The technique is also able to provide information on the role of the outer cell membrane permeation on the kinetics of the electron-transfer processes and to obtain more insight into the nature of the species involved.

Chromatophores↗

PubMed1995

Ion pair formation between basic residues at 144 of the Cyt b polypeptide and the ubiquinones at the Qo site of the Cyt bc1 complex.

Loci of spontaneous Qo site inhibitor resistant mutants in the cyt bc1 complex of the photosynthetic bacterium Rhodobacter capsulatus are M140, F144, G152, G158, and T163 of the cyt b polypeptide. In this report, we have studied the effects of arginine (R) substitution at these positions with a view to test for specific interactions with the [2Fe-2S] cluster, cyt bL with Qo site ubiquinone (Q), or hydroquinone (QH2). All the arginine mutants displayed severely or completely impeded photosynthetic growth resulting from dysfunctional cyt bc1 complexes. The source of dysfunction in G158R and T163R was identified by a > 1000-fold decrease in the Qo site affinity for QH2 and Q, sufficient to empty the site in the presence of the 30 mM ubiquinone pool of the chromatophore membrane; they appear similar to the class of mutants described in the preceding paper [Ding, H., Moser, C. C., Robertson, D. E., Tokito, M., Daldal, F., & Dutton, P. L (1995) Biochemistry 34, 15979-15996]. The source(s) of dysfunction of M140R and G152R is not so apparent since they possess Qo sites with normal QH2/Q affinity; they appear to be members to the class of mutants identified and characterized in the following paper [Saribaş, S., Ding, H., Dutton, P. L., & Daldal, F. (1995) Biochemistry 34, 16004-16012]. The present paper focuses on the unique affects of F144R. Redox potential and EPR spectral properties of the Qo site of F144R showed that arginine forms an ion pair with the head group of an anionic ubiquinone, tentatively suggested to be a ubihydroquinone anion (QH-) in the Qos domain.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗

PubMed1992

Cytochrome bc1 complex [2Fe-2S] cluster and its interaction with ubiquinone and ubihydroquinone at the Qo site: a double-occupancy Qo site model.

The ubiquinone complement of Rhodobacter capsulatus chromatophore membranes has been characterized by its isooctane solvent extractability and electrochemistry; we find that the main ubiquinone pool (Qpool) amounts to about 80% of the total ubiquinone and has an Em7 value close to 90 mV. To investigate the interactions of ubiquinone with the cyt bc1 complex, we have examined the distinctive EPR line shapes of the [2Fe-2S] cluster of the cyt bc1 complex when the Qpool-cyt bc1 complex interactions are modulated by changing the numbers of Q or QH2 present (by solvent extraction and reconstitution), by the exposure of the [2Fe-2S] to the Qpool in different redox states, by the presence of inhibitors specific for the Qo site (myxothiazol and stigmatellin) and Qi site (antimycin), and by site-specific mutations of side chains of the cyt b polypeptide (mutants F144L and F144G) previously identified as important for Qo site structure. Evidence suggests that the Qo site can accommodate two ubiquinone molecules. One (designated Qos) is bound relatively strongly and is second only to the ubiquinone of the QA site of the reaction center in its resistance to solvent extraction. In this strong interaction, the Qo site binds Q and QH2 with approximately equal affinities. Their bound states are distinguished by their effects on the [2Fe-2S] cluster spectral feature at gx at 1.783 (Q) and gx at 1.777 (QH2); titration of the line-shape change reveals an Em7 value of approximately 95 mV. The other molecule (Qow) is bound more weakly, in the same range as the ubiquinone of the QB site of the reaction center. Again, the affinities of the Q form (gx at 1.800) and QH2 form (gx at 1.777) are nearly equal, and the Em7 value measured is approximately 80 mV. These results are discussed in terms of earlier EPR analyses of the cyt bc1 complexes of other systems. A Qo site double-occupancy model is considered that builds on the previous model based on Qo site mutants [Robertson, D. E., Daldal, F.,& Dutton, P. L. (1990) Biochemistry 29, 11249-11260] and includes the recent suggestion that two of the [2F3-2S] cluster ligands of the R. capsulatus cyt bc1 complex are histidines [Gurbiel, R. J. Ohnishi, T., Robertson, D. E. Daldal, F., & Hoffman, B. M. (1991) Biochemistry 30, 11579-11584]. We speculate that the cyt bc1 complex complexes a full enzymatic turnover without necessary exchange of ubiquinone with the Qpool.

Bacterial Chromatophores↗