bacteriochlorophylls and spheroidenone

We used elastic incoherent neutron scattering (EINS) to find out if structural changes accompanying local hydrogen bond rupture are also reflected in global dynamical response of the protein complex. Chromatophore membranes from LH2-only strains of the photosynthetic bacterium Rhodobacter sphaeroides, with spheroidenone or neurosporene as the major carotenoids, were subjected to high hydrostatic pressure at ambient temperature. Optical spectroscopy conducted at high pressure confirmed rupture of tertiary structure hydrogen bonds. In parallel, we used EINS to follow average motions of the hydrogen atoms in LH2, which reflect the flexibility of this complex. A decrease of the average atomic mean square displacements of hydrogen atoms was observed up to a pressure of 5 kbar in both carotenoid samples due to general stiffening of protein structures, while at higher pressures a slight increase of the displacements was detected in the neurosporene mutant LH2 sample only. These data show a correlation between the local pressure-induced breakage of H-bonds, observed in optical spectra, with the altered protein dynamics monitored by EINS. The slightly higher compressibility of the neurosporene mutant sample shows that even subtle alterations of carotenoids are manifested on a larger scale and emphasize a close connection between the local structure and global dynamics of this membrane protein complex.

Topics: Bacteriochlorophylls; Carotenoids; Hydrogen Bonding; Hydrostatic Pressure; Light-Harvesting Protein Complexes; Rhodobacter sphaeroides

Photosystem formation in anaerobic anoxygenic phototrophic bacteria (APB) is repressed by oxygen but is de-repressed when oxygen tension decreases. Under semiaerobic conditions, the synthesis of photopigments and pigment protein complexes in Rhodobacter (Rba.) sphaeroides are repressed by light. AppA, a blue-light receptor, mediates this regulation. In the present study, it was showed that the synthesis of bacteriochlorophyll, carotenoid, and pigment protein complexes in Rba. azotoformans 134K20 was significantly repressed by oxygen. Oxygen exposure also led to a conversion of spheroidene to spheroidenone. In semiaerobically growing cells, light irradiation resulted in a decrease in the formation of photosystem, and blue light was found to be the most effective light source. Blue light reduced the contents of bacteriochlorophyll and carotenoid slightly, but had negligible effects on light harvesting complex (LH) 1 content, whereas the content of LH2 was significantly decreased indicating that blue light selectively repressed the synthesis of LH2 in semiaerobically growing 134K20. It was concluded that, similar to Rba. sphaeroides, a blue light receptor presented in strain 134K20 played important roles in its light-dependent repression. A possible mechanism involved in controlling the differential inhibitory of blue light on the synthesis of photosystem was discussed.

Topics: Bacteriochlorophylls; Carotenoids; Light; Light-Harvesting Protein Complexes; Rhodobacter; Rhodobacter sphaeroides

A Rhodobacter capsulatus strain, designated XJ-1, isolated from saline soil, accumulated almost only one kind of bacteriochlorophyll a anaerobically in the light, aerobically in the light and dark, and the relative contents of the bacteriochlorophyll a were 44.61, 74.89, and 77.53% of the total pigments, respectively. A new purple pigment appeared only in aerobic-light grown cells, exhibited absorption maxima at 355, 389, 520, 621, and 755 nm, especially distinctly unusual peak at 621 nm, whereas vanished in anaerobic-light and in aerobic-dark culture. Spheroidene and OH-spheroidene predominated in anaerobic phototrophic cultures. Spheroidenone was the sole carotenoid when exposed to both light and oxygen. The second keto-carotenoids, OH-spheroidenone, presented only in aerobic-dark culture in addition to spheroidenone. Strain XJ-1 would be a good model organism for the further illustration of the regulation of bacteriochlorophyll biosynthesis gene expression in response to unique habitat.

Topics: Bacterial Proteins; Bacteriochlorophylls; Carotenoids; Light; Mass Spectrometry; Oxygen; Pheophytins; Rhodobacter capsulatus; Salinity; Sodium Chloride; Soil; Soil Microbiology

Light-harvesting complexes ensure necessary flow of excitation energy into photosynthetic reaction centers. In the present work, transient absorption measurements were performed on LH1-RC complexes isolated from two aerobic anoxygenic phototrophs (AAPs), Roseobacter sp. COL2P containing the carotenoid spheroidenone, and Erythrobacter sp. NAP1 which contains the carotenoids zeaxanthin and bacteriorubixanthinal. We show that the spectroscopic data from the LH1-RC complex of Roseobacter sp. COL2P are very similar to those previously reported for Rhodobacter sphaeroides, including the transient absorption spectrum originating from the intramolecular charge-transfer (ICT) state of spheroidenone. Although the ICT state is also populated in LH1-RC complexes of Erythrobacter sp. NAP1, its appearance is probably related to the polarity of the bacteriorubixanthinal environment rather than to the specific configuration of the carotenoid, which we hypothesize is responsible for populating the ICT state of spheroidenone in LH1-RC of Roseobacter sp. COL2P. The population of the ICT state enables efficient S1/ICT-to-bacteriochlorophyll (BChl) energy transfer which would otherwise be largely inhibited for spheroidenone and bacteriorubixanthinal due to their low energy S1 states. In addition, the triplet states of these carotenoids appear well-tuned for efficient quenching of singlet oxygen or BChl-a triplets, which is of vital importance for oxygen-dependent organisms such as AAPs.

Topics: Bacterial Proteins; Bacteriochlorophylls; Carotenoids; Energy Transfer; Kinetics; Light-Harvesting Protein Complexes; Rhodobacter sphaeroides; Roseobacter; Sphingomonadaceae; Xanthophylls; Zeaxanthins

Carotenoids are known to offer protection against the potentially damaging combination of light and oxygen encountered by purple phototrophic bacteria, but the efficiency of such protection depends on the type of carotenoid. Rhodobacter sphaeroides synthesizes spheroidene as the main carotenoid under anaerobic conditions whereas, in the presence of oxygen, the enzyme spheroidene monooxygenase catalyses the incorporation of a keto group forming spheroidenone. We performed ultrafast transient absorption spectroscopy on membranes containing reaction center-light-harvesting 1-PufX (RC-LH1-PufX) complexes and showed that when oxygen is present the incorporation of the keto group into spheroidene, forming spheroidenone, reconfigures the energy transfer pathway in the LH1, but not the LH2, antenna. The spheroidene/spheroidenone transition acts as a molecular switch that is suggested to twist spheroidenone into an s-trans configuration increasing its conjugation length and lowering the energy of the lowest triplet state so it can act as an effective quencher of singlet oxygen. The other consequence of converting carotenoids in RC-LH1-PufX complexes is that S(2)/S(1)/triplet pathways for spheroidene is replaced with a new pathway for spheroidenone involving an activated intramolecular charge-transfer (ICT) state. This strategy for RC-LH1-PufX-spheroidenone complexes maintains the light-harvesting cross-section of the antenna by opening an active, ultrafast S(1)/ICT channel for energy transfer to LH1 Bchls while optimizing the triplet energy for singlet oxygen quenching. We propose that spheroidene/spheroidenone switching represents a simple and effective photoprotective mechanism of likely importance for phototrophic bacteria that encounter light and oxygen.

Topics: Bacterial Proteins; Bacteriochlorophylls; Carotenoids; Cell Membrane; Energy Transfer; Light; Light-Harvesting Protein Complexes; Molecular Structure; Oxygen; Proteobacteria; Rhodobacter sphaeroides; Spectrophotometry

A peripheral light-harvesting complex from the aerobic purple bacterium Roseobacter (R.) denitrificans was purified and its photophysical properties characterized. The complex contains two types of pigments, bacteriochlorophyll (BChl) a and the carotenoid (Car) spheroidenone and possesses unique spectroscopic properties. It appears to lack the B850 bacteriochlorophyll a Q(y) band that is typical for similar light-harvesting complex 2 antennas. Circular dichroism and low temperature steady-state absorption spectroscopy revealed that the B850 band is present but is shifted significantly to shorter wavelengths and overlaps with the B800 band at room temperature. Such a spectral signature classifies this protein as a member of the light-harvesting complex 4 class of peripheral light-harvesting complexes, along with the previously known light-harvesting complex 4 from Rhodopseudomonas palustris. The influence of the spectral change on the light-harvesting ability was studied using steady-state absorption, fluorescence, circular dichroism, femtosecond and microsecond time-resolved absorption and time-resolved fluorescence spectroscopies. The results were compared to the properties of the similar (in pigment composition) light-harvesting complex 2 from aerobically grown Rhodobacter sphaeroides and are understood within the context of shared similarities and differences and the putative influence of the pigments on the protein structure and its properties.

Topics: Amino Acid Sequence; Bacteriochlorophylls; Carotenoids; Circular Dichroism; Energy Transfer; Light-Harvesting Protein Complexes; Molecular Sequence Data; Roseobacter; Spectrometry, Fluorescence

Aerobic anoxygenic phototrophs (AAPs) are prokaryotic microorganisms capable of harvesting light using bacteriochlorophyll-based reaction centres. Marine AAP communities are generally dominated by species belonging to the Roseobacter clade. For this reason, we used marine Roseobacter-related strain COL2P as a model organism to characterize its photosynthetic apparatus, level of pigmentation and expression of photosynthetic complexes. This strain contained functional photosynthetic reaction centres with bacteriochlorophyll a and spheroidenone as the main light-harvesting pigments, but the expression of the photosynthetic apparatus was significantly reduced when compared to truly photoautotrophic species. Moreover, the absence of peripheral light-harvesting complexes largely reduced its light-harvesting capacity. The size of the photosynthetic unit was limited to 35.4 +/- 1.0 BChl a molecules supplemented by the same number of spheroidenone molecules. The contribution of oxidative phosphorylation and photophosphorylation was analysed by respiration and fluorometric measurements. Our results indicate that even with a such reduced photosynthetic apparatus, photophosphorylation provides up to three times higher electron fluxes than aerobic respiration. These results suggest that light-derived energy can provide a substantial fraction of COL2P metabolic needs.

Topics: Aerobiosis; Amino Acid Sequence; Anaerobiosis; Bacterial Proteins; Bacteriochlorophyll A; Bacteriochlorophylls; Carotenoids; Conjugation, Genetic; Gene Expression Regulation, Bacterial; Genes, Bacterial; Light; Oxygen Consumption; Photophosphorylation; Photosynthesis; Photosynthetic Reaction Center Complex Proteins; Phylogeny; RNA, Bacterial; RNA, Messenger; Roseobacter; Seawater

Steady-state and ultrafast time-resolved optical spectroscopic investigations have been carried out at 293 and 10 K on LH2 pigment-protein complexes isolated from three different strains of photosynthetic bacteria: Rhodobacter (Rb.) sphaeroides G1C, Rb. sphaeroides 2.4.1 (anaerobically and aerobically grown), and Rps. acidophila 10050. The LH2 complexes obtained from these strains contain the carotenoids, neurosporene, spheroidene, spheroidenone, and rhodopin glucoside, respectively. These molecules have a systematically increasing number of pi-electron conjugated carbon-carbon double bonds. Steady-state absorption and fluorescence excitation experiments have revealed that the total efficiency of energy transfer from the carotenoids to bacteriochlorophyll is independent of temperature and nearly constant at approximately 90% for the LH2 complexes containing neurosporene, spheroidene, spheroidenone, but drops to approximately 53% for the complex containing rhodopin glucoside. Ultrafast transient absorption spectra in the near-infrared (NIR) region of the purified carotenoids in solution have revealed the energies of the S1 (2(1)Ag-)-->S2 (1(1)Bu+) excited-state transitions which, when subtracted from the energies of the S0 (1(1)Ag-)-->S2 (1(1)Bu+) transitions determined by steady-state absorption measurements, give precise values for the positions of the S1 (2(1)Ag-) states of the carotenoids. Global fitting of the ultrafast spectral and temporal data sets have revealed the dynamics of the pathways of de-excitation of the carotenoid excited states. The pathways include energy transfer to bacteriochlorophyll, population of the so-called S* state of the carotenoids, and formation of carotenoid radical cations (Car*+). The investigation has found that excitation energy transfer to bacteriochlorophyll is partitioned through the S1 (1(1)Ag-), S2 (1(1)Bu+), and S* states of the different carotenoids to varying degrees. This is understood through a consideration of the energies of the states and the spectral profiles of the molecules. A significant finding is that, due to the low S1 (2(1)Ag-) energy of rhodopin glucoside, energy transfer from this state to the bacteriochlorophylls is significantly less probable compared to the other complexes. This work resolves a long-standing question regarding the cause of the precipitous drop in energy transfer efficiency when the extent of pi-electron conjugation of the carotenoid is extended from ten to eleven conjugated ca

Topics: Algorithms; Bacterial Proteins; Bacteriochlorophylls; Carotenoids; Cold Temperature; Energy Transfer; Kinetics; Light-Harvesting Protein Complexes; Models, Molecular; Rhodobacter sphaeroides; Rhodopseudomonas; Spectrometry, Fluorescence; Spectrophotometry; Spectroscopy, Near-Infrared; Temperature; Time Factors

In LH2 complexes of Rhodobacter sphaeroides the formation of a carotenoid radical cation has recently been observed upon photoexcitation of the carotenoid S2 state. To shed more light onto the yet unknown molecular mechanism leading to carotenoid radical formation in LH2, the interactions between carotenoid and bacteriochlorophyll in LH2 are investigated by means of quantum chemical calculations for three different carotenoids--neurosporene, spheroidene, and spheroidenone--using time-dependent density functional theory. Crossings of the calculated potential energy curve of the electron transfer state with the bacteriochlorophyll Qx state and the carotenoid S1 and S2 states occur along an intermolecular distance coordinate for neurosporene and spheroidene, but for spheroidenone no crossing of the electron transfer state with the carotenoid S1 state could be found. By comparison with recent experiments where no formation of a spheroidenone radical cation has been observed, a molecular mechanism for carotenoid radical cation formation is proposed in which it is formed via a vibrationally excited carotenoid S1 or S*state. Arguments are given why the formation of the carotenoid radical cation does not proceed via the Qx, S2, or higher excited electron transfer states.

Topics: Algorithms; Bacterial Proteins; Bacteriochlorophylls; Carotenoids; Cations; Electron Transport; Energy Transfer; Free Radicals; Light; Light-Harvesting Protein Complexes; Protein Conformation; Proteobacteria; Quantum Theory; Rhodobacter sphaeroides; Time Factors

A novel group of aerobic anoxygenic phototrophic bacteria was isolated from marine dinoflagellates, and two strains were characterized in detail. Cells were Gram-negative cocci or ovoid rods and were motile by means of a single, polarly inserted flagellum. They were obligate aerobes requiring 1-7 % salinity. The optimal pH range for growth was 6.5-9.0 and the temperature optimum was 33 degrees C. The bacteria contained bacteriochlorophyll a and spheroidenone as the only carotenoid. The in vivo absorption spectrum displayed two maxima in the infrared region at 804 and 868 nm. The distinct 804 nm band indicates the presence of light-harvesting system 2. Various organic carbon sources were assimilated, including many carboxylic acids, glucose and glycerol, but not butyrate, ethanol or methanol. Dissimilatory nitrate reduction was found for both strains. The physiological characteristics of the new strains resembled those of Roseobacter denitrificans, but there were differences in the lipid composition. Based on 16S rRNA gene sequence analysis the new strains are relatively distant from other recognized species, with the closest relatives Jannaschia helgolandensis, Ruegeria atlantica and Rhodobacter veldkampii showing 94.1-93.4 % similarity. Similarity to Roseobacter denitrificans was only 92.2 %, in line with numerous other species of the Roseobacter group. Therefore, it is proposed to classify the strains in a new genus and species within the Roseobacter clade, Dinoroseobacter shibae gen. nov., sp. nov. The type strain is DFL 12(T) (=DSM 16493(T)=NCIMB 14021(T)).

Topics: Aerobiosis; Animals; Bacterial Typing Techniques; Bacteriochlorophyll A; Carbon; Carotenoids; Dinoflagellida; DNA, Bacterial; DNA, Ribosomal; Fatty Acids; Flagella; Genes, rRNA; Gentian Violet; Growth Inhibitors; Hydrogen-Ion Concentration; Molecular Sequence Data; Movement; Nitrates; Oxidation-Reduction; Phenazines; Photosystem II Protein Complex; Phylogeny; Rhodobacteraceae; RNA, Bacterial; RNA, Ribosomal, 16S; Sequence Analysis, DNA; Sodium Chloride; Spectrum Analysis; Temperature

Using the bacteriochlorophyll a (Bchl) cofactors as intrinsic probes to monitor changes in membrane protein structure, we investigate the response to high-pressure of the LH2 complexes from the photosynthetic bacteria Rhodobacter sphaeroides 2.4.1 and Rhodopseudomonas acidophila 10050. By FT-Raman spectroscopy, we demonstrate that high pressure does not induce significant distortion of the protein-bound 850 nm-absorbing bacteriochlorophyll molecules, or break of the hydrogen bond they are involved in. This indicates in particular that the oligomerization of the polypeptides is not perturbed up to 0.6 GPa. The pressure-induced changes in the Bchl absorption spectra are attributed to pigment-pigment interactions. In contrast, the loss of 800 nm-absorbing bacteriochlorophyll reflects pressure-induced alterations to the tertiary structure of the protein in proximity to the membrane/cytosol interface. This suggests that the LH2 protein does have two independent structural domains. The first domain is pressure independent and comprises mostly the C-terminal domain. The second domain located on the N-terminal side exhibits sensitivity to pressure and pH reminiscent of soluble proteins. The LH2 thus constitutes a suitable model system for studying in detail the stability of membrane-embedded hydrophobic helices and helices located at or close to the solvent/membrane interface.

Topics: Amino Acid Sequence; Bacterial Proteins; Bacteriochlorophylls; Carotenoids; Light-Harvesting Protein Complexes; Membrane Proteins; Molecular Sequence Data; Pressure; Protein Binding; Protein Structure, Quaternary; Protein Structure, Tertiary; Rhodobacter sphaeroides; Rhodopseudomonas; Spectrophotometry; Spectrum Analysis, Raman; Thermodynamics