Publications
277 results found
Sugiura M, Kimura M, Shimamoto N, et al., 2024, Tuning of the ChlD1 and ChlD2 properties in photosystem II by site-directed mutagenesis of neighbouring amino acids, BBA: Bioenergetics, Vol: 1865, ISSN: 0005-2728
Photosystem II is the water/plastoquinone photo-oxidoreductase of photosynthesis. The photochemistry and catalysis occur in a quasi-symmetrical heterodimer, D1D2, that evolved from a homodimeric ancestor. Here, we studied site-directed mutants in PSII from the thermophilic cyanobacterium Thermosynechoccocus elongatus, focusing on the primary electron donor chlorophyll a in D1, ChlD1, and on its symmetrical counterpart in D2, ChlD2, which does not play a direct photochemical role. The main conserved amino acid specific to ChlD1 is D1/T179, which H-bonds the water ligand to its Mg2+, while its counterpart near ChlD2 is the non-H-bonding D2/I178. The symmetrical-swapped mutants, D1/T179I and D2/I178T, and a second ChlD2 mutant, D2/I178H, were studied. The D1 mutations affected the 686 nm absorption attributed to ChlD1, while the D2 mutations affected a 663 nm feature, tentatively attributed to ChlD2. The mutations had little effect on enzyme activity and forward electron transfer, reflecting the robustness of the overall enzyme function. In contrast, the mutations significantly affected photodamage and protective mechanisms, reflecting the importance of redox tuning in these processes. In D1/T179I, the radical pair recombination triplet on ChlD1 was shared onto a pheophytin, presumably PheD1 and the detection of 3PheD1 supports the proposed mechanism for the anomalously short lifetime of 3ChlD1; e.g. electron transfer quenching by QA− of 3PheD1 after triplet transfer from 3ChlD1. In D2/I178T, a charge separation could occur between ChlD2 and PheD2, a reaction that is thought to occur in ancestral precursors of PSII. These mutants help understand the evolution of asymmetry in PSII.
Boussac A, Sugiura M, Nakamura M, et al., 2023, Absorption changes in Photosystem II in the Soret band region upon the formation of the chlorophyll cation radical [PD1PD2]., Photosynth Res
Flash-induced absorption changes in the Soret region arising from the [PD1PD2]+ state, the chlorophyll cation radical formed upon light excitation of Photosystem II (PSII), were measured in Mn-depleted PSII cores at pH 8.6. Under these conditions, TyrD is i) reduced before the first flash, and ii) oxidized before subsequent flashes. In wild-type PSII, when TyrD● is present, an additional signal in the [PD1PD2]+-minus-[PD1PD2] difference spectrum was observed when compared to the first flash when TyrD is not oxidized. The additional feature was "W-shaped" with troughs at 434 nm and 446 nm. This feature was absent when TyrD was reduced, but was present (i) when TyrD was physically absent (and replaced by phenylalanine) or (ii) when its H-bonding histidine (D2-His189) was physically absent (replaced by a Leucine). Thus, the simple difference spectrum without the double trough feature at 434 nm and 446 nm, seemed to require the native structural environment around the reduced TyrD and its H bonding partners to be present. We found no evidence of involvement of PD1, ChlD1, PheD1, PheD2, TyrZ, and the Cytb559 heme in the W-shaped difference spectrum. However, the use of a mutant of the PD2 axial His ligand, the D2-His197Ala, shows that the PD2 environment seems involved in the formation of "W-shaped" signal.
Fantuzzi A, Haniewicz P, Farci D, et al., 2023, Bicarbonate activation of the monomeric photosystem II-PsbS/Psb27 complex, Plant Physiology, Vol: 192, Pages: 2656-2671, ISSN: 0032-0889
In thylakoid membranes, Photosystem II (PSII) monomers from the stromal lamellae contain the subunits PsbS and Psb27 (PSIIm-S/27), while PSII monomers from granal regions (PSIIm) lack these subunits. Here, we have isolated and characterised these two types of Photosystem II complexes in tobacco (Nicotiana tabacum). PSIIm-S/27 showed enhanced fluorescence, the near-absence of oxygen evolution, as well as limited and slow electron transfer from QA to QB compared to the near-normal activities in the granal PSIIm. However, when bicarbonate was added to PSIIm-S/27, water splitting and QA to QB electron transfer rates were comparable to those in granal PSIIm. The findings suggest that the binding of PsbS and/or Psb27 inhibits forward electron transfer and lowers the binding affinity for bicarbonate. This can be rationalized in terms of the recently discovered photoprotection role played by bicarbonate binding via the redox tuning of the QA/QA•– couple, which controls the charge recombination route, and this limits chlorophyll triplet mediated 1O2 formation. These findings suggest that PSIIm-S/27 is an intermediate in the assembly of PSII in which PsbS and/or Psb27 restrict PSII activity while in transit using a bicarbonate-mediated switch and protective mechanism.
Oliver T, Kim TD, Trinugroho JP, et al., 2023, The evolution and evolvability of photosystem II, Annual Review of Plant Biology, Vol: 74, ISSN: 1040-2519
Photosystem II is the water-oxidizing and O2 -evolving enzyme of photosynthesis. How and when this remarkable enzyme arose are fundamental questions in the history of life that have remained difficult to answer. Here, recent advances in our understanding of the origin andevolution of photosystem II are reviewed and discussed in detail. The evolution of photosystem II indicates that water oxidation originated early in the history of life, long before the diversification of cyanobacteria and other major groups of prokaryotes, challenging and transforming current paradigms on the evolution of photosynthesis. We show that photosystem II has remained virtually unchanged for well over three billion years, and yet the nonstop duplication process of the D1 subunit of photosystem II, which controls photochemistry and catalysis, has enabled the enzyme to become adaptable to variable environmental conditions, and even to innovate enzymatic functions beyond water oxidation. It is suggested that this evolvability can be exploited to develop novel light-powered enzymes with the capacity to carry out complex multi-step oxidative transformations for sustainable biocatalysis.
Langley J, Purchase R, Viola S, et al., 2022, Simulating the low-temperature, metastable electrochromism of Photosystem I: Applications to <i>Thermosynechococcus vulcanus</i> and <i>Chroococcidiopsis thermalis</i>, JOURNAL OF CHEMICAL PHYSICS, Vol: 157, ISSN: 0021-9606
Viola S, Roseby W, Santabarbara S, et al., 2022, Impact of energy limitations on function and resilience in long-wavelength photosystem II, eLife, Vol: 11, ISSN: 2050-084X
Photosystem II (PSII) uses the energy from red light to split water and reduce quinone, an energy-demanding process based on chlorophyll a (Chl-a) photochemistry. Two types of cyanobacterial PSII can use chlorophyll d (Chl-d) and chlorophyll f (Chl-f) to perform the same reactions using lower energy, far-red light. PSII from Acaryochloris marina has Chl-d replacing all but one of its 35 Chl-a, while PSII from Chroococcidiopsis thermalis, a facultative far-red species, has just 4 Chl-f and 1 Chl-d and 30 Chl-a. From bioenergetic considerations, the far-red PSII were predicted to lose photochemical efficiency and/or resilience to photodamage. Here, we compare enzyme turnover efficiency, forward electron transfer, back-reactions and photodamage in Chl-f-PSII, Chl-d-PSII and Chl-a-PSII. We show that: i) all types of PSII have a comparable efficiency in enzyme turnover; ii) the modified energy gaps on the acceptor side of Chl-d-PSII favour recombination via PD1+Phe- repopulation, leading to increased singlet oxygen production and greater sensitivity to high-light damage compared to Chl-a-PSII and Chl-f-PSII; iii) the acceptor-side energy gaps in Chl-f-PSII are tuned to avoid harmful back reactions, favouring resilience to photodamage over efficiency of light usage. The results are explained by the differences in the redox tuning of the electron transfer cofactors Phe and QA and in the number and layout of the chlorophylls that share the excitation energy with the primary electron donor. PSII has adapted to lower energy in two distinct ways, each appropriate for its specific environment but with different functional penalties.
Allgower F, Gamiz-Hernandez AP, Rutherford AW, et al., 2022, Molecular Mechanisms of Redox-Coupled Protonation Dynamics in Photosystem II, 21st European Bioenergetics Conference (EBEC), Publisher: ELSEVIER, Pages: 83-83, ISSN: 0005-2728
Viola S, Roseby W, Santabarabara S, et al., 2022, Abstract The energetics of the two types of far-red Photosystem II, 21st European Bioenergetics Conference (EBEC), Publisher: ELSEVIER, Pages: 81-81, ISSN: 0005-2728
Kim TD, Oliver T, Trinugroho JP, et al., 2022, Evolution and evolvability of photosystem II, 21st European Bioenergetics Conference (EBEC), Publisher: ELSEVIER, Pages: 80-80, ISSN: 0005-2728
Allgower F, Gamiz-Hernandez AP, Rutherford AW, et al., 2022, Molecular Principles of Redox-Coupled Protonation Dynamics in Photosystem II, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 144, Pages: 7171-7180, ISSN: 0002-7863
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- Citations: 24
Viola S, Roseby W, Santabarabara S, et al., 2022, Impact of energy limitations on function and resilience in long-wavelength Photosystem II
<jats:title>Abstract</jats:title><jats:p>Photosystem II (PSII) uses the energy from red light to split water and reduce quinone, an energy-demanding process based on chlorophyll a (Chl-a) photochemistry. Two kinds of cyanobacterial PSII can use Chl-d and Chl-f to perform the same reactions using lower energy, far-red light. PSII from <jats:italic>Acaryochloris marina</jats:italic> has Chl-d replacing all but one of its 35 Chl-a, while PSII from <jats:italic>Chroococcidiopsis thermalis</jats:italic>, a facultative far-red species, has just 4 Chl-f and 1 Chl-d and 30 Chl-a. From bioenergetic considerations, the far-red PSII were predicted to lose photochemical efficiency and/or resilience to photodamage. Here, we compare enzyme turnover efficiency, forward electron transfer, back-reactions and photodamage in Chl-f-PSII, Chl-d-PSII and Chl-a-PSII. We show that: i) all types of PSII have a comparable efficiency in enzyme turnover; ii) the modified energy gaps on the acceptor side of Chl-d-PSII favor recombination via P<jats:sub>D1</jats:sub><jats:sup>+</jats:sup>Phe<jats:sup>-</jats:sup> repopulation, leading to increased singlet oxygen production and greater sensitivity to high-light damage compared to Chl-a-PSII and Chl-f-PSII; ii) the acceptor-side energy gaps in Chl-f-PSII are tuned to avoid harmful back reactions, favoring resilience to photodamage over efficiency of light usage. The results are explained by the differences in the redox tuning of the electron transfer cofactors Phe and Q<jats:sub>A</jats:sub> and in the number and layout of the chlorophylls that share the excitation energy with the primary electron donor. PSII has adapted to lower energy in two distinct ways, each appropriate for its specific environment but with different functional penalties.</jats:p>
Fantuzzi A, Allgower F, Baker H, et al., 2022, Bicarbonate-controlled reduction of oxygen by the QA semiquinone in Photosystem II in membranes, Proceedings of the National Academy of Sciences of USA, Vol: 119, ISSN: 0027-8424
Photosystem II (PSII), the water/plastoquinone photo-oxidoreductase, plays a key energy input role in the biosphere. Q∙−A, the reduced semiquinone form of the nonexchangeable quinone, is often considered capable of a side reaction with O2, forming superoxide, but this reaction has not yet been demonstrated experimentally. Here, using chlorophyll fluorescence in plant PSII membranes, we show that O2 does oxidize Q∙−A at physiological O2 concentrations with a t1/2 of 10 s. Superoxide is formed stoichiometrically, and the reaction kinetics are controlled by the accessibility of O2 to a binding site near Q∙−A, with an apparent dissociation constant of 70 ± 20 µM. Unexpectedly, Q∙−A could only reduce O2 when bicarbonate was absent from its binding site on the nonheme iron (Fe2+) and the addition of bicarbonate or formate blocked the O2-dependant decay of Q∙−A. These results, together with molecular dynamics simulations and hybrid quantum mechanics/molecular mechanics calculations, indicate that electron transfer from Q∙−A to O2 occurs when the O2 is bound to the empty bicarbonate site on Fe2+. A protective role for bicarbonate in PSII was recently reported, involving long-lived Q∙−A triggering bicarbonate dissociation from Fe2+ [Brinkert et al., Proc. Natl. Acad. Sci. U.S.A. 113, 12144–12149 (2016)]. The present findings extend this mechanism by showing that bicarbonate release allows O2 to bind to Fe2+ and to oxidize Q∙−A. This could be beneficial by oxidizing Q∙−A and by producing superoxide, a chemical signal for the overreduced state of the electron transfer chain.
MacGregor-Chatwin C, Nurnberg DJ, Jackson PJ, et al., 2022, Changes in supramolecular organization of cyanobacterial thylakoid membrane complexes in response to far-red light photoacclimation, SCIENCE ADVANCES, Vol: 8, ISSN: 2375-2548
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- Citations: 3
Oliver T, Sanchez-Baracaldo P, Larkum AW, et al., 2021, Time-resolved comparative molecular evolution of oxygenic photosynthesis, BBA: Bioenergetics, Vol: 1862, Pages: 1-20, ISSN: 0005-2728
Oxygenic photosynthesis starts with the oxidation of water to O2, a light-driven reaction catalysed by photosystem II. Cyanobacteria are the only prokaryotes capable of water oxidation and therefore, it is assumed that the origin of oxygenic photosynthesis is a late innovation relative to the origin of life and bioenergetics. However, when exactly water oxidation originated remains an unanswered question. Here we use phylogenetic analysis to study a gene duplication event that is unique to photosystem II: the duplication that led to the evolution of the core antenna subunits CP43 and CP47. We compare the changes in the rates of evolution of this duplication with those of some of the oldest well-described events in the history of life: namely, the duplication leading to the Alpha and Beta subunits of the catalytic head of ATP synthase, and the divergence of archaeal and bacterial RNA polymerases and ribosomes. We also compare it with more recent events such as the duplication of Cyanobacteria-specific FtsH metalloprotease subunits and the radiation leading to Margulisbacteria, Sericytochromatia, Vampirovibrionia, and other clades containing anoxygenic phototrophs. We demonstrate that the ancestral core duplication of photosystem II exhibits patterns in the rates of protein evolution through geological time that are nearly identical to those of the ATP synthase, RNA polymerase, or the ribosome. Furthermore, we use ancestral sequence reconstruction in combination with comparative structural biology of photosystem subunits, to provide additional evidence supporting the premise that water oxidation had originated before the ancestral core duplications. Our work suggests that photosynthetic water oxidation originated closer to the origin of life and bioenergetics than can be documented based on phylogenetic or phylogenomic species trees alone.
Sjodin M, Hjelm J, Rutherford AW, et al., 2020, Reprint of Proton-coupled electron transfer from an interfacial phenol monolayer (Reprinted from Journal of Electroanalytical Chemistry, vol 859, 113856, 2020), JOURNAL OF ELECTROANALYTICAL CHEMISTRY, Vol: 875, ISSN: 1572-6657
Murray JW, Rutherford AW, Nixon PJ, 2020, Photosystem II in a state of disassembly, Joule, Vol: 4, Pages: 2082-2084, ISSN: 2542-4351
The light-driven oxidation of water to oxygen characteristic of oxygenic photosynthesis is catalyzed by a redox-active manganese/calcium cluster embedded in the Photosystem II (PSII) complex. How the cluster is assembled during the biogenesis and repair of PSII is unclear. Cryo-electron microscopy data have now provided new insights into the structure of a PSII complex lacking the cluster and have identified features that might be important for delivery and stabilization of Mn during assembly.
Judd M, Morton J, Nurnberg D, et al., 2020, The primary donor of far-red photosystem II: Chl<sub>D1</sub> or P<sub>D2</sub>?, BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS, Vol: 1861, ISSN: 0005-2728
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- Citations: 7
Babacan O, De Causmaecker S, Gambhir A, et al., 2020, Assessing the feasibility of carbon dioxide mitigation options in terms of energy usage, Nature Energy, Vol: 5, Pages: 720-728, ISSN: 2058-7546
Measures to mitigate the emissions of carbon dioxide (CO2) can vary substantially in terms of the energy required. Some proposed CO2 mitigation options involve energy-intensive processes that compromise their viability as routes to mitigation, especially if deployed at a global scale. Here we provide an assessment of different mitigation options in terms of their energy usage. We assess the relative effectiveness of several CO2 mitigation routes by calculating the energy cost of carbon abatement (kilowatt-hour spent per kilogram CO2-equivalent, or kWh kgCO2e–1) mitigated. We consider energy efficiency measures, decarbonizing electricity, heat, chemicals and fuels, and also capturing CO2 from air. Among the routes considered, switching to renewable energy technologies (0.05–0.53 kWh kgCO2e–1 mitigated) offer more energy-effective mitigation than carbon embedding or carbon removal approaches, which are more energy intensive (0.99–10.03 kWh kgCO2e–1 and 0.78–2.93 kWh kgCO2e–1 mitigated, respectively), whereas energy efficiency measures, such as improving building lighting, can offer the most energy-effective mitigation.
Zamzam N, Rakowski R, Kaucikas M, et al., 2020, Femtosecond visible transient absorption spectroscopy of chlorophyll f- containing Photosystem II, Proceedings of the National Academy of Sciences of USA, Vol: 117, Pages: 1-7, ISSN: 0027-8424
The recently discovered, chlorophyll-f-containing, far-red photosystem II (FR-PSII) supports far-red light photosynthesis. Participation and kinetics of spectrally shifted far-red pigments are directly observable and separated from that of bulk chlorophyll-a. We present an ultrafast transient absorption study of FR-PSII, investigating energy transfer and charge separation processes. Results show a rapid subpicosecond energy transfer from chlorophyll-a to the long-wavelength chlorophylls-f/d. The data demonstrate the decay of an ∼720-nm negative feature on the picosecond-to-nanosecond timescales, coinciding with charge separation, secondary electron transfer, and stimulated emission decay. An ∼675-nm bleach attributed to the loss of chl-a absorption due to the formation of a cation radical, PD1+•, is only fully developed in the nanosecond spectra, indicating an unusually delayed formation. A major spectral feature on the nanosecond timescale at 725 nm is attributed to an electrochromic blue shift of a FR-chlorophyll among the reaction center pigments. These time-resolved observations provide direct experimental support for the model of Nürnberg et al. [D. J. Nürnberg et al., Science 360, 1210–1213 (2018)], in which the primary electron donor is a FR-chlorophyll and the secondary donor is chlorophyll-a (PD1 of the central chlorophyll pair). Efficient charge separation also occurs using selective excitation of long-wavelength chlorophylls-f/d, and the localization of the excited state on P720* points to a smaller (entropic) energy loss compared to conventional PSII, where the excited state is shared over all of the chlorin pigments. This has important repercussions on understanding the overall energetics of excitation energy transfer and charge separation reactions in FR-PSII.
Shevela D, Do H-N, Fantuzzi A, et al., 2020, Bicarbonate-Mediated CO<sub>2</sub> Formation on Both Sides of Photosystem II., Biochemistry, Vol: 59, Pages: 2442-2449, ISSN: 0006-2960
The effect of bicarbonate (HCO<sub>3</sub><sup>-</sup>) on photosystem II (PSII) activity was discovered in the 1950s, but only recently have its molecular mechanisms begun to be clarified. Two chemical mechanisms have been proposed. One is for the electron-donor side, in which mobile HCO<sub>3</sub><sup>-</sup> enhances and possibly regulates water oxidation by acting as proton acceptor, after which it dissociates into CO<sub>2</sub> and H<sub>2</sub>O. The other is for the electron-acceptor side, in which (i) reduction of the Q<sub>A</sub> quinone leads to the release of HCO<sub>3</sub><sup>-</sup> from its binding site on the non-heme iron and (ii) the <i>E</i><sub>m</sub> potential of the Q<sub>A</sub>/Q<sub>A</sub><sup>•-</sup> couple increases when HCO<sub>3</sub><sup>-</sup> dissociates. This suggested a protective/regulatory role of HCO<sub>3</sub><sup>-</sup> as it is known that increasing the <i>E</i><sub>m</sub> of Q<sub>A</sub> decreases the extent of back-reaction-associated photodamage. Here we demonstrate, using plant thylakoids, that time-resolved membrane-inlet mass spectrometry together with <sup>13</sup>C isotope labeling of HCO<sub>3</sub><sup>-</sup> allows donor- and acceptor-side formation of CO<sub>2</sub> by PSII to be demonstrated and distinguished, which opens the door for future studies of the importance of both mechanisms under <i>in vivo</i> conditions.
Oliver T, Sánchez-Baracaldo P, Larkum AW, et al., 2020, Time-resolved comparative molecular evolution of oxygenic photosynthesis
<jats:title>Abstract</jats:title><jats:p>Oxygenic photosynthesis starts with the oxidation of water to O<jats:sub>2</jats:sub>, a light-driven reaction catalysed by photosystem II. Cyanobacteria are the only prokaryotes capable of water oxidation and therefore, it is assumed that relative to the origin of life and bioenergetics, the origin of oxygenic photosynthesis is a late innovation. However, when exactly water oxidation originated remains an unanswered question. Here we use relaxed molecular clocks to compare one of the two ancestral core duplications that are unique to water-oxidizing photosystem II, that leading to CP43 and CP47, with some of the oldest well-described events in the history of life. Namely, the duplication leading to the Alpha and Beta subunits of the catalytic head of ATP synthase, and the divergence of archaeal and bacterial RNA polymerases and ribosomes. We also compare it with more recent events such as the duplication of cyanobacteria-specific FtsH metalloprotease subunits, of CP43 variants used in a variety of photoacclimation responses, and the speciation events leading to Margulisbacteria, Sericytochromatia, Vampirovibrionia, and other clades containing anoxygenic phototrophs. We demonstrate that the ancestral core duplication of photosystem II exhibits patterns in the rates of protein evolution through geological time that are nearly identical to those of the ATP synthase, RNA polymerase, or the ribosome. Furthermore, we use ancestral sequence reconstruction in combination with comparative structural biology of photosystem subunits, to provide additional evidence supporting the premise that water oxidation had originated before the ancestral core duplications. Our work suggests that photosynthetic water oxidation originated closer to the origin of life and bioenergetics than can be documented based on species trees alone.</jats:p>
Sjodin M, Hjelm J, Rutherford AW, et al., 2020, Proton-coupled electron transfer from an interfacial phenol monolayer, JOURNAL OF ELECTROANALYTICAL CHEMISTRY, Vol: 859, ISSN: 1572-6657
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- Citations: 2
Cardona Londono T, Rutherford AW, 2019, Evolution of photochemical reaction centres: more twists?, Trends in Plant Science, Vol: 24, Pages: 1008-1021, ISSN: 1878-4372
One of the earliest events in the molecular evolution of photosynthesis is the structural and functional specialisation of Type I (ferredoxin-reducing) and Type II (quinone-reducing) reaction centres. In this opinion article we point out that the homodimeric Type I reaction centre of Heliobacteria has a calcium-binding site with striking structural similarities to the Mn4CaO5 cluster of Photosystem II. These similarities indicate that most of the structural elements required to evolve water oxidation chemistry were present in the earliest reaction centres. We suggest that the divergence of Type I and Type II reaction centres was made possible by a drastic structural shift linked to a change in redox properties that coincided with or facilitated the origin of photosynthetic water oxidation.
De Causmaecker S, Douglass J, Fantuzzi A, et al., 2019, Energetics of the exchangeable quinone, QB, in Photosystem II, Proceedings of the National Academy of Sciences of USA, Vol: 116, Pages: 19458-19463, ISSN: 0027-8424
Photosystem II (PSII), the light-driven water/plastoquinone photo-oxidoreductase, is of central importance in the planetary energy cycle. The product of the reaction, plastohydroquinone (PQH2), is released into the membrane from the QB-site, where it is formed. A plastoquinone (PQ) from the membrane pool then binds into the QB-site. Despite their functional importance, the thermodynamic properties of the PQ in the QB-site, QB, in its different redox forms have received relatively little attention. Here we report the midpoint potentials (Em) of QB in PSII from Thermosynechococcus elongatus using EPR spectroscopy: Em QB/QB•−≈ 90 mV and Em QB•−/QBH2≈ 40 mV. These data allow the following conclusions: 1) the semiquinone, QB•−, is stabilized thermodynamically; 2) the resulting Em QB/QBH2 (~ 65 mV) is lower than the EmPQ/PQH2 (~117 mV), and the difference (ΔE ~50 meV) represents the driving force for QBH2 release into the pool; 3) PQ is ~ 50x more tightly bound than PQH2; 4) the difference between the Em QB/QB•− measured here and the Em QA/QA•− from the literature is ~234 meV, in principle corresponding to the driving force for electron transfer from QA•− to QB. The pH-dependence of the thermoluminescence associated with QB•− provided a functional estimate for this energy gap and gave a similar value (≥180 meV). These estimates are larger than the generally accepted value (~70 meV) and this is discussed. The energetics of QB in PSII are comparable to those in the homologous purple bacterial reaction center.
Varghese F, Kabasakal BV, Cotton CA, et al., 2019, A low-potential terminal oxidase associated with the iron-only nitrogenase from the nitrogen-fixing bacterium Azotobacter vinelandii, Journal of Biological Chemistry, Vol: 294, Pages: 9367-9376, ISSN: 0021-9258
The biological route for nitrogen gas entering the biosphere is reduction to ammonia by the nitrogenase enzyme, which is inactivated by oxygen. Three types of nitrogenase exist, the least studied of which is the iron-only nitrogenase. The Anf3 protein in the bacterium Rhodobacter capsulatus is essential for diazotrophic (i.e. nitrogen-fixing) growth with the iron-only nitrogenase, but its enzymatic activity and function are unknown. Here, we biochemically and structurally characterize Anf3 from the model diazotrophic bacterium Azotobacter vinelandii. Determining the Anf3 crystal structure to atomic resolution, we observed that it is a dimeric flavocytochrome with an unusually close interaction between the heme and the flavin adenine dinucleotide cofactors. Measuring the reduction potentials by spectroelectrochemical redox titration, we observed values of -420 ± 10 mV and -330 ± 10 mV for the two FAD potentials and -340 ± 1 mV for the heme. We further show that Anf3 accepts electrons from spinach ferredoxin and that Anf3 consumes oxygen without generating superoxide or hydrogen peroxide. We predict that Anf3 protects the iron-only nitrogenase from oxygen inactivation by functioning as an oxidase in respiratory protection, with flavodoxin or ferredoxin as the physiological electron donors.
Takegawa Y, Nakamura M, Nakamura S, et al., 2019, New insights on ChlD1 function in Photosystem II from site-directed mutants of D1/T179 in Thermosynechococcus elongatus, Biochimica et Biophysica Acta - Bioenergetics, Vol: 1860, Pages: 297-309, ISSN: 0005-2728
The monomeric chlorophyll, ChlD1, which is located between the PD1PD2 chlorophyll pair and the pheophytin, PheoD1, is the longest wavelength chlorophyll in the heart of Photosystem II and is thought to be the primary electron donor. Its central Mg2+ is liganded to a water molecule that is H-bonded to D1/T179. Here, two site-directed mutants, D1/T179H and D1/T179V, were made in the thermophilic cyanobacterium, Thermosynechococcus elongatus, and characterized by a range of biophysical techniques. The Mn4CaO5 cluster in the water-splitting site is fully active in both mutants. Changes in thermoluminescence indicate that i) radiative recombination occurs via the repopulation of *ChlD1 itself; ii) non-radiative charge recombination reactions appeared to be faster in the T179H-PSII; and iii) the properties of PD1PD2 were unaffected by this mutation, and consequently iv) the immediate precursor state of the radiative excited state is the ChlD1+PheoD1- radical pair. Chlorophyll bleaching due to high intensity illumination correlated with the amount of 1O2 generated. Comparison of the bleaching spectra with the electrochromic shifts attributed to ChlD1 upon QA- formation, indicates that in the T179H-PSII and in the WT*3-PSII, the ChlD1 itself is the chlorophyll that is first damaged by 1O2, whereas in the T179V-PSII a more red chlorophyll is damaged, the identity of which is discussed. Thus, ChlD1 appears to be one of the primary damage site in recombination-mediated photoinhibition. Finally, changes in the absorption of ChlD1 very likely contribute to the well-known electrochromic shifts observed at ~430 nm during the S-state cycle.
Cardona Londono T, Sanchez-Baracaldo P, Rutherford AW, et al., 2019, Early Archean origin of Photosystem II, Geobiology, Vol: 17, Pages: 127-150, ISSN: 1472-4669
Photosystem II is a photochemical reaction center that catalyzes the light‐driven oxidation of water to molecular oxygen. Water oxidation is the distinctive photochemical reaction that permitted the evolution of oxygenic photosynthesis and the eventual rise of eukaryotes. At what point during the history of life an ancestral photosystem evolved the capacity to oxidize water still remains unknown. Here, we study the evolution of the core reaction center proteins of Photosystem II using sequence and structural comparisons in combination with Bayesian relaxed molecular clocks. Our results indicate that a homodimeric photosystem with sufficient oxidizing power to split water had already appeared in the early Archean about a billion years before the most recent common ancestor of all described Cyanobacteria capable of oxygenic photosynthesis, and well before the diversification of some of the known groups of anoxygenic photosynthetic bacteria. Based on a structural and functional rationale, we hypothesize that this early Archean photosystem was capable of water oxidation to oxygen and had already evolved protection mechanisms against the formation of reactive oxygen species. This would place primordial forms of oxygenic photosynthesis at a very early stage in the evolutionary history of life.
Zamzam N, Kaucikas M, Nurnberg D, et al., 2019, Femtosecond infrared spectroscopy of chlorophyll f-containing photosystem I, Physical Chemistry Chemical Physics, Vol: 21, Pages: 1224-1234, ISSN: 1463-9076
The recent discovery of extremely red-shifted chlorophyll f pigments in both photosystem I (PSI) and photosystem II has led to the conclusion that chlorophyll f plays a role not only in the energy transfer, but also in the charge separation processes [Nürnberg et al., Science, 2018, 360, 1210–1213]. We have employed ultrafast transient infrared absorption spectroscopy to study the contribution of far-red light absorbing chlorophyll f to energy transfer and charge separation processes in far-red light-grown PSI (FRL-PSI) from the cyanobacterium Chroococcidiopsis thermalis PCC 7203. We compare the kinetics and spectra of FRL-grown PSI excited at 670 nm and 740 nm wavelengths to those of white light-grown PSI (WL-PSI) obtained at 675 nm excitation. We report a fast decay of excited state features of chlorophyll a and complete energy transfer from chlorophyll a to chlorophyll f in FRL-PSI upon 670 nm excitation, as indicated by a frequency shift in a carbonyl absorption band occurring within a 1 ps timescale. While the WL-PSI measurements support the assignment of initial charge separation to A−1+˙A0−˙ [Di Donato et al., Biochemistry, 2011, 50, 480–490] from the kinetics of a distinct cation feature at 1710 cm−1, in the case of FRL-PSI, small features at 1715 cm−1 from the chlorophyll cation are present from sub-ps delays instead, supporting the replacement of the A−1 pigment with chlorophyll f. Comparisons of nanosecond spectra show that charge separation proceeds with 740 nm excitation, which selectively excites chlorophyll f, and modifications in specific carbonyl absorption bands assigned to P700+˙ minus P700 and A1−˙ minus A1 indicate dielectric differences of FRL-PSI compared to WL-PSI in one or both of the two electron transfer branches of FRL-PSI.
Kornienko N, Zhang JZ, Sokol K, et al., 2018, Oxygenic photoreactivity in photosystem II studied by rotating ring disk electrochemistry, Journal of the American Chemical Society, Vol: 140, Pages: 17923-17931, ISSN: 1520-5126
Protein film photoelectrochemistry has previously been used to monitor the activity of Photosystem II, the water-plastoquinone photooxidoreductase, but the mechanistic information attainable from a three-electrode setup has remained limited. Here we introduce the four-electrode rotating ring disk electrode technique for quantifying light-driven reaction kinetics and mechanistic pathways in real time at the enzyme-electrode interface. This setup allows us to study photochemical H2O oxidation in Photosystem II and to gain in-depth understanding of pathways that generate reactive oxygen species. The results show that Photosystem II reacts with O2 through two main pathways that both involve a superoxide intermediate to produce H2O2. The first pathway involves the established chlorophyll triplet-mediated formation of singlet oxygen, which is followed by its reduction to superoxide at the electrode surface. The second pathway is specific for the enzyme/electrode interface: an exposed antenna chlorophyll is sufficiently close to the electrode for rapid injection of an electron to form a highly reducing chlorophyll anion, which reacts with O2 in solution to produce O2•-. Incomplete H2O oxidation does not significantly contribute to reactive oxygen formation in our conditions. The rotating ring disk electrode technique allows the chemical reactivity of Photosystem II to be studied electrochemically and opens several avenues for future investigation.
Cardona T, Rutherford AW, 2018, Evolution of photochemical reaction centres: more twists?
<jats:p>The earliest event recorded in the molecular evolution of photosynthesis is the structural and functional specialisation of Type I (ferredoxin-reducing) and Type II (quinone-reducing) reaction centres. Here we point out that the homodimeric Type I reaction centre of Heliobacteria has a Ca2+-binding site with a number of striking parallels to the Mn4CaO5 cluster of cyanobacterial Photosystem II. This structural parallels indicate that water oxidation chemistry originated at the divergence of Type I and Type II reaction centres. We suggests that this divergence was triggered by a structural rearrangement of a core transmembrane helix resulting in a shift of the redox potential of the electron donor side and electron acceptor side at the same time and in the same redox direction.</jats:p>
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