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The present investigation concerns the potentiality of Rhodopseudomonas sp. cells to produce clean energy such as molecular hydrogen (H[sub.2] ). The abovementioned goal could be reached by improving the capability of purple non-sulfur bacteria to produce H[sub.2] via a photofermentative process through the enzyme nitrogenase. Rhodopseudomonas sp. cells were immobilized in calcium alginate gel beads and cultured in a cylindrical photobioreactor at a working volume of 0.22 L. The semi-continuous process, which lasted for 11 days, was interspersed with the washing of the beads with the aim of increasing the H[sub.2] production rate. The maximum H[sub.2] production rate reached 5.25 ± 0.93 mL/h with a total output of 505 mL. The productivity was 40.9 μL (of H[sub.2] )/mg (of cells)/h or 10.2 mL (of H[sub.2] )/L (of culture)/h with a light conversion efficiency of 1.20%.

Photosynthetic reaction centres harvest the energy content of sunlight by transporting electrons across an energy-transducing biological membrane. Here we use time-resolved serial femtosecond crystallography 1 using an X-ray free-electron laser 2 to observe light-induced structural changes in the photosynthetic reaction centre of Blastochloris viridis on a timescale of picoseconds. Structural perturbations first occur at the special pair of chlorophyll molecules of the photosynthetic reaction centre that are photo-oxidized by light. Electron transfer to the menaquinone acceptor on the opposite side of the membrane induces a movement of this cofactor together with lower amplitude protein rearrangements. These observations reveal how proteins use conformational dynamics to stabilize the charge-separation steps of electron-transfer reactions. Time-resolved serial femtosecond crystallography is used to reveal the structural changes that stabilize the charge-separation steps of electron-transfer reactions in the photosynthetic reaction centre of Blastochloris viridis on a timescale of picoseconds.

The light-harvesting 1–reaction centre (LH1–RC) complex is a key functional component of bacterial photosynthesis. Here we present a 2.9 Å resolution cryo-electron microscopy structure of the bacteriochlorophyll b -based LH1–RC complex from Blastochloris viridis that reveals the structural basis for absorption of infrared light and the molecular mechanism of quinone migration across the LH1 complex. The triple-ring LH1 complex comprises a circular array of 17 β-polypeptides sandwiched between 17 α- and 16 γ-polypeptides. Tight packing of the γ-apoproteins between β-polypeptides collectively interlocks and stabilizes the LH1 structure; this, together with the short Mg–Mg distances of bacteriochlorophyll b pairs, contributes to the large redshift of bacteriochlorophyll b absorption. The ‘missing’ 17th γ-polypeptide creates a pore in the LH1 ring, and an adjacent binding pocket provides a folding template for a quinone, Q P , which adopts a compact, export-ready conformation before passage through the pore and eventual diffusion to the cytochrome bc 1 complex. A cryo-electron microscopy structure of the light-harvesting–reaction centre (LH1–RC) complex of the photosynthetic bacterium Blastochloris viridis suggests factors that underlie the large redshift in the absorption spectrum of bacteriochlorophyll in the complex and that promote quinone–quinol translocation across the LH1 ring.

All purple photosynthetic bacteria contain RC–LH1 ‘Core’ complexes. The structure of this complex from Rhodobacter sphaeroides, Rhodopseudomonas palustris and Thermochromatium tepidum has been solved using X-ray crystallography. Recently, the application of single particle cryo-EM has revolutionised structural biology and the structure of the RC–LH1 ‘Core’ complex from Blastochloris viridis has been solved using this technique, as well as the complex from the non-purple Chloroflexi species, Roseiflexus castenholzii. It is apparent that these structures are variations on a theme, although with a greater degree of structural diversity within them than previously thought. Furthermore, it has recently been discovered that the only phototrophic representative from the phylum Gemmatimonadetes, Gemmatimonas phototrophica, also contains a RC–LH1 ‘Core’ complex. At present only a low-resolution EM-projection map exists but this shows that the Gemmatimonas phototrophica complex contains a double LH1 ring. This short review compares these different structures and looks at the functional significance of these variations from two main standpoints: energy transfer and quinone exchange.

Geranylgeranyl reductase (GGR) encoded by the bchP gene catalyzes the reductions of three unsaturated C = C double bonds (C6 = C7, C10 = C11, and C14 = C15) in a geranylgeranyl (GG) group of the esterifying moiety in 17-propionate residue of bacteriochlorophyll (BChl) molecules. It was recently reported that GGR in Halorhodospira halochloris potentially catalyzes two hydrogenations, yielding BChl with a tetrahydrogeranylgeranyl (THGG) tail. Furthermore, its engineered GGR, in which N-terminal insertion peptides characteristic for H. halochloris were deleted, performed single hydrogenation, producing BChl with a dihydrogeranylgeranyl (DHGG) tail. In some of these enzymatic reactions, it remained unclear in which order the C = C double bond in a GG group was first reduced. In this study, we demonstrated that the (variant) GGR from H. halochloris catalyzed an initial reduction of the C6 = C7 double bond to yield a 6,7-DHGG tail. The intact GGR of H. halochloris catalyzed the further hydrogenation of the C14 = C15 double bonds to give a 6,7,14,15-THGG group, whereas deleting the characteristic peptide region from the GGR suppressed the C14 = C15 reduction. We also verified that in a model bacterium, Blastochloris viridis producing standard BChl-b, the reduction of a GG to phytyl group occurred via 10,11-DHGG and 6,7,10,11-THGG. The high-performance liquid chromatographic elution profiles of BChls-a/b employed in this study are essential for identifying the regioisomeric diterpenoid tails in the BChls of phototrophic bacteria distributed in nature and elucidating GGR enzymatic reactions.

Quinones serve as redox active cofactors in bacterial photosynthetic reaction centers: photosystem I, photosystem II, cytochrome bc 1 , and cytochrome b 6 f . In particular, ubiquinone is ubiquitous in animals and most bacteria and plays a key role in several cellular processes, e.g., mitochondrial electron transport. Their experimentally measured redox potential values for one-electron reduction E m (Q/Q ·− ) were already reported in dimethylformamide (DMF) versus saturated calomel electrode but not in water versus normal hydrogen electrode (NHE). We calculated E m (Q/Q ·− ) of 1,4-quinones using a quantum chemical approach. The calculated energy differences of reduction of Q to Q ·− in DMF and water for 1,4-quinone derivatives correlated highly with the experimentally measured E m (Q/Q ·− ) in DMF and water, respectively. E m (Q/Q ·− ) were calculated to be −163 mV for ubiquinone, −260 mV for menaquinone and phylloquinone, and −154 mV for plastoquinone in water versus NHE.

Photosynthetic reaction centers convert the energy content of light into a transmembrane potential difference and so provide the major pathway for energy input into the biosphere. We applied time-resolved Laue diffraction to study light-induced conformational changes in the photosynthetic reaction center complex of Blastochlorís virídis. The side chain of TyrL162, which lies adjacent to the special pair of bacteriochlorophyll molecules that are photooxidized in the primary light conversion event of photosynthesis, was observed to move 1.3 angstroms closer to the special pair after photoactivation. Free energy calculations suggest that this movement results from the deprotonation of this conserved tyrosine residue and provides a mechanism for stabilizing the primary charge separation reactions of photosynthesis.

Rhodopseudomonas is recognized for its versatile metabolic capabilities that enable it to effectively degrade pollutants and survive various environmental stresses. In this study, we conducted a genome analysis of Rhodopseudomonas sp. P1 to investigate its genetic potential for wastewater treatment processes. Phylogenetic and genome-relatedness analyses confirmed that strain P1 is genetically distinct from other species within the Rhodopseudomonas genus, establishing it as a novel species. The genome sequences obtained and analyzed focused on genes related to carbon and nutrient removal, photosynthetic capabilities, nitrate and nitrite reduction, and the biodegradation of common wastewater pollutants. The identification of wastewater treatment-related genes followed an extensive review of the existing literature that helped in selecting genes involved in various wastewater treatment mechanisms. The genome of Rhodopseudomonas sp. P1 contains a diverse array of genes involved in carbon and nutrient cycling, pollutant biodegradation, and metal resistance, all of which are crucial for its survival in the complex wastewater environment. Specifically, the strain contains genes responsible for the denitrification, nitrogen fixation, sulfur cycling, and detoxification of toxic metals such as copper and arsenic. These findings highlight the potential application of Rhodopseudomonas sp. P1 in wastewater treatment, particularly in environments contaminated with organic pollutants and heavy metals. However, while the genomic features indicate significant promise, the practical implementation of Rhodopseudomonas sp. P1 in real-world wastewater treatment systems will require further investigation, optimization, and validation to fully harness its potential for sustainable and efficient wastewater treatment.

Quinones can accept two electrons and two protons, and are involved in electron transfer and proton transfer reactions in photosynthetic reaction centers. To date, the pK of these quinones in aqueous solution have not been reported. We calculated the pK of the initial protonation (Q to QH ) and the second protonation (QH to QH ) of 1,4-quinones using a quantum chemical approach. The calculated energy differences of the protonation reactions Q to QH and QH to QH in the aqueous phase for nine 1,4-quinones were highly correlated with the experimentally measured pK (Q /QH ) and pK (QH /QH ), respectively. In the present study, we report the pK (Q /QH ) and pK (QH /QH ) of ubiquinone, menaquinone, phylloquinone, plastoquinone, and rhodoquinone in aqueous solution.

Studies on membrane development in purple bacteria during adaptation to alterations in light intensity and oxygen tension are reviewed. Anoxygenic phototrophic such as the purple α-proteobacterium Rhodobacter sphaeroides have served as simple, dynamic, and experimentally accessible model organisms for studies of the photosynthetic apparatus. A major landmark in photosynthesis research, which dramatically illustrates this point, was provided by the determination of the X-ray structure of the reaction center (RC) in Blastochloris viridis (Deisenhofer and Michel, EMBO J 8:2149–2170, 1989), once it was realized that this represented the general structure for the photosystem II RC present in all oxygenic phototrophs. This seminal advance, together with a considerable body of subsequent research on the light-harvesting (LH) and electron transfer components of the photosynthetic apparatus has provided a firm basis for the current understanding of how phototrophs acclimate to alterations in light intensity and quality. Oxygenic phototrophs adapt to these changes by extensive thylakoid membrane remodeling, which results in a dramatic supramolecular reordering to assure that an appropriate flow of quinone redox species occurs within the membrane bilayer for efficient and rapid electron transfer. Despite the high level of photosynthetic unit organization in Rba. sphaeroides as observed by atomic force microscopy (AFM), fluorescence induction/relaxation measurements have demonstrated that the addition of the peripheral LH2 antenna complex in cells adapting to low-intensity illumination results in a slowing of the rate of electron transfer turnover by the RC of up to an order of magnitude. This is ascribed to constraints in quinone redox species diffusion between the RC and cytochrome bc ₁ complexes arising from the increased packing density as the intracytoplasmic membrane (ICM) bilayer becomes crowded with LH2 rings. In addition to downshifts in light intensity as a paradigm for membrane development studies in Rba. sphaeroides, the lowering of oxygen tension in chemoheterotropically growing cells results in a gratuitous formation of the ICM by an extensive membrane biogenesis process. These membrane alterations in response to lowered illumination and oxygen levels in purple bacteria are under the control of a number of interrelated two-component regulatory circuits reviewed here, which act at the transcriptional level to regulate the formation of both the pigment and apoprotein components of the LH, RC, and respiratory complexes. We have performed a proteomic examination of the ICM development process in which membrane proteins have been identified that are temporally expressed both during adaptation to low light intensity and ICM formation at low aeration and are spatially localized in both growing and mature ICM regions. For these proteomic analyses, membrane growth initiation sites and mature ICM vesicles were isolated as respective upper-pigmented band (UPB) and chromatophore fractions and subjected to clear native electrophoresis for isolation of bands containing the LH2 and RC–LH1 core complexes. In chromatophores, increasing levels of LH2 polypeptides relative to those of the RC–LH1 complex were observed as ICM membrane development proceeded during light-intensity downshifts, along with a large array of other associated proteins including high spectral counts for the F₁FO–ATP synthase subunits and the cytochrome bc ₁ complex, as well as RSP6124, a protein of unknown function, that was correlated with increasing LH2 spectral counts. In contrast, the UPB was enriched in cytoplasmic membrane (CM) markers, including electron transfer and transport proteins, as well as general membrane protein assembly factors confirming the origin of the UPB from both peripheral respiratory membrane and sites of active CM invagination that give rise to the ICM. The changes in ICM vesicles were correlated to AFM mapping results (Adams and Hunter, Biochim Biophys Acta 1817:1616–1627, 2012), in which the increasing LH2 levels were shown to form densely packed LH2-only domains, representing the light-responsive antenna complement formed under low illumination. The advances described here could never have been envisioned when the author was first introduced in the mid-1960s to the intricacies of the photosynthetic apparatus during a lecture delivered in a graduate Biochemistry course at the University of Illinois by Govindjee, to whom this volume is dedicated on the occasion of his 80th birthday.