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In photosystem II (PSII), the Mn 4 CaO 5 cluster catalyses the water splitting reaction. The crystal structure of PSII shows the presence of a hydrogen-bonded water molecule directly linked to O4. Here we show the detailed properties of the H-bonds associated with the Mn 4 CaO 5 cluster using a quantum mechanical/molecular mechanical approach. When O4 is taken as a μ-hydroxo bridge acting as a hydrogen-bond donor to water539 (W539), the S 0 redox state best describes the unusually short O4–O W539 distance (2.5 Å) seen in the crystal structure. We find that in S 1 , O4 easily releases the proton into a chain of eight strongly hydrogen-bonded water molecules. The corresponding hydrogen-bond network is absent for O5 in S 1 . The present study suggests that the O4-water chain could facilitate the initial deprotonation event in PSII. This unexpected insight is likely to be of real relevance to mechanistic models for water oxidation. The availability of crystal structures of photosystem II opens up the possibility of gaining insights into its mechanism. Here, the authors use a computational approach and propose a deprotonation event at O4 followed by long-range proton-transfer along a chain of strongly bonded water molecules.

Plants and cyanobacteria use chlorophyll-rich photosystem complexes to convert light energy into chemical energy. Some organisms have developed adaptations to take advantage of longer-wavelength photons. Nürnberg et al. studied photosystem complexes from cyanobacteria grown in the presence of far-red light. The authors identified the primary donor chlorophyll as one of a few chlorophyll molecules in the far-red light–adapted enzymes that were chemically altered to shift their absorption spectrum. Kinetic measurements demonstrated that far-red light is capable of directly driving water oxidation, despite having less energy than the red light used by most photosynthetic organisms. Science , this issue p. 1210 A chlorophyll variant with far-red absorption is involved in photosynthesis in cyanobacteria adapted to far red light. Photosystems I and II convert solar energy into the chemical energy that powers life. Chlorophyll a photochemistry, using red light (680 to 700 nm), is near universal and is considered to define the energy “red limit” of oxygenic photosynthesis. We present biophysical studies on the photosystems from a cyanobacterium grown in far-red light (750 nm). The few long-wavelength chlorophylls present are well resolved from each other and from the majority pigment, chlorophyll a. Charge separation in photosystem I and II uses chlorophyll f at 745 nm and chlorophyll f (or d) at 727 nm, respectively. Each photosystem has a few even longer-wavelength chlorophylls f that collect light and pass excitation energy uphill to the photochemically active pigments. These photosystems function beyond the red limit using far-red pigments in only a few key positions.

The midpoint potential (Em ) of Q A /Q A -• , the one-electron acceptor quinone of Photosystem II (PSII), provides the thermodynamic reference for calibrating PSII bioenergetics. Uncertainty exists in the literature, with two values differing by ∼80 mV. Here, we have resolved this discrepancy by using spectroelectrochemistry on plant PSII-enriched membranes. Removal of bicarbonate (HCO₃⁻) shifts the Em from ∼−145 mV to −70 mV. The higher values reported earlier are attributed to the loss of HCO₃⁻ during the titrations (pH 6.5, stirred under argon gassing). These findings mean that HCO₃⁻ binds less strongly when QA −• is present. Light-induced QA −• formation triggered HCO₃⁻ loss as manifest by the slowed electron transfer and the upshift in the Em of QA. HCO₃⁻-depleted PSII also showed diminished light-induced ¹O₂ formation. This finding is consistent with a model in which the increase in the Em of Q A /Q A -• promotes safe, direct P +• Q A -• charge recombination at the expense of the damaging back-reaction route that involves chlorophyll triplet-mediated ¹O₂ formation [Johnson GN, et al. (1995) Biochim Biophys Acta 1229:202–207]. These findings provide a redox tuning mechanism, in which the interdependence of the redox state of QA and the binding by HCO₃⁻ regulates and protects PSII. The potential for a sink (CO₂) to source (PSII) feedback mechanism is discussed.

Photosystem II, the water oxidizing enzyme, altered the course of evolution by filling the atmosphere with oxygen. Here, we reconstruct the origin and evolution of water oxidation at an unprecedented level of detail by studying the phylogeny of all D1 subunits, the main protein coordinating the water oxidizing cluster (Mn4CaO5) of Photosystem II. We show that D1 exists in several forms making well-defined clades, some of which could have evolved before the origin of water oxidation and presenting many atypical characteristics. The most ancient form is found in the genome of Gloeobacter kilaueensis JS-1 and this has a C-terminus with a higher sequence identity to D2 than to any other D1. Two other groups of early evolving D1 correspond to those expressed under prolonged far-red illumination and in darkness. These atypical D1 forms are characterized by a dramatically different Mn4CaO5 binding site and a Photosystem II containing such a site may assemble an unconventional metal cluster. The first D1 forms with a full set of ligands to the Mn4CaO5 cluster are grouped with D1 proteins expressed only under low oxygen concentrations and the latest evolving form is the dominant type of D1 found in all cyanobacteria and plastids. In addition, we show that the plastid ancestor had a D1 more similar to those in early branching Synechococcus. We suggest each one of these forms of D1 originated from transitional forms at different stages toward the innovation and optimization of water oxidation before the last common ancestor of all known cyanobacteria.

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 electron paramagnetic resonance (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 Em PQ/PQH₂ (∼117 mV), and the difference (ΔE ≈ 50 meV) represents the driving force for QBH₂ release into the pool; 3) PQ is ∼50× more tightly bound than PQH₂; and 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.

Photosystem II uses light to drive water oxidation and plastoquinone (PQ) reduction. PQ reduction involves two PQ cofactors, QA and QB, working in series. QA is a one-electron carrier, whereas QB undergoes sequential reduction and protonation to form QBH₂. QBH₂ exchanges with PQ from the pool in the membrane. Based on the atomic coordinates of the Photosystem II crystal structure, we analyzed the proton transfer (PT) energetics adopting a quantum mechanical/molecular mechanical approach. The potential-energy profile suggests that the initial PT to QB•- occurs from the protonated, D1-His252 to QB•- via D1-Ser264. The second PT is likely to occur from D1-His215 to QB•- via an H-bond with an energy profile with a single well, resulting in the formation of QBH₂ and the D1-His215 anion. The pathway for reprotonation of D1-His215⁻ may involve bicarbonate, D1-Tyr246 and water in the QB site. Formate ligation to Fe²⁺ did not significantly affect the protonation of reduced QB, suggesting that formate inhibits QBH₂ release rather than its formation. The presence of carbonate rather than bicarbonate seems unlikely because the calculations showed that this greatly perturbed the potential of the nonheme iron, stabilizing the Fe³⁺ state in the presence of QB•- a situation not encountered experimentally. H-bonding from D1-Tyr246 and D2-Tyr244 to the bicarbonate ligand of the nonheme iron contributes to the stability of the semiquinones. A detailed mechanistic model for QB reduction is presented.

There is considerable interest in improving plant productivity by altering the dynamic responses of photosynthesis in tune with natural conditions. This is exemplified by the 'energy-dependent' form of non-photochemical quenching (qE), the formation and decay of which can be considerably slower than natural light fluctuations, limiting photochemical yield. In addition, we recently reported that rapidly fluctuating light can produce field recombinationinduced photodamage (FRIP), where large spikes in electric field across the thylakoid membrane (Δψ) induce photosystem II recombination reactions that produce damaging singlet oxygen (¹O₂). Both qE and FRIP are directly linked to the thylakoid proton motive force (pmf), and in particular, the slow kinetics of partitioning pmf into its ΔpH and Δψ components. Using a series of computational simulations, we explored the possibility of 'hacking' pmf partitioning as a target for improving photosynthesis. Under a range of illumination conditions, increasing the rate of counter-ion fluxes across the thylakoid membrane should lead to more rapid dissipation of Δψ and formation of ΔpH. This would result in increased rates for the formation and decay of qE while resulting in a more rapid decline in the amplitudes of Δψ-spikes and decreasing ¹O₂ production. These results suggest that ion fluxes may be a viable target for plant breeding or engineering. However, these changes also induce transient, but substantial mismatches in the ATP: NADPH output ratio as well as in the osmotic balance between the lumen and stroma, either of which may explain why evolution has not already accelerated thylakoid ion fluxes. Overall, though the model is simplified, it recapitulates many of the responses seen in vivo, while spotlighting critical aspects of the complex interactions between pmf components and photosynthetic processes. By making the programme available, we hope to enable the community of photosynthesis researchers to further explore and test specific hypotheses. This article is part of the themed issue 'Enhancing photosynthesis in crop plants: targets for improvement'.

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 P D1 + 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 Q A 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. Algae, plants and cyanobacteria perform a process called photosynthesis, in which carbon dioxide and water are converted into oxygen and energy-rich carbon compounds. The first step of this process involves an enzyme called photosystem II, which uses light energy to extract electrons from water to help capture the carbon dioxide. If the photosystem absorbs too much light, compounds known as reactive oxygen species are produced in quantities that damage the photosystem and kill the cell. To ensure that the photosystem works efficiently and to protect it from damage, about half of the energy from the absorbed light is dissipated as heat, while the rest of the energy is stored in the products of photosynthesis. The standard form of photosystem II uses the energy of visible light, but some cyanobacteria contain different types of photosystem II, which do the same chemical reactions using lower energy far-red light. One type of far-red photosystem II is found in Acaryochloris marina , a cyanobacterium living in stable levels of far-red light, shaded from visible light. The other type is found in a cyanobacterium called Chroococcidiopsis thermalis, which can switch between using its far-red photosystem II when shaded from visible light and using its standard photosystem II when exposed to it. Being able to work with less energy, the two types of far-red photosystem II appear to be more efficient than the standard one, but it has been unclear if there were any downsides to this trait. Viola et al. compared the standard photosystem II with the far-red photosystem II types from C. thermalis and A. marina by measuring the efficiency of these enzymes, the quantity of reactive oxygen species produced, and the resulting light-induced damage. The experiments revealed that the far-red photosystem II of A. marina is highly efficient but produces elevated levels of reactive oxygen species if exposed to high light conditions. On the other hand, the far-red photosystem II of C. thermalis is less efficient in collecting and using far-red light, but is more robust, producing fewer reactive oxygen species. Despite these tradeoffs, engineering crop plants or algae that could use far-red photosynthesis may help boost food and biomass production. A better understanding of the trade-offs between efficiency and resilience in the two types of far-red photosystem II could determine which features would be beneficial, and under what conditions. This work also improves our knowledge of how the standard photosystem II balances light absorption and damage limitation to work efficiently in a variable environment.

The thylakoid proton motive force (pmf) generated during photosynthesis is the essential driving force for ATP production; it is also a central regulator of light capture and electron transfer. We investigated the effects of elevated pmf on photosynthesis in a library of Arabidopsis thaliana mutants with altered rates of thylakoid lumen proton efflux, leading to a range of steady-state pmf extents. We observed the expected pmf-dependent alterations in photosynthetic regulation, but also strong effects on the rate of photosystem II (PSII) photodamage. Detailed analyses indicate this effect is related to an elevated electric field (Δψ) component of the pmf, rather than lumen acidification, which in vivo increased PSII charge recombination rates, producing singlet oxygen and subsequent photodamage. The effects are seen even in wild type plants, especially under fluctuating illumination, suggesting that Δψ-induced photodamage represents a previously unrecognized limiting factor for plant productivity under dynamic environmental conditions seen in the field.

Population declines of the monarch butterfly (Danaus plexippus plexippus) in North America have largely been attributed to the distribution and condition of species‐specific preferred nectar sources. In 2020, the US Fish and Wildlife Service (USFWS) listed the monarch butterfly in the US Federal Register as a candidate species under the Endangered Species Act of 1973. The USFWS ranked the availability, quality, and spatial distribution of nectar plants during autumn migration as the fourth most contributing factor to US monarch population declines. During the autumn migration through the Great Plains, monarchs seek nectar plants to accumulate lipid reserves for further migration to and overwintering in Mexico. We applied vegetation and rangeland health data from the US Department of Agriculture, Natural Resources Conservation Service, National Resources Inventory (NRI) to quantify species density and richness of monarch‐preferred nectar plants, associated rangeland conditions, and diversity of nectar sources along this autumn migration pathway. We focused specifically on longitudinal gradients W‐095‐100 and W‐100‐105 and discrete 5° latitudinal–longitudinal cells within those gradients. The respective NRI dataset spans 8211 rangeland sites sampled between 2009 and 2018. Approximately 84.4% of sites in W‐095‐100 and 72.5% of sites in W‐100‐105 contained monarch‐preferred nectar plants. Preferred nectar plants made up 7.4% of 2438 identified plant species in W‐095‐100 and 6.1% of 2371 identified plant species in W‐100‐105. For W‐095‐100, preferred nectar plant densities were highest for the 5° cell covering portions of US states Oklahoma and Kansas and lowest for the 5° cell at the US–Mexico border. In W‐100‐105, preferred nectar plant densities decreased linearly from north to south. Preferred nectar plant densities were greater for 5° cells in W‐100‐105 (50.5 billion plants) as compared with W‐095‐100 (44.4 billion plants). Consistent with trends in preferred nectar source density, rangeland conditions assessed by similarity indices and rangeland health protocols were generally lowest for 5° cells spanning the US–Mexico border. The results provide the most comprehensive assessment to date for preferred nectar sources of the monarch butterfly along the Great Plains autumn migration to Mexico and document generally decreasing nectar sources and habitat conditions at southern latitudes in this ecologically important pathway.