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Nature May 2023Photosynthesis fuels life on Earth by storing solar energy in chemical form. Today's oxygen-rich atmosphere has resulted from the splitting of water at the protein-bound...
Photosynthesis fuels life on Earth by storing solar energy in chemical form. Today's oxygen-rich atmosphere has resulted from the splitting of water at the protein-bound manganese cluster of photosystem II during photosynthesis. Formation of molecular oxygen starts from a state with four accumulated electron holes, the S state-which was postulated half a century ago and remains largely uncharacterized. Here we resolve this key stage of photosynthetic O formation and its crucial mechanistic role. We tracked 230,000 excitation cycles of dark-adapted photosystems with microsecond infrared spectroscopy. Combining these results with computational chemistry reveals that a crucial proton vacancy is initally created through gated sidechain deprotonation. Subsequently, a reactive oxygen radical is formed in a single-electron, multi-proton transfer event. This is the slowest step in photosynthetic O formation, with a moderate energetic barrier and marked entropic slowdown. We identify the S state as the oxygen-radical state; its formation is followed by fast O-O bonding and O release. In conjunction with previous breakthroughs in experimental and computational investigations, a compelling atomistic picture of photosynthetic O formation emerges. Our results provide insights into a biological process that is likely to have occurred unchanged for the past three billion years, which we expect to support the knowledge-based design of artificial water-splitting systems.
Topics: Electrons; Oxidation-Reduction; Oxygen; Photosynthesis; Photosystem II Protein Complex; Protons; Water
PubMed: 37138082
DOI: 10.1038/s41586-023-06008-5 -
Biochemistry. Biokhimiia Nov 2017This review considers the state-of-the-art on mechanisms and alternative pathways of electron transfer in photosynthetic electron transport chains of chloroplasts and... (Review)
Review
This review considers the state-of-the-art on mechanisms and alternative pathways of electron transfer in photosynthetic electron transport chains of chloroplasts and cyanobacteria. The mechanisms of electron transport control between photosystems (PS) I and II and the Calvin-Benson cycle are considered. The redistribution of electron fluxes between the noncyclic, cyclic, and pseudocyclic pathways plays an important role in the regulation of photosynthesis. Mathematical modeling of light-induced electron transport processes is considered. Particular attention is given to the electron transfer reactions on the acceptor side of PS I and to interactions of PS I with exogenous acceptors, including molecular oxygen. A kinetic model of PS I and its interaction with exogenous electron acceptors has been developed. This model is based on experimental kinetics of charge recombination in isolated PS I. Kinetic and thermodynamic parameters of the electron transfer reactions in PS I are scrutinized. The free energies of electron transfer between quinone acceptors A/A in the symmetric redox cofactor branches of PS I and iron-sulfur clusters F, F, and F have been estimated. The second-order rate constants of electron transfer from PS I to external acceptors have been determined. The data suggest that byproduct formation of superoxide radical in PS I due to the reduction of molecular oxygen in the A site (Mehler reaction) can exceed 0.3% of the total electron flux in PS I.
Topics: Chloroplasts; Cyanobacteria; Electron Transport; Iron-Sulfur Proteins; Kinetics; Models, Chemical; Oxygen; Photosystem I Protein Complex; Quinones
PubMed: 29223152
DOI: 10.1134/S0006297917110037 -
Toxics Aug 2022Mercury (Hg) poses high toxicity to organisms including algae. Studies showed that the growth and photosynthesis of green algae such as Chlorella are vulnerable to Hg...
Mercury (Hg) poses high toxicity to organisms including algae. Studies showed that the growth and photosynthesis of green algae such as Chlorella are vulnerable to Hg stress. However, the differences between the activities and tolerance of photosystem I and II (PSI and PSII) of green microalgae under Hg exposure are still little known. Responses of quantum yields and electron transport rates (ETRs) of PSI and PSII of Chlorella pyrenoidosa to 0.05−1 mg/L Hg2+ were simultaneously measured for the first time by using the Dual-PAM-100 system. The photosystems were isolated to analyze the characteristics of toxicity of Hg during the binding process. The inhibition of Hg2+ on growth and photosystems was found. PSII was more seriously affected by Hg2+ than PSI. After Hg2+ exposure, the photochemical quantum yield of PSII [Y(II)] decreased with the increase in non-photochemical fluorescence quenching [Y(NO) and Y(NPQ)]. The toxic effects of Hg on the photochemical quantum yield and ETR in PSI were lower than those of PSII. The stimulation of cyclic electron yield (CEF) was essential for the stability and protection of PSI under Hg stress and played an important role in the induction of non-photochemical quenching (NPQ). The results showed a strong combination ability of Hg ions and photosystem particles. The number of the binding sites (n) of Hg on PSII was more than that of PSI, which may explain the different toxicity of Hg on PSII and PSI.
PubMed: 36006134
DOI: 10.3390/toxics10080455 -
Journal of Bacteriology Feb 1998Cyanothece sp. strain ATCC 51142, a unicellular, diazotrophic cyanobacterium, demonstrated extensive metabolic periodicities of photosynthesis, respiration, and nitrogen...
Transcriptional and translational regulation of photosystem I and II genes in light-dark- and continuous-light-grown cultures of the unicellular cyanobacterium Cyanothece sp. strain ATCC 51142.
Cyanothece sp. strain ATCC 51142, a unicellular, diazotrophic cyanobacterium, demonstrated extensive metabolic periodicities of photosynthesis, respiration, and nitrogen fixation when grown under N2-fixing conditions. This report describes the relationship of the biosynthesis of photosynthesis genes to changes in the oligomerization state of the photosystems. Transcripts of the psbA gene family, encoding the photosystem II (PSII) reaction center protein D1, accumulated primarily during the light period, and net transcription reached a peak between 2 to 6 h in the light in light-dark (LD) growth and between 4 to 10 h in the subjective light when grown under continuous light (LL). The relative amount of the D1 protein (form 1 versus form 2) appeared to change during this diurnal cycle, along with changes in the PSII monomer/dimer ratio. D1 form 1 accumulated at approximately equal levels throughout the 24-h cycle, whereas D1 form 2 accumulated at significantly higher levels at approximately 8 to 10 h in the light or subjective light. The psbD gene, encoding the reaction center protein D2, also demonstrated differences between the two copies of this gene, with one copy transcribed more heavily around 6 to 8 h in the light. Accumulation of the PSI reaction center proteins PsaA and PsaB was maximal in the dark or subjective-dark periods, a period during which PSI was primarily in the trimeric form. We conclude that photosystem organization changes during the diurnal cycle to favor either noncyclic electron flow, which leads to O2 evolution and CO2 fixation, or cyclic electron flow, which favors ATP synthesis.
Topics: Bacterial Proteins; Culture Media; Cyanobacteria; Darkness; Gene Expression Regulation, Bacterial; Light; Membrane Proteins; Nitrogen Fixation; Photosynthesis; Photosynthetic Reaction Center Complex Proteins; Photosystem I Protein Complex; Photosystem II Protein Complex; Protein Biosynthesis; Transcription, Genetic
PubMed: 9457853
DOI: 10.1128/JB.180.3.519-526.1998 -
Biochimica Et Biophysica Acta.... Apr 2023Knowledge about the exact abundance and ratio of photosynthetic protein complexes in thylakoid membranes is central to understanding structure-function relationships in...
Knowledge about the exact abundance and ratio of photosynthetic protein complexes in thylakoid membranes is central to understanding structure-function relationships in energy conversion. Recent modeling approaches for studying light harvesting and electron transport reactions rely on quantitative information on the constituent complexes in thylakoid membranes. Over the last decades several quantitative methods have been established and refined, enabling precise stoichiometric information on the five main energy-converting building blocks in the thylakoid membrane: Light-harvesting complex II (LHCII), Photosystem II (PSII), Photosystem I (PSI), cytochrome bf complex (cyt bf complex), and ATPase. This paper summarizes a few quantitative spectroscopic and biochemical methods that are currently available for quantification of plant thylakoid protein complexes. Two new methods are presented for quantification of LHCII and the cyt bf complex, which agree well with established methods. In addition, recent improvements in mass spectrometry (MS) allow deeper compositional information on thylakoid membranes. The comparison between mass spectrometric and more classical protein quantification methods shows similar quantities of complexes, confirming the potential of thylakoid protein complex quantification by MS. The quantitative information on PSII, PSI, and LHCII reveal that about one third of LHCII must be associated with PSI for a balanced light energy absorption by the two photosystems.
Topics: Thylakoids; Cytochrome b6f Complex; Cytochromes b; Light-Harvesting Protein Complexes; Photosystem I Protein Complex; Plant Proteins
PubMed: 36442511
DOI: 10.1016/j.bbabio.2022.148945 -
The Journal of General and Applied... Feb 2024Although n-butanol (BuOH) is an ideal fuel because of its superior physical properties, it has toxicity to microbes. Previously, a Synechococcus elongatus PCC 7942...
Although n-butanol (BuOH) is an ideal fuel because of its superior physical properties, it has toxicity to microbes. Previously, a Synechococcus elongatus PCC 7942 derivative strain that produces BuOH from CO was developed by introducing six heterologous genes (BUOH-SE strain). To identify the bottleneck in BuOH production, the effects of BuOH production and its toxicity on central metabolism and the photosystem were investigated. Parental (WT) and BUOH-SE strains were cultured under autotrophic conditions. Consistent with the results of a previous study, BuOH production was observed only in the BUOH-SE strain. Isotopically non-stationary C-metabolic flux analysis revealed that the CO fixation rate was much larger than the BuOH production rate in the BUOH-SE strain (1.70 vs 0.03 mmol gDCW h), implying that the carbon flow for BuOH biosynthesis was less affected by the entire flux distribution. No large difference was observed in the flux of metabolism between the WT and BUOH-SE strains. Contrastingly, in the photosystem, the chlorophyll content and maximum O evolution rate per dry cell weight of the BUOH-SE strain were decreased to 81% and 43% of the WT strain, respectively. Target proteome analysis revealed that the amounts of some proteins related to antennae (ApcA, ApcD, ApcE, and CpcC), photosystem II (PsbB, PsbU, and Psb28-2), and cytochrome bf complex (PetB and PetC) in photosystems decreased in the BUOH-SE strain. The activation of photosynthesis would be a novel approach for further enhancing BuOH production in S. elongatus PCC 7942.
Topics: 1-Butanol; Proteome; Cytochrome b6f Complex; Carbon Dioxide; Photosynthesis; Butanols
PubMed: 36935115
DOI: 10.2323/jgam.2023.03.002 -
Plants (Basel, Switzerland) Mar 2021A significant increase in atmospheric CO concentration and associated climate aridization and soil salinity are factors affecting the growth, development, productivity,...
A significant increase in atmospheric CO concentration and associated climate aridization and soil salinity are factors affecting the growth, development, productivity, and stress responses of plants. In this study, the effect of ambient (400 ppm) and elevated (800 ppm) CO concentrations were evaluated on the C xero-halophyte treated with moderate salinity (200 mM NaCl) and polyethylene glycol (PEG)-induced osmotic stress. Our results indicated that plants grown at elevated CO concentration had different responses to osmotic stress and salinity. The synergistic effect of elevated CO and osmotic stress increased proline accumulation, but elevated CO did not mitigate the negative effects of osmotic stress on dark respiration intensity and photosystem II (PSII) efficiency. This indicates a stressful state, which is accompanied by a decrease in the efficiency of light reactions of photosynthesis and significant dissipative respiratory losses, thereby resulting in growth inhibition. Plants grown at elevated CO concentration and salinity showed high Na and proline contents, high water-use efficiency and time required to reach the maximum P700 oxidation level (PSI), and low dark respiration. Maintaining stable water balance, the efficient functioning of cyclic transport of PSI, and the reduction of dissipation costs contributed to an increase in dry shoot biomass (2-fold, compared with salinity at 400 ppm CO). The obtained experimental data and PCA showed that elevated CO concentration improved the physiological parameters of under salinity.
PubMed: 33807685
DOI: 10.3390/plants10030491 -
Biochimica Et Biophysica Acta Jun 2009The chlorophyll a/b light-harvesting complex of photosystem II (LHC-II) collects most of the solar energy in the biosphere. LHC-II is the prototype of a highly conserved... (Review)
Review
The chlorophyll a/b light-harvesting complex of photosystem II (LHC-II) collects most of the solar energy in the biosphere. LHC-II is the prototype of a highly conserved family of membrane proteins that fuels plant photosynthesis in the conversion of excitation energy into biologically useful chemical energy. In addition, LHC-II plays an important role in the organisation of the thylakoid membrane, the structure of the photosynthetic apparatus, the regulation of energy flow between the two photosystems, and in the controlled dissipation of excess excitation energy under light stress. Our current understanding of the sophisticated mechanisms behind each of these processes has profited greatly from the progress made over the past two decades in determining the structure of the complex. This review presents the developments and breakthroughs that ultimately lead to the high-resolution structure of LHC-II. Based on an alignment of the remarkably well engineered and highly conserved LHC polypeptide, we propose several key features of the LHC-II structure that are likely to be present in all members of the LHC family. Finally, some recently proposed mechanisms of energy-dependent non-photochemical quenching (NPQ) are examined from a structural perspective.
Topics: Amino Acid Sequence; Arabidopsis Proteins; Binding Sites; Crystallization; Crystallography, X-Ray; Freeze Fracturing; Light-Harvesting Protein Complexes; Microscopy, Electron, Transmission; Models, Molecular; Molecular Sequence Data; Photochemical Processes; Photosystem II Protein Complex; Plant Proteins; Protein Structure, Quaternary; Sequence Homology, Amino Acid
PubMed: 19327340
DOI: 10.1016/j.bbabio.2009.03.012 -
Nature Communications Dec 2023Phycobilisomes (PBS) are antenna megacomplexes that transfer energy to photosystems II and I in thylakoids. PBS likely evolved from a basic, inefficient form into the...
Phycobilisomes (PBS) are antenna megacomplexes that transfer energy to photosystems II and I in thylakoids. PBS likely evolved from a basic, inefficient form into the predominant hemidiscoidal shape with radiating peripheral rods. However, it has been challenging to test this hypothesis because ancestral species are generally inaccessible. Here we use spectroscopy and cryo-electron microscopy to reveal a structure of a "paddle-shaped" PBS from a thylakoid-free cyanobacterium that likely retains ancestral traits. This PBS lacks rods and specialized ApcD and ApcF subunits, indicating relict characteristics. Other features include linkers connecting two chains of five phycocyanin hexamers (CpcN) and two core subdomains (ApcH), resulting in a paddle-shaped configuration. Energy transfer calculations demonstrate that chains are less efficient than rods. These features may nevertheless have increased light absorption by elongating PBS before multilayered thylakoids with hemidiscoidal PBS evolved. Our results provide insights into the evolution and diversification of light-harvesting strategies before the origin of thylakoids.
Topics: Thylakoids; Phycobilisomes; Cryoelectron Microscopy; Photosystem I Protein Complex; Bacterial Proteins; Cyanobacteria
PubMed: 38049400
DOI: 10.1038/s41467-023-43646-9 -
Frontiers in Plant Science 2020Recruitment of HO as the final donor of electrons for light-governed reactions in photosynthesis has been an utmost breakthrough, bursting the evolution of life and... (Review)
Review
Recruitment of HO as the final donor of electrons for light-governed reactions in photosynthesis has been an utmost breakthrough, bursting the evolution of life and leading to the accumulation of O molecules in the atmosphere. O molecule has a great potential to accept electrons from the components of the photosynthetic electron transfer chain (PETC) (so-called the Mehler reaction). Here we overview the Mehler reaction mechanisms, specifying the changes in the structure of the PETC of oxygenic phototrophs that probably had occurred as the result of evolutionary pressure to minimize the electron flow to O. These changes are warranted by the fact that the efficient electron flow to O would decrease the quantum yield of photosynthesis. Moreover, the reduction of O leads to the formation of reactive oxygen species (ROS), namely, the superoxide anion radical and hydrogen peroxide, which cause oxidative stress to plant cells if they are accumulated at a significant amount. From another side, hydrogen peroxide acts as a signaling molecule. We particularly zoom in into the role of photosystem I (PSI) and the plastoquinone (PQ) pool in the Mehler reaction.
PubMed: 32231675
DOI: 10.3389/fpls.2020.00211