Showing papers on "Photosynthetic reaction centre published in 2018"

Effects of drought stress on photosynthesis and photosynthetic electron transport chain in young apple tree leaves.

Abstract: In our study, the effects of water stress on photosynthesis and photosynthetic electron transport chain (PETC) were studied in several ways, including monitoring the change of gas exchange parameters, modulated chlorophyll fluorescence, rapid fluorescence induction kinetics, reactive oxygen species (ROS), antioxidant enzyme activities and D1 protein levels in apple leaves. Our results show that when leaf water potential ( ψ w ) is above –1.5 MPa, the stomatal limitation should be the main reason for a drop of photosynthesis. In this period, photosynthetic rate ( P N ), stomatal conductance ( G s ), transpiration rate ( E ) and intercellular CO 2 concentration ( C i ) all showed a strong positive correlation with ψ w . Modulated chlorophyll fluorescence parameters related to photosynthetic biochemistry activity including maximum photochemical efficiency (F v /F m ), actual photochemical efficiency of PSII (Φ PSII ), photochemical quenching coefficient ( q P ) and coefficient of photochemical fluorescence quenching assuming interconnected PSII antennae ( q L ) also showed a strong positive correlation as ψ w gradually decreased. On the other hand, in this period, Stern-Volmer type non-photochemical quenching coefficient (NPQ) and quantum yield of light-induced non-photochemical fluorescence quenching [ Y (NPQ) ] kept going up, which shows an attempt to dissipate excess energy to avoid damage to plants. When ψ w was below –1.5 MPa, P N continued to decrease linearly, while C i increased and a ‘V’ model presents the correlation between C i and ψ w by polynomial regression. This implies that, in this period, the drop in photosynthesis activity might be caused by non-stomatal limitation. F v /F m , Φ PSII , q P and q L in apple leaves treated with water stress were much lower than in control, while NPQ and Y (NPQ) started to go down. This demonstrates that excess energy might exceed the tolerance ability of apple leaves. Consistent with changes of these parameters, excess energy led to an increase in the production of ROS including H 2 O 2 and O 2 • − . Although the activities of antioxidant enzymes like catalase (CAT), superoxide dismutase (SOD) and peroxidase (POD) increased dramatically and ascorbate peroxidase (APX) decreased in apple leaves with drought stress, it was still not sufficient to scavenge ROS. Consequently, the accumulation of ROS triggered a reduction of net D1 protein content, a core protein in the PSII reaction center. As D1 is responsible for the photosynthetic electron transport from plastoquinone A (Q A ) to plastoquinone B (Q B ), the capacity of PETC between Q A and Q B was considerably downregulated. The decline of photosynthesis and activity of PETC may result in the shortage of adenosine triphosphate (ATP) and limitation the regeneration of RuBP ( J max ), a key enzyme in CO 2 assimilation. These are all non-stomatal factors and together contributed to decreased CO 2 assimilation under severe water stress.

A physiological perspective on the origin and evolution of photosynthesis

Abstract: The origin and early evolution of photosynthesis are reviewed from an ecophysiological perspective. Earth's first ecosystems were chemotrophic, fueled by geological H2 at hydrothermal vents and, required flavin-based electron bifurcation to reduce ferredoxin for CO2 fixation. Chlorophyll-based phototrophy (chlorophototrophy) allowed autotrophs to generate reduced ferredoxin without electron bifurcation, providing them access to reductants other than H2. Because high-intensity, short-wavelength electromagnetic radiation at Earth's surface would have been damaging for the first chlorophyll (Chl)-containing cells, photosynthesis probably arose at hydrothermal vents under low-intensity, long-wavelength geothermal light. The first photochemically active pigments were possibly Zn-tetrapyrroles. We suggest that (i) after the evolution of red-absorbing Chl-like pigments, the first light-driven electron transport chains reduced ferredoxin via a type-1 reaction center (RC) progenitor with electrons from H2S; (ii) photothioautotrophy, first with one RC and then with two, was the bridge between H2-dependent chemolithoautotrophy and water-splitting photosynthesis; (iii) photothiotrophy sustained primary production in the photic zone of Archean oceans; (iv) photosynthesis arose in an anoxygenic cyanobacterial progenitor; (v) Chl a is the ancestral Chl; and (vi), anoxygenic chlorophototrophic lineages characterized so far acquired, by horizontal gene transfer, RCs and Chl biosynthesis with or without autotrophy, from the architects of chlorophototrophy-the cyanobacterial lineage.

COMT1 Silencing Aggravates Heat Stress-Induced Reduction in Photosynthesis by Decreasing Chlorophyll Content, Photosystem II Activity, and Electron Transport Efficiency in Tomato.

Abstract: Despite a range of initiatives to reduce global carbon emission, the mean global temperature is increasing due to climate change. Since rising temperatures pose a serious threat of food insecurity, it is important to further explore important biological molecules that can confer thermotolerance to plants. Recently, melatonin has emerged as a universal abiotic stress regulator that can enhance plant tolerance to high temperature. Nonetheless, such regulatory roles of melatonin were unraveled mainly by assessing the effect of exogenous melatonin on plant tolerance to abiotic stress. Here, we generated melatonin deficient tomato plants by silencing of a melatonin biosynthetic gene, CAFFEIC ACID O-METHYLTRANSFERASE 1 (COMT1), to unveil the role of endogenous melatonin in photosynthesis under heat stress. We examined photosynthetic pigment content, leaf gas exchange, and a range of chlorophyll fluorescence parameters. The results showed that silencing of COMT1 aggravated heat stress by inhibiting both the light reactions and the carbon fixation reactions of photosynthesis. The photosynthetic pigment content, light absorption flux, trapped energy flux, energy dissipation, density of active reaction center per photosystem II (PSII) cross-section, the photosynthetic electron transport rate, the maximum photochemical efficiency of PSII photochemistry, and the rate of CO2 assimilation all decreased in COMT1-silenced plants compared with that of non-silenced plants particularly under heat stress. However, exogenous melatonin alleviated heat-induced photosynthetic inhibition in both genotypes, indicating that melatonin is essential for maintaining photosynthetic capacity under stressful conditions. These findings provide genetic evidence on the vital role of melatonin in photosynthesis and thus may have useful implication in horticultural crop management in the face of climate change.

Oxidation of P700 Ensures Robust Photosynthesis.

Abstract: In the light, photosynthetic cells can potentially suffer from oxidative damage derived from reactive oxygen species. Nevertheless, a variety of oxygenic photoautotrophs, including cyanobacteria, algae, and plants, manage their photosynthetic systems successfully. In the present article, we review previous research on how these photoautotrophs safely utilize light energy for photosynthesis without photo-oxidative damage to photosystem I (PSI). The reaction center chlorophyll of PSI, P700, is kept in an oxidized state in response to excess light, under high light and low CO2 conditions, to tune the light utilization and dissipate the excess photo-excitation energy in PSI. Oxidation of P700 is co-operatively regulated by a number of molecular mechanisms on both the electron donor and acceptor sides of PSI. The strategies to keep P700 oxidized are diverse among a variety of photoautotrophs, which are evolutionarily optimized for their ecological niche.

Primary processes in the bacterial reaction center probed by two-dimensional electronic spectroscopy.

Abstract: In the initial steps of photosynthesis, reaction centers convert solar energy to stable charge-separated states with near-unity quantum efficiency. The reaction center from purple bacteria remains an important model system for probing the structure-function relationship and understanding mechanisms of photosynthetic charge separation. Here we perform 2D electronic spectroscopy (2DES) on bacterial reaction centers (BRCs) from two mutants of the purple bacterium Rhodobacter capsulatus, spanning the Q y absorption bands of the BRC. We analyze the 2DES data using a multiexcitation global-fitting approach that employs a common set of basis spectra for all excitation frequencies, incorporating inputs from the linear absorption spectrum and the BRC structure. We extract the exciton energies, resolving the previously hidden upper exciton state of the special pair. We show that the time-dependent 2DES data are well-represented by a two-step sequential reaction scheme in which charge separation proceeds from the excited state of the special pair (P*) to P+HA- via the intermediate P+BA- When inhomogeneous broadening and Stark shifts of the B* band are taken into account we can adequately describe the 2DES data without the need to introduce a second charge-separation pathway originating from the excited state of the monomeric bacteriochlorophyll BA*.

Chloroplast Acetyltransferase NSI Is Required for State Transitions in Arabidopsis thaliana

Abstract: The amount of light energy received by the photosynthetic reaction centers photosystem II (PSII) and photosystem I (PSI) is balanced through state transitions. Reversible phosphorylation of a light-harvesting antenna trimer (L-LHCII) orchestrates the association between L-LHCII and the photosystems, thus adjusting the amount of excitation energy received by the reaction centers. In this study, we identified the enzyme NUCLEAR SHUTTLE INTERACTING (NSI; AT1G32070) as an active lysine acetyltransferase in the chloroplasts of Arabidopsis thaliana. Intriguingly, nsi knockout mutant plants were defective in state transitions, even though they had a similar LHCII phosphorylation pattern as the wild type. Accordingly, nsi plants were not able to accumulate the PSI-LHCII state transition complex, even though the LHCII docking site of PSI and the overall amounts of photosynthetic protein complexes remained unchanged. Instead, the nsi mutants showed a decreased Lys acetylation status of specific photosynthetic proteins including PSI, PSII, and LHCII subunits. Our work demonstrates that the chloroplast acetyltransferase NSI is needed for the dynamic reorganization of thylakoid protein complexes during photosynthetic state transitions.

The mechanism of photosystem‐II inactivation during sulphur deprivation‐induced H2 production in Chlamydomonas reinhardtii

Abstract: Sulphur limitation may restrain cell growth and viability. In the green alga, Chlamydomonas reinhardtii, sulphur limitation may induce H2 production lasting for several days, to be exploited as a renewable energy source. Sulphur limitation causes a large number of physiological changes, including the inactivation of photosystem II (PSII), leading to the establishment of hypoxia, essential for the increase in hydrogenase expression and activity. The inactivation of PSII has been long assumed to be caused by the sulphur-limited turnover of its reaction center protein, PsbA. Here we reinvestigated this issue in detail and show that i) upon transferring Chlamydomonas cells to sulphur-free media, the amount of cellular sulphur content decreases only by about 25%, ii) as demonstrated by lincomycin treatments, PsbA has a significant turnover and other photosynthetic subunits, namely RbcL and CP43, are degraded more rapidly than PsbA. On the other hand, sulphur limitation imposes oxidative stress early on, most probably involving the formation of singlet oxygen in PSII, which leads to an increase in the expression of GDP-L-galactose phosphorylase, playing an essential role in ascorbate biosynthesis. When accumulated to the millimolar concentration range, ascorbate may inactivate the oxygen-evolving complex and provide electrons to PSII albeit at a low rate. In the absence of a functional donor side and sufficient electron transport, PSII reaction centers get inactivated and degraded. We therefore demonstrate that the inactivation of PSII is a complex and multistep process, which may serve to mitigate the damaging effects of sulphur limitation. This article is protected by copyright. All rights reserved.

Structure of the plant photosystem I

Abstract: Plant photosystem I (PSI) is one of the most intricate membrane complexes in nature. It comprises two complexes, a reaction center and light-harvesting complex (LHC), which together form the PSI-LHC supercomplex. The crystal structure of plant PSI was solved with two distinct crystal forms. The first, crystallized at pH 6.5, exhibited P21 symmetry; the second, crystallized at pH 8.5, exhibited P212121 symmetry. The surfaces involved in binding plastocyanin and ferredoxin are identical in both forms. The crystal structure at 2.6 A resolution revealed 16 subunits, 45 transmembrane helices, and 232 prosthetic groups, including 143 chlorophyll a, 13 chlorophyll b, 27 β-carotene, 7 lutein, 2 xanthophyll, 1 zeaxanthin, 20 monogalactosyl diglyceride, 7 phosphatidyl diglyceride, 5 digalactosyl diglyceride, 2 calcium ions, 2 phylloquinone, and 3 iron sulfur clusters. The model reveals detailed interactions, providing mechanisms for excitation energy transfer and its modulation in one of nature's most efficient photochemical machine.

Dynamics of Long-Distance Hydrogen-Bond Networks in Photosystem II

Abstract: Photosystem II uses the energy of absorbed light to split water molecules, generating molecular oxygen, electrons, and protons. The four protons generated during each reaction cycle are released to the lumen via mechanisms that are poorly understood. Given the complexity of photosystem II, which consists of multiple protein subunits and cofactor molecules and hosts numerous waters, a fundamental issue is finding transient networks of hydrogen bonds that bridge potential proton donor and acceptor groups. Here, we address this issue by performing all-atom molecular dynamics simulations of wild-type and mutant photosystem II monomers, which we analyze using a new protocol designed to facilitate efficient analysis of hydrogen-bond networks. Our computations reveal that local protein/water hydrogen-bond networks can assemble transiently in photosystem II such that the reaction center connects to the lumen. The dynamics of the hydrogen-bond networks couple to the protonation state of specific carboxylate groups...

Ultrafast Energy Transfer Involving the Red Chlorophylls of Cyanobacterial Photosystem I Probed through Two-Dimensional Electronic Spectroscopy

Abstract: Photosystem I (PSI) is a naturally occurring light-harvesting complex that drives oxygenic photosynthesis through a series of photoinitiated transmembrane electron transfer reactions that occur with a high quantum efficiency. Understanding the mechanism by which this process occurs is fundamental to understanding the near-unity quantum efficiency of PSI and in turn could lead to further insight into PSI-based technologies for solar energy conversion. In this article, we have applied two-dimensional electronic spectroscopy to PSI complexes isolated from two different cyanobacterial strains to gain further insight into the ultrafast energy transfer in PSI. The PSI complexes studied differ in the number and absorption of the red chlorophylls, chlorophylls that lie to lower energies than the reaction center. By applying a global analysis to the 2D electronic spectra of the PSI complexes we extract 2D decay associated spectra (2D-DAS). Through analysis of the 2D-DAS we observe a 50 fs relaxation among the bulk...

Energetic insights into two electron transfer pathways in light-driven energy-converting enzymes

Abstract: We report redox potentials (Em) for one-electron reduction for all chlorophylls in the two electron-transfer branches of water-oxidizing enzyme photosystem II (PSII), photosystem I (PSI), and purple bacterial photosynthetic reaction centers (PbRC). In PSI, Em values for the accessory chlorophylls were similar in both electron-transfer branches. In PbRC, the corresponding Em value was 170 mV less negative in the active L-branch (BL) than in the inactive M-branch (BM), favoring BL˙− formation. This contrasted with the corresponding chlorophylls, ChlD1 and ChlD2, in PSII, where Em(ChlD1) was 120 mV more negative than Em(ChlD2), implying that to rationalize electron transfer in the D1-branch, ChlD1 would need to serve as the primary electron donor. Residues that contributed to Em(ChlD1) < Em(ChlD2) simultaneously played a key role in (i) releasing protons from the substrate water molecules and (ii) contributing to the larger cationic population on the chlorophyll closest to the Mn4CaO5 cluster (PD1), favoring electron transfer from water molecules. These features seem to be the nature of PSII, which needs to possess the proton-exit pathway to use a protonated electron source—water molecules.

LOW PHOTOSYNTHETIC EFFICIENCY 1 is required for light-regulated photosystem II biogenesis in Arabidopsis.

Abstract: Photosystem II (PSII), a multisubunit protein complex of the photosynthetic electron transport chain, functions as a water-plastoquinone oxidoreductase, which is vital to the initiation of photosynthesis and electron transport. Although the structure, composition, and function of PSII are well understood, the mechanism of PSII biogenesis remains largely elusive. Here, we identified a nuclear-encoded pentatricopeptide repeat (PPR) protein LOW PHOTOSYNTHETIC EFFICIENCY 1 (LPE1; encoded by At3g46610) in Arabidopsis , which plays a crucial role in PSII biogenesis. LPE1 is exclusively targeted to chloroplasts and directly binds to the 5′ UTR of psbA mRNA which encodes the PSII reaction center protein D1. The loss of LPE1 results in less efficient loading of ribosome on the psbA mRNA and great synthesis defects in D1 protein. We further found that LPE1 interacts with a known regulator of psbA mRNA translation HIGH CHLOROPHYLL FLUORESCENCE 173 (HCF173) and facilitates the association of HCF173 with psbA mRNA. More interestingly, our results indicate that LPE1 associates with psbA mRNA in a light-dependent manner through a redox-based mechanism. This study enhances our understanding of the mechanism of light-regulated D1 synthesis, providing important insight into PSII biogenesis and the functional maintenance of efficient photosynthesis in higher plants.

Ycf48 involved in the biogenesis of the oxygen-evolving photosystem II complex is a seven-bladed beta-propeller protein.

Abstract: Robust photosynthesis in chloroplasts and cyanobacteria requires the participation of accessory proteins to facilitate the assembly and maintenance of the photosynthetic apparatus located within the thylakoid membranes. The highly conserved Ycf48 protein acts early in the biogenesis of the oxygen-evolving photosystem II (PSII) complex by binding to newly synthesized precursor D1 subunit and by promoting efficient association with the D2 protein to form a PSII reaction center (PSII RC) assembly intermediate. Ycf48 is also required for efficient replacement of damaged D1 during the repair of PSII. However, the structural features underpinning Ycf48 function remain unclear. Here we show that Ycf48 proteins encoded by the thermophilic cyanobacterium Thermosynechococcus elongatus and the red alga Cyanidioschyzon merolae form seven-bladed beta-propellers with the 19-aa insertion characteristic of eukaryotic Ycf48 located at the junction of blades 3 and 4. Knowledge of these structures has allowed us to identify a conserved "Arg patch" on the surface of Ycf48 that is important for binding of Ycf48 to PSII RCs but also to larger complexes, including trimeric photosystem I (PSI). Reduced accumulation of chlorophyll in the absence of Ycf48 and the association of Ycf48 with PSI provide evidence of a more wide-ranging role for Ycf48 in the biogenesis of the photosynthetic apparatus than previously thought. Copurification of Ycf48 with the cyanobacterial YidC protein insertase supports the involvement of Ycf48 during the cotranslational insertion of chlorophyll-binding apopolypeptides into the membrane.

Intramolecular Vibrations Complement the Robustness of Primary Charge Separation in a Dimer Model of the Photosystem II Reaction Center.

Abstract: The energy conversion of oxygenic photosynthesis is triggered by primary charge separation in proteins at the photosystem II reaction center. Here, we investigate the impacts of the protein environment and intramolecular vibrations on primary charge separation at the photosystem II reaction center. This is accomplished by combining the quantum dynamic theories of condensed phase electron transfer with quantum chemical calculations to evaluate the vibrational Huang–Rhys factors of chlorophyll and pheophytin molecules. We report that individual vibrational modes play a minor role in promoting charge separation, contrary to the discussion in recent publications. Nevertheless, these small contributions accumulate to considerably influence the charge separation rate, resulting in subpicosecond charge separation almost independent of the driving force and temperature. We suggest that the intramolecular vibrations complement the robustness of the charge separation in the photosystem II reaction center against th...

Changing frequency of fluctuating light reveals the molecular mechanism for P700 oxidation in plant leaves.

Abstract: Natural sunlight exceeds the demand of photosynthesis such that it can cause plants to produce reactive oxygen species (ROS), which subsequently cause photo-oxidative damage. Because photosystem I (PSI) is a major source of ROS, plants actively maintain the reaction center chlorophyll of PSI(P700) oxidized under excessive light conditions to alleviate the ROS production. P700 oxidation is universally recognized in photosynthetic organisms as a physiological response to excessive light. However, it is still poorly understood how P700 oxidation is induced in response to fluctuating light with a variety of frequencies. Here, we investigated the relationships of photosynthetic parameters with P700 oxidation in Arabidopsis thaliana under a sine fluctuating light with different frequencies. As the photon flux density of the light increased, P700 was oxidized concurrently with the chlorophyll fluorescence parameter qL unless the electron acceptor side of PSI was limited. Conversely, we did not observe a proportional relationship of non-photochemical quenching with P700 oxidation. The mutant crr-2, which lacks chloroplast NADPH dehydrogenase, was impaired in P700 oxidation during light fluctuation at high, but not low frequency, unlike the pgrl1 mutant deficient in PGR5 and PGRL1 proteins, which could not oxidize P700 during light fluctuation at both high and low frequencies. Taken together, our findings suggested that the changing frequency of fluctuating light reveals the tracking performance of molecular mechanisms underlying P700 oxidation.

Artificial Photosynthesis for Production of ATP, NAD(P)H, and Hydrogen Peroxide

Abstract: The initial product of photosynthesis is NADPH (dihydronicotinamide adenine dinucleotide phosphate), which is produced from the oxidized form (NADP+) by reduction with two electrons and one proton released from Photosystem I (PSI) via ferredoxin. The proton gradient generated across the thylakoid membrane produces a proton-motive force, which is utilized to synthesize ATP by the use of ATP synthase. NADPH is used as a hydride source in the Calvin–Benson cycle to produce sugars by photosynthesis. In addition to NADP+, PSI can reduce O2 by two electrons with two protons to produce hydrogen peroxide (H2O2), which can be used as a fuel in H2O2 fuel cells. This Minireview focuses on artificial photosynthetic systems to produce the products of natural photosynthesis, such as ATP, NAD(P)H and H2O2 from NAD+ and O2 with water using solar energy, respectively. ATP was produced by use of an artificial photosynthetic membrane, composed of a photosynthetic reaction center mimic that pumps protons into the interior of the liposome, where F-type ATP synthase was incorporated. Solar-driven catalytic water splitting produces hydrogen, which can reduce NAD+ to NADH with an iridium complex catalyst in a slightly alkaline solution at room temperature. H2O2 has been produced by the combination of four-electron oxidation of H2O with four protons to evolve O2 and two-electron/two-proton reduction of O2 under sun-light irradiation. H2O2 can also be produced by direct reaction of H2 and O2 by the combination of an iridium complex catalyst and flavin coenzyme.

Mimicry and functions of photosynthetic reaction centers.

Abstract: The structure and function of photosynthetic reaction centers (PRCs) have been modeled by designing and synthesizing electron donor-acceptor ensembles including electron mediators, which can mimic multi-step photoinduced charge separation occurring in PRCs to obtain long-lived charge-separated states. PRCs in photosystem I (PSI) or/and photosystem II (PSII) have been utilized as components of solar cells to convert solar energy to electric energy. Biohybrid photoelectrochemical cells composed of PSII have also been developed for solar-driven water splitting into H2 and O2 Such a strategy to bridge natural photosynthesis with artificial photosynthesis is discussed in this minireview.

Phycocyanin: One Complex, Two States, Two Functions.

Abstract: Solar energy captured by pigments embedded in light-harvesting complexes can be transferred to neighboring pigments, dissipated, or emitted as fluorescence. Only when it reaches a reaction center is the excitation energy stabilized in the form of a charge separation and converted into chemical energy. Well-directed and regulated energy transfer within the network of pigments is therefore of crucial importance for the success of the photosynthetic processes. Using single-molecule spectroscopy, we show that phycocyanin can dynamically switch between two spectrally distinct states originating from two different conformations. Unexpectedly, one of the two states has a red-shifted emission spectrum. This state is not involved in energy dissipation; instead, we propose that it is involved in direct energy transfer to photosystem I. Finally, our findings suggest that the function of linker proteins in phycobilisomes is to stabilize one state or the other, thus controlling the light-harvesting functions of phycocyanin.

Chlorophyll Composition, Chlorophyll Fluorescence, and Grain Yield Change in esl Mutant Rice

Abstract: To evaluate the effect of changes in chlorophyll (Chl) composition and fluorescence on final yield formation, early senescence leaf (esl) mutant rice and its wild-type cultivar were employed to investigate the genotype-dependent differences in Chl composition, Chl fluorescence, and yield characteristics during the grain-filling stage. However, the temporal expression patterns of key genes involved in the photosystem II (PSII) reaction center in the leaves of two rice genotypes were analyzed by quantitative real-time polymerase chain reaction (qRT-PCR). Results showed that the seed-setting rate, 1000-grain weight, and yield per plant remarkably decreased, and the increase in the 1000-grain weight during the grain-filling stage was retarded in esl mutant rice. Chl composition, maximal fluorescence yield (Fm), variable fluorescence (Fv), a maximal quantum yield of PSII photochemistry (Fv/Fm), and net photosynthetic rate (Pn) in esl mutant rice considerably decreased, thereby indicating the weakened abilities of light energy harvesting and transferring in senescent leaves. The esl mutant rice showed an increase in the minimal fluorescence yield (F0) and 1 − Fv/Fm and decreases in the expression levels of light-harvesting Chl a/b binding protein (Cab) and photosystem II binding protein A (PsbA), PsbB, PsbC, and PsbD encoding for the reaction center of the PSII complex during the grain-filling stage. These results indicated the PSII reaction centers were severely damaged in the mesophyll cells of senescent leaves, which resulted in the weakened harvesting quantum photon and transferring light energy to PSI and PSII for carbon dioxide assimilation, leading to enhanced heat dissipation of light energy and a decrease in Pn.

Impact of the lipid bilayer on energy transfer kinetics in the photosynthetic protein LH2

Abstract: Photosynthetic purple bacteria convert solar energy to chemical energy with near unity quantum efficiency. The light-harvesting process begins with absorption of solar energy by an antenna protein called Light-Harvesting Complex 2 (LH2). Energy is subsequently transferred within LH2 and then through a network of additional light-harvesting proteins to a central location, termed the reaction center, where charge separation occurs. The energy transfer dynamics of LH2 are highly sensitive to intermolecular distances and relative organizations. As a result, minor structural perturbations can cause significant changes in these dynamics. Previous experiments have primarily been performed in two ways. One uses non-native samples where LH2 is solubilized in detergent, which can alter protein structure. The other uses complex membranes that contain multiple proteins within a large lipid area, which make it difficult to identify and distinguish perturbations caused by protein–protein interactions and lipid–protein interactions. Here, we introduce the use of the biochemical platform of model membrane discs to study the energy transfer dynamics of photosynthetic light-harvesting complexes in a near-native environment. We incorporate a single LH2 from Rhodobacter sphaeroides into membrane discs that provide a spectroscopically amenable sample in an environment more physiological than detergent but less complex than traditional membranes. This provides a simplified system to understand an individual protein and how the lipid–protein interaction affects energy transfer dynamics. We compare the energy transfer rates of detergent-solubilized LH2 with those of LH2 in membrane discs using transient absorption spectroscopy and transient absorption anisotropy. For one key energy transfer step in LH2, we observe a 30% enhancement of the rate for LH2 in membrane discs compared to that in detergent. Based on experimental results and theoretical modeling, we attribute this difference to tilting of the peripheral bacteriochlorophyll in the B800 band. These results highlight the importance of well-defined systems with near-native membrane conditions for physiologically-relevant measurements.

Light-driven quinone reduction in heliobacterial membranes

Abstract: Photosynthetic reaction centers (RCs) evolved > 3 billion years ago and have diverged into Type II RCs reducing quinones and Type I RCs reducing soluble acceptors via iron–sulfur clusters. Photosystem I (PSI), the exemplar Type I RC, uses modified menaquinones as intermediate electron transfer cofactors, but it has been controversial if the Type I RC of heliobacteria (HbRC) uses its two bound menaquinones in the same way. The sequence of the quinone-binding site in PSI is not conserved in the HbRC, and the recently solved crystal structure of the HbRC does not reveal a quinone in the analogous site. We found that illumination of heliobacterial membranes resulted in reduction of menaquinone to menaquinol, suggesting that the HbRC can perform a function thought restricted to Type II RCs. Experiments on membranes and live cells are consistent with the hypothesis that the HbRC preferentially reduces soluble electron acceptors (e.g., ferredoxins) in low light, but switches to reducing lipophilic quinones in high light, when the soluble acceptor pool becomes full. Thus, the HbRC may represent a functional evolutionary intermediate between PSI and the Type II RCs.

Photosystem I-LHCII megacomplexes respond to high light and aging in plants.

Abstract: Photosystem II is known to be a highly dynamic multi-protein complex that participates in a variety of regulatory and repair processes. In contrast, photosystem I (PSI) has, until quite recently, been thought of as relatively static. We report the discovery of plant PSI-LHCII megacomplexes containing multiple LHCII trimers per PSI reaction center. These PSI-LHCII megacomplexes respond rapidly to changes in light intensity, as visualized by native gel electrophoresis. PSI-LHCII megacomplex formation was found to require thylakoid stacking, and to depend upon growth light intensity and leaf age. These factors were, in turn, correlated with changes in PSI/PSII ratios and, intriguingly, PSI-LHCII megacomplex dynamics appeared to depend upon PSII core phosphorylation. These findings suggest new functions for PSI and a new level of regulation involving specialized subpopulations of photosystem I which have profound implications for current models of thylakoid dynamics.

Lipid-Controlled Stabilization of Charge-Separated States (P+QB-) and Photocurrent Generation Activity of a Light-Harvesting-Reaction Center Core Complex (LH1-RC) from Rhodopseudomonas palustris.

Abstract: The photosynthetic light-harvesting–reaction center core complex (LH1-RC) is a natural excitonic and photovoltaic device embedded in a lipid membrane. In order to apply LH1-RCs as a biohybrid energy-producing material, some important issues must be addressed, including how to make LH1-RCs function as efficiently as possible. In addition, they should be characterized to evaluate how many active LH1-RCs efficiently work in artificial systems. We report here that an anionic phospholipid, phosphatidylglycerol (PG), stabilizes the charge-separated state (a photooxidized electron donor and reduced quinone pair, P+QB–) of LH1-RC (from Rhodopseudomonas palustris) and enhances its activity in photocurrent generation. Steady-state fluorometric analysis demonstrated that PG enhances the formation of the P+QB– state at lower irradiances. The photocurrent generation activity was analyzed via Michaelis–Menten kinetics, revealing that 38% of LH1-RCs reconstituted into the PG membrane generated photocurrent at a turnover...

Photochemically induced dynamic nuclear polarization NMR on photosystem II: donor cofactor observed in entire plant.

Abstract: The solid-state photo-CIDNP (photochemically induced dynamic nuclear polarization) effect allows for increase of signal and sensitivity in magic-angle spinning (MAS) NMR experiments. The effect occurs in photosynthetic reaction centers (RC) proteins upon illumination and induction of cyclic electron transfer. Here we show that the strength of the effect allows for observation of the cofactors forming the spin-correlated radical pair (SCRP) in isolated proteins, in natural photosynthetic membranes as well as in entire plants. To this end, we measured entire selectively 13C isotope enriched duckweed plants (Spirodela oligorrhiza) directly in the MAS rotor. Comparison of 13C photo-CIDNP MAS NMR spectra of photosystem II (PS2) obtained from different levels of RC isolation, from entire plant to isolated RC complex, demonstrates the intactness of the photochemical machinery upon isolation. The SCRP in PS2 is structurally and functionally very similar in duckweed and spinach (Spinacia oleracea). The analysis of the photo-CIDNP MAS NMR spectra reveals a monomeric Chl a donor. There is an experimental evidence for matrix involvement, most likely due to the axial donor histidine, in the formation of the SCRP. Data do not suggest a chemical modification of C-131 carbonyl position of the donor cofactor.

Z-scheme solar water splitting via self-assembly of photosystem I-catalyst hybrids in thylakoid membranes.

Abstract: Nature's solar energy converters, the Photosystem I (PSI) and Photosystem II (PSII) reaction center proteins, flawlessly manage photon capture and conversion processes in plants, algae, and cyanobacteria to drive oxygenic water-splitting and carbon fixation. Herein, we utilize the native photosynthetic Z-scheme electron transport chain to drive hydrogen production from thylakoid membranes by directional electron transport to abiotic catalysts bound at the stromal end of PSI. Pt-nanoparticles readily self-assemble with PSI in spinach and cyanobacterial membranes as evidenced by light-driven H2 production in the presence of a mediating electron shuttle protein and the sacrificial electron donor sodium ascorbate. EPR characterization confirms placement of the Pt-nanoparticles on the acceptor end of PSI. In the absence of sacrificial reductant, H2 production at PSI occurs via coupling to light-induced PSII O2 evolution as confirmed by correlation of catalytic activity to the presence or absence of the PSII inhibitor DCMU. To create a more sustainable system, first-row transition metal molecular cobaloxime and nickel diphosphine catalysts were found to perform photocatalysis when bound in situ to cyanobacterial thylakoid membranes. Thus, the self-assembly of abiotic catalysts with photosynthetic membranes demonstrates a tenable method for accomplishing solar overall water splitting to generate H2, a renewable and clean fuel. This work benchmarks a significant advance toward improving photosynthetic efficiency for solar fuel production.