Electron transfer in photosystem I

Photo-oxidative damage of photosystem I by repetitive flashes and chilling stress in cucumber leaves

Photosystem I (PSI) is an essential protein complex for oxygenic photosynthesis and is also known to be an important source of reactive oxygen species (ROS) in the light. When ROS are generated within PSI, the photosystem can be damaged. The so-called PSI photoinhibition is a lethal event for oxygenic phototrophs, and it is prevented by keeping the reaction center chlorophyll (P700) oxidized in excess light conditions. Whereas regulatory mechanisms for controlling P700 oxidation have been discovered already, the molecular mechanism of PSI photoinhibition is still unclear. Here, we characterized the damage mechanism of PSI photoinhibition by in vitro transient absorption and electron paramagnetic resonance (EPR) spectroscopy in isolated PSI from cucumber leaves that had been subjected to photoinhibition treatment. Photodamage to PSI was induced by two different light treatments: 1. continuous illumination with high light at low (chilling) temperature (C/LT) and 2. repetitive flashes at room temperature (F/RT). These samples were compared to samples that had been illuminated with high light at room temperature (C/RT). The [FeS] clusters FX and (FA FB) were destructed in C/LT but not in F/RT. Transient absorption spectroscopy indicated that half of the charge separation was impaired in F/RT, however, low-temperature EPR revealed the light-induced FX signal at a similar size as in the case of C/RT. This indicates that the two branches of electron transfer in PSI were affected differently. Electron transfer at the A-branch was inhibited in F/RT and also partially in C/LT, while the B-branch remained active.

Presence of low-energy chlorophylls d in photosystem I trimer and monomer cores isolated from Acaryochloris sp. NBRC 102871

Acaryochloris species belong to a special category of cyanobacteria possessing chlorophyll (Chl) d. One of the photosynthetic characteristics of Acaryochloris marina MBIC11017 is that the absorption spectra of photosystem I (PSI) showed almost no bands and shoulders of low-energy Chls d over 740 nm. In contrast, the absorption spectra of other Acaryochloris species showed a shoulder around 740 nm, suggesting that low-energy Chls d within PSI are diversified among Acaryochloris species. In this study, we purified PSI trimer and monomer cores from Acaryochloris sp. NBRC 102871 and examined their protein and pigment compositions and spectral properties. The protein bands and pigment compositions of the PSI trimer and monomer of NBRC102871 were virtually identical to those of MBIC11017. The absorption spectra of the NBRC102871 PSIs exhibited a shoulder around 740 nm, whereas the fluorescence spectra of PSI trimer and monomer displayed maximum peaks at 754 and 767 nm, respectively. These spectral properties were different from those of MBIC11017, indicating the presence of low-energy Chls d within the NBRC102871 PSIs. Moreover, we analyzed the NBRC102871 genome to identify amino acid sequences of PSI proteins and compared them with those of the A. marina MBIC11017 and MBIC10699 strains whose genomes are available. The results showed that some of the sequences in NBRC102871 were distinct from those in MBIC11017 and MBIC10699. These findings provide insights into the variety of low-energy Chls d with respect to the protein environments of PSI cores among the three Acaryochloris strains.

Strong interaction of CpcL with photosystem I cores induced in heterocysts of Anabaena sp. PCC 7120

Phycobilisomes (PBSs) are photosynthetic light-harvesting antennae and appear to be loosely bound to photosystem I (PSI). We previously found unique protein bands in each PSI fraction in heterocysts of Anabaena sp. PCC 7120 by two-dimensional blue native/SDS-PAGE; however, the protein bands have not been identified. Here we analyzed the protein bands by mass spectrometry, which were identified as CpcL, one of the components in PBSs. As different composition and organization of Anabaena PSI-PBS supercomplexes were observed, the expression and binding properties of PBSs including CpcL to PSIs in this cyanobacterium may be diversified in response to its living environments.

Physiological and Proteomic Responses of the Tetraploid Robinia pseudoacacia L. to High CO2 Levels

The increase in atmospheric CO2 concentration is a significant factor in triggering global warming. CO2 is essential for plant photosynthesis, but excessive CO2 can negatively impact photosynthesis and its associated physiological and biochemical processes. The tetraploid Robinia pseudoacacia L., a superior and improved variety, exhibits high tolerance to abiotic stress. In this study, we investigated the physiological and proteomic response mechanisms of the tetraploid R. pseudoacacia under high CO2 treatment. The results of our physiological and biochemical analyses revealed that a 5% high concentration of CO2 hindered the growth and development of the tetraploid R. pseudoacacia and caused severe damage to the leaves. Additionally, it significantly reduced photosynthetic parameters such as Pn, Gs, Tr, and Ci, as well as respiration. The levels of chlorophyll (Chl a and b) and the fluorescent parameters of chlorophyll (Fm, Fv/Fm, qP, and ETR) also significantly decreased. Conversely, the levels of ROS (H2O2 and O2·-) were significantly increased, while the activities of antioxidant enzymes (SOD, CAT, GR, and APX) were significantly decreased. Furthermore, high CO2 induced stomatal closure by promoting the accumulation of ROS and NO in guard cells. Through a proteomic analysis, we identified a total of 1652 DAPs after high CO2 treatment. GO functional annotation revealed that these DAPs were mainly associated with redox activity, catalytic activity, and ion binding. KEGG analysis showed an enrichment of DAPs in metabolic pathways, secondary metabolite biosynthesis, amino acid biosynthesis, and photosynthetic pathways. Overall, our study provides valuable insights into the adaptation mechanisms of the tetraploid R. pseudoacacia to high CO2.

Is the A-1 Pigment in Photosystem I Part of P700? A (P700+-P700) FTIR Difference Spectroscopy Study of A-1 Mutants

The involvement of the second pair of chlorophylls, termed A-1A and A-1B, in light-induced electron transfer in photosystem I (PSI) is currently debated. Asparagines at PsaA600 and PsaB582 are involved in coordinating the A-1B and A-1A pigments, respectively. Here we have mutated these asparagine residues to methionine in two single mutants and a double mutant in PSI from Synechocystis sp. PCC 6803, which we term NA600M, NB582M, and NA600M/NB582M mutants. (P700+-P700) FTIR difference spectra (DS) at 293 K were obtained for the wild-type and the three mutant PSI samples. The wild-type and mutant FTIR DS differ considerably. This difference indicates that the observed changes in the (P700+-P700) FTIR DS cannot be due to only the PA and PB pigments of P700. Comparison of the wild-type and mutant FTIR DS allows the assignment of different features to both A-1 pigments in the FTIR DS for wild-type PSI and assesses how these features shift upon cation formation and upon mutation. While the exact role the A-1 pigments play in the species we call P700 is unclear, we demonstrate that the vibrational modes of the A-1A and A-1B pigments are modified upon P700+ formation. Previously, we showed that the A-1 pigments contribute to P700 in green algae. In this manuscript, we demonstrate that this is also the case in cyanobacterial PSI. The nature of the mutation-induced changes in algal and cyanobacterial PSI is similar and can be considered within the same framework, suggesting a universality in the nature of P700 in different photosynthetic organisms.

Impact of Peripheral Hydrogen Bond on Electronic Properties of the Primary Acceptor Chlorophyll in the Reaction Center of Photosystem I

Photosystem I (PS I) is a photosynthetic pigment-protein complex that absorbs light and uses the absorbed energy to initiate electron transfer. Electron transfer has been shown to occur concurrently along two (A- and B-) branches of reaction center (RC) cofactors. The electron transfer chain originates from a special pair of chlorophyll a molecules (P700), followed by two chlorophylls and one phylloquinone in each branch (denoted as A-1, A0, A1, respectively), converging in a single iron-sulfur complex Fx. While there is a consensus that the ultimate electron donor-acceptor pair is P700+A0-, the involvement of A-1 in electron transfer, as well as the mechanism of the very first step in the charge separation sequence, has been under debate. To resolve this question, multiple groups have targeted electron transfer cofactors by site-directed mutations. In this work, the peripheral hydrogen bonds to keto groups of A0 chlorophylls have been disrupted by mutagenesis. Four mutants were generated: PsaA-Y692F; PsaB-Y667F; PsaB-Y667A; and a double mutant PsaA-Y692F/PsaB-Y667F. Contrary to expectations, but in agreement with density functional theory modeling, the removal of the hydrogen bond by Tyr → Phe substitution was found to have a negligible effect on redox potentials and optical absorption spectra of respective chlorophylls. In contrast, Tyr → Ala substitution was shown to have a fatal effect on the PS I function. It is thus inferred that PsaA-Y692 and PsaB-Y667 residues have primarily structural significance, and their ability to coordinate respective chlorophylls in electron transfer via hydrogen bond plays a minor role.

Photophysics of a Monoannulated Indigo: Intra- and Intermolecular Charge Transfer

In the present work, the photoinduced charge-transfer (CT) behavior of 7-phenyl-6H-pyrido[1,2-a:3,4-b']diindole-6,13(12H)-dione (HCB) as a function of solvent polarity is reported by UV-vis absorption, steady-state and time-resolved fluorescence, and quantum chemical calculations. Calculated excited state energies of HCB at the B3PW91/6-31+G* level in vacuo and in solvents fulfill the energy requirements for singlet fission, which is the most promising path for the generation of highly efficient solar cells. The calculated potential energy curve for the compound reveals that the keto form is the predominant form in the ground state. Large bathochromic shifts in fluorescence with decreasing trends of quantum yield and lifetime indicate the occurrence of intramolecular CT from the indole bicycle to the indolinone moiety of HCB in highly polar solvents. The observed quenching of HCB fluorescence in different solvents without altering the spectral shape upon addition of a donor, triethylamine, is attributed to intermolecular CT, and it was examined in terms of the Stern-Volmer kinetics. The thermodynamics of photoinduced CT processes in HCB was analyzed using the measured photophysical data and cyclic voltammetric redox potentials via the Rehm-Weller equation. Analyses with the semiclassical Marcus theory suggest that both the CT processes fall under the Marcus normal region.

Establishing an efficient membrane bioreactor for simultaneous pollutant removal and purple bacteria production under salinity stress

Recovering resources from wastewater to alleviate the energy crisis has become the prevailing trend of technological development. Purple phototrophic bacteria (PPB), a group of fast-growing microbes, have been widely noticed for their potential in producing value-added products from waste streams. However, saline contents in these waste streams, such as food processing wastewater pose a big challenge, which not only restrain the pollutant removal efficiency, but also hinder the growth of functional microbes. To overcome this, a photo anaerobic membrane bioreactor cultivating PPB (PPB-MBR) was constructed and its performance upon long-term salinity stress was investigated. PPB-MBR achieved desirable pollutants removal performance with the average COD and NH4+ removal efficiency being 87% (±8%, n = 87) and 89% (±10%, n = 87), respectively during long-term exposure to salinity stress of 1-80 g NaCl L-1. PPB were predominant during the entire operation period of 87 days (60%-80%), obtaining maximum biomass yield of 0.67 g biomass g-1 CODremoved and protein productivity of 0.18 g L-1 d-1 at the salinity level of 20 g NaCl L-1 and 60 g NaCl L-1, respectively. The sum of value-added products in proportion to the biomass reached 58% at maximum at the salinity level of 60 g NaCl L-1 with protein, pigments and trehalose contributing to 44%, 8.7%, and 5%, respectively. Based on economic analysis, the most cost-saving scenario treating food processing wastewater was revealed at salinity level of around 20 g NaCl L-1. However, more optimization tools are needed to boost the production efficiency so that the profit from value-added products can outweigh the additional cost by excess salinity in the future implication.

High-Resolution Frequency-Domain Spectroscopic and Modeling Studies of Photosystem I (PSI), PSI Mutants and PSI Supercomplexes

Photosystem I (PSI) is one of the two main pigment-protein complexes where the primary steps of oxygenic photosynthesis take place. This review describes low-temperature frequency-domain experiments (absorption, emission, circular dichroism, resonant and non-resonant hole-burned spectra) and modeling efforts reported for PSI in recent years. In particular, we focus on the spectral hole-burning studies, which are not as common in photosynthesis research as the time-domain spectroscopies. Experimental and modeling data obtained for trimeric cyanobacterial Photosystem I (PSI3), PSI3 mutants, and PSI3-IsiA18 supercomplexes are analyzed to provide a more comprehensive understanding of their excitonic structure and excitation energy transfer (EET) processes. Detailed information on the excitonic structure of photosynthetic complexes is essential to determine the structure-function relationship. We will focus on the so-called "red antenna states" of cyanobacterial PSI, as these states play an important role in photochemical processes and EET pathways. The high-resolution data and modeling studies presented here provide additional information on the energetics of the lowest energy states and their chlorophyll (Chl) compositions, as well as the EET pathways and how they are altered by mutations. We present evidence that the low-energy traps observed in PSI are excitonically coupled states with significant charge-transfer (CT) character. The analysis presented for various optical spectra of PSI3 and PSI3-IsiA18 supercomplexes allowed us to make inferences about EET from the IsiA18 ring to the PSI3 core and demonstrate that the number of entry points varies between sample preparations studied by different groups. In our most recent samples, there most likely are three entry points for EET from the IsiA18 ring per the PSI core monomer, with two of these entry points likely being located next to each other. Therefore, there are nine entry points from the IsiA18 ring to the PSI3 trimer. We anticipate that the data discussed below will stimulate further research in this area, providing even more insight into the structure-based models of these important cyanobacterial photosystems.

Inverted region in the reaction of the quinone reduction in the A1-site of photosystem I from cyanobacteria

Photosystem I from the menB strain of Synechocystis sp. PCC 6803 containing foreign quinones in the A1 sites was used for studying the primary steps of electron transfer by pump-probe femtosecond laser spectroscopy. The free energy gap (- ΔG) of electron transfer between the reduced primary acceptor A0 and the quinones bound in the A1 site varied from 0.12 eV for the low-potential 1,2-diamino-anthraquinone to 0.88 eV for the high-potential 2,3-dichloro-1,4-naphthoquinone, compared to 0.5 eV for the native phylloquinone. It was shown that the kinetics of charge separation between the special pair chlorophyll P700 and the primary acceptor A0 was not affected by quinone substitutions, whereas the rate of A0 → A1 electron transfer was sensitive to the redox-potential of quinones: the decrease of - ΔG by 400 meV compared to the native phylloquinone resulted in a ~ fivefold slowing of the reaction The presence of the asymmetric inverted region in the ΔG dependence of the reaction rate indicates that the electron transfer in photosystem I is controlled by nuclear tunneling and should be treated in terms of quantum electron-phonon interactions. A three-mode implementation of the multiphonon model, which includes modes around 240 cm-1 (large-scale protein vibrations), 930 cm-1 (out-of-plane bending of macrocycles and protein backbone vibrations), and 1600 cm-1 (double bonds vibrations) was applied to rationalize the observed dependence. The modes with a frequency of at least 1600 cm-1 make the predominant contribution to the reorganization energy, while the contribution of the "classical" low-frequency modes is only 4%.

Photosynthetic and transcriptome responses to fluctuating light in Arabidopsis thylakoid ion transport triple mutant

Fluctuating light intensity challenges fluent photosynthetic electron transport in plants, inducing photoprotection while diminishing carbon assimilation and growth, and also influencing photosynthetic signaling for regulation of gene expression. Here, we employed in vivo chlorophyll-a fluorescence and P700 difference absorption measurements to demonstrate the enhancement of photoprotective energy dissipation of both photosystems in wild-type Arabidopsis thaliana after 6 h exposure to fluctuating light as compared with constant light conditions. This acclimation response to fluctuating light was hampered in a triple mutant lacking the thylakoid ion transport proteins KEA3, VCCN1, and CLCe, leading to photoinhibition of photosystem I. Transcriptome analysis revealed upregulation of genes involved in biotic stress and defense responses in both genotypes after exposure to fluctuating as compared with constant light, yet these responses were demonstrated to be largely upregulated in triple mutant already under constant light conditions compared with wild type. The current study illustrates the rapid acclimation of plants to fluctuating light, including photosynthetic, transcriptomic, and metabolic adjustments, and highlights the connection among thylakoid ion transport, photosynthetic energy balance, and cell signaling.

Photoactive and conductive biohybrid films by polymerization of pyrrole through voids in photosystem I multilayer films

The combination of conducting polymers with electro- and photoactive proteins into thin films holds promise for advanced energy conversion materials and devices. The emerging field of protein electronics requires conductive soft materials in a composite with electrically insulating proteins. The electropolymerization of pyrrole through voids in a drop-casted photosystem I (PSI) multilayer film enables the straightforward fabrication of photoactive and conductive biohybrid films. The rate of polypyrrole (PPy) growth is reduced by the presence of the PSI film but is insensitive to its thickness, suggesting that rapid diffusion of pyrrole through the voids within the PSI film enables initiation at vacant areas on the gold surface. The base thickness of the composite tends to increase with time, as PPy chains propagate through and beyond the PSI film, coalescing to exhibit a tubule-like morphology as observed by scanning electron microscopy. Increasing amounts of PPy greatly increase the capacitance of the composite films in a manner almost identical to that of pure PPy films grown from unmodified gold, consistent with a high polymer/aqueous interfacial area and a conductive composite film. While PPy is not photoactive here, all composite films, including those with large amounts of PPy, exhibit photocurrents when irradiated by white light in the presence of redox mediator species. Optimization of the Py electropolymerization time is necessary, as increasing amounts of PPy lead to decreased photocurrent density due to a combination of light absorbance by the polymer and reduced accessibility of redox species to active PSI sites.

Isolation and characterization of trimeric and monomeric PSI cores from Acaryochloris marina MBIC11017

Photosystem I (PSI) catalyzes light-induced electron-transfer reactions and has been observed to exhibit various oligomeric states and different energy levels of chlorophylls (Chls) in response to oligomerization. However, the biochemical and spectroscopic properties of a PSI monomer containing Chls d are not well understood. In this study, we successfully isolated and characterized PSI monomers from the cyanobacterium Acaryochloris marina MBIC11017, and compared their properties with those of the A. marina PSI trimer. The PSI trimers and monomers were prepared using trehalose density gradient centrifugation after anion-exchange and hydrophobic interaction chromatography. The polypeptide composition of the PSI monomer was found to be consistent with that of the PSI trimer. The absorption spectrum of the PSI monomer showed the Qy band of Chl d at 704 nm, which was blue-shifted from the peak at 707 nm observed in the PSI-trimer spectrum. The fluorescence-emission spectrum of the PSI monomer measured at 77 K exhibited a peak at 730 nm without a broad shoulder in the range of 745-780 nm, which was clearly observed in the PSI-trimer spectrum. These spectroscopic properties of the A. marina PSI trimer and monomer suggest different formations of low-energy Chls d between the two types of PSI cores. Based on these findings, we discuss the location of low-energy Chls d in A. marina PSIs.

Diverse biological functions of vitamin K: from coagulation to ferroptosis

Vitamin K is essential for several physiological processes, such as blood coagulation, in which it serves as a cofactor for the conversion of peptide-bound glutamate to γ-carboxyglutamate in vitamin K-dependent proteins. This process is driven by the vitamin K cycle facilitated by γ-carboxyglutamyl carboxylase, vitamin K epoxide reductase and ferroptosis suppressor protein-1, the latter of which was recently identified as the long-sought-after warfarin-resistant vitamin K reductase. In addition, vitamin K has carboxylation-independent functions. Akin to ubiquinone, vitamin K acts as an electron carrier for ATP production in some organisms and prevents ferroptosis, a type of cell death hallmarked by lipid peroxidation. In this Perspective, we provide an overview of the diverse functions of vitamin K in physiology and metabolism and, at the same time, offer a perspective on its role in ferroptosis together with ferroptosis suppressor protein-1. A comparison between vitamin K and ubiquinone, from an evolutionary perspective, may offer further insights into the manifold roles of vitamin K in biology.

Biochemical and spectroscopic characterization of PSI-LHCI from the red alga Cyanidium caldarium

Light-harvesting complexes (LHCs) have been diversified in oxygenic photosynthetic organisms, and play an essential role in capturing light energy which is transferred to two types of photosystem cores to promote charge-separation reactions. Red algae are one of the groups of photosynthetic eukaryotes, and their chlorophyll (Chl) a-binding LHCs are specifically associated with photosystem I (PSI). In this study, we purified three types of preparations, PSI-LHCI supercomplexes, PSI cores, and isolated LHCIs, from the red alga Cyanidium caldarium, and examined their properties. The polypeptide bands of PSI-LHCI showed characteristic PSI and LHCI components without contamination by other proteins. The carotenoid composition of LHCI displayed zeaxanthins, β-cryptoxanthins, and β-carotenes. Among the carotenoids, zeaxanthins were enriched in LHCI. On the contrary, both zeaxanthins and β-cryptoxanthins could not be detected from PSI, suggesting that zeaxanthins and β-cryptoxanthins are bound to LHCI but not PSI. A Qy peak of Chl a in the absorption spectrum of LHCI was shifted to a shorter wavelength than those in PSI and PSI-LHCI. This tendency is in line with the result of fluorescence-emission spectra, in which the emission maxima of PSI-LHCI, PSI, and LHCI appeared at 727, 719, and 677 nm, respectively. Time-resolved fluorescence spectra of LHCI represented no 719 and 727-nm fluorescence bands from picoseconds to nanoseconds. These results indicate that energy levels of Chls around/within LHCIs and within PSI are changed by binding LHCIs to PSI. Based on these findings, we discuss the expression, function, and structure of red algal PSI-LHCI supercomplexes.

Thermal site energy fluctuations in photosystem I: new insights from MD/QM/MM calculations

Cyanobacterial photosystem I (PSI) is one of the most efficient photosynthetic machineries found in nature. Due to the large scale and complexity of the system, the energy transfer mechanism from the antenna complex to the reaction center is still not fully understood. A central element is the accurate evaluation of the individual chlorophyll excitation energies (site energies). Such an evaluation must include a detailed treatment of site specific environmental influences on structural and electrostatic properties, but also their evolution in the temporal domain, because of the dynamic nature of the energy transfer process. In this work, we calculate the site energies of all 96 chlorophylls in a membrane-embedded model of PSI. The employed hybrid QM/MM approach using the multireference DFT/MRCI method in the QM region allows to obtain accurate site energies under explicit consideration of the natural environment. We identify energy traps and barriers in the antenna complex and discuss their implications for energy transfer to the reaction center. Going beyond previous studies, our model also accounts for the molecular dynamics of the full trimeric PSI complex. Via statistical analysis we show that the thermal fluctuations of single chlorophylls prevent the formation of a single prominent energy funnel within the antenna complex. These findings are also supported by a dipole exciton model. We conclude that energy transfer pathways may form only transiently at physiological temperatures, as thermal fluctuations overcome energy barriers. The set of site energies provided in this work sets the stage for theoretical and experimental studies on the highly efficient energy transfer mechanisms in PSI.

Drop-casted Photosystem I/cytochrome c multilayer films for biohybrid solar energy conversion

One of the main barriers to making efficient Photosystem I-based biohybrid solar cells is the need for an electrochemical pathway to facilitate electron transfer between the P₇₀₀ reaction center of Photosystem I and an electrode. To this end, nature provides inspiration in the form of cytochrome c₆, a natural electron donor to the P₇₀₀ site. Its natural ability to access the P₇₀₀ binding pocket and reduce the reaction center can be mimicked by employing cytochrome c, which has a similar protein structure and redox chemistry while also being compatible with a variety of electrode surfaces. Previous research has incorporated cytochrome c to improve the photocurrent generation of Photosystem I using time consuming and/or specialized electrode preparation. While those methods lead to high protein areal density, in this work we use the quick and facile vacuum-assisted drop-casting technique to construct a Photosystem I/cytochrome c photoactive composite film with micron-scale thickness. We demonstrate that this simple fabrication technique can result in high cytochrome c loading and improvement in cathodic photocurrent over a drop-casted Photosystem I film without cytochrome c. In addition, we analyze the behavior of the cytochrome c/Photosystem I system at varying applied potentials to show that the improvement in performance can be attributed to enhancement of the electron transfer rate to P₇₀₀ sites and therefore the PSI turnover rate within the composite film.

Structural insights into photosynthetic cyclic electron transport

During photosynthesis, light energy is utilized to drive sophisticated biochemical chains of electron transfers, converting solar energy into chemical energy that feeds most life on earth. Cyclic electron transfer/flow (CET/CEF) plays an essential role in efficient photosynthesis, as it balances the ATP/NADPH ratio required in various regulatory and metabolic pathways. Photosystem I, cytochrome b₆f, and NADH dehydrogenase (NDH) are large multisubunit protein complexes embedded in the thylakoid membrane of the chloroplast and key players in NDH-dependent CEF pathway. Furthermore, small mobile electron carriers serve as shuttles for electrons between these membrane protein complexes. Efficient electron transfer requires transient interactions between these electron donors and acceptors. Structural biology has been a powerful tool to advance our knowledge of this important biological process. A number of structures of the membrane-embedded complexes, soluble electron carrier proteins, and transient complexes composed of both have now been determined. These structural data reveal detailed interacting patterns of these electron donor-acceptor pairs, thus allowing us to visualize the different parts of the electron transfer process. This review summarizes the current state of structural knowledge of three membrane complexes and their interaction patterns with mobile electron carrier proteins.

Cyanobacterial Bioenergetics in Relation to Cellular Growth and Productivity

Cyanobacteria, the evolutionary originators of oxygenic photosynthesis, have the capability to convert CO2, water, and minerals into biomass using solar energy. This process is driven by intricate bioenergetic mechanisms that consist of interconnected photosynthetic and respiratory electron transport chains coupled. Over the last few decades, advances in physiochemical analysis, molecular genetics, and structural analysis have enabled us to gain a more comprehensive understanding of cyanobacterial bioenergetics. This includes the molecular understanding of the primary energy conversion mechanisms as well as photoprotective and other dissipative mechanisms that prevent photodamage when the rates of photosynthetic output, primarily in the form of ATP and NADPH, exceed the rates that cellular assimilatory processes consume these photosynthetic outputs. Despite this progress, there is still much to learn about the systems integration and the regulatory circuits that control expression levels for optimal cellular abundance and activity of the photosynthetic complexes and the cellular components that convert their products into biomass. With an improved understanding of these regulatory principles and mechanisms, it should be possible to optimally modify cyanobacteria for enhanced biotechnological purposes.

Sustainable Production of Pigments from Cyanobacteria

Pigments are intensely coloured compounds used in many industries to colour other materials. The demand for naturally synthesised pigments is increasing and their production can be incorporated into circular bioeconomy approaches. Natural pigments are produced by bacteria, cyanobacteria, microalgae, macroalgae, plants and animals. There is a huge unexplored biodiversity of prokaryotic cyanobacteria which are microscopic phototrophic microorganisms that have the ability to capture solar energy and CO2 and use it to synthesise a diverse range of sugars, lipids, amino acids and biochemicals including pigments. This makes them attractive for the sustainable production of a wide range of high-value products including industrial chemicals, pharmaceuticals, nutraceuticals and animal-feed supplements. The advantages of cyanobacteria production platforms include comparatively high growth rates, their ability to use freshwater, seawater or brackish water and the ability to cultivate them on non-arable land. The pigments derived from cyanobacteria and microalgae include chlorophylls, carotenoids and phycobiliproteins that have useful properties for advanced technical and commercial products. Development and optimisation of strain-specific pigment-based cultivation strategies support the development of economically feasible pigment biorefinery scenarios with enhanced pigment yields, quality and price. Thus, this chapter discusses the origin, properties, strain selection, production techniques and market opportunities of cyanobacterial pigments.

A two-site triplet exciton hopping model: Application to 3P700

A model is presented describing the effect on spin-polarized transient EPR signals caused by incoherent state hopping between two sites. It is shown that the size of the spin state space can be reduced by half to the subspace described by the site-average Hamiltonian and that the dynamics of the system results in a redistribution of the population between its eigenstates. Analytical expressions for the rates of population redistribution and the line shape are derived for the general case in which the back-and-forth rates are unequal. The EPR signals calculated using these expressions are in very good agreement with those obtained by direct numerical solution of the density matrix rate equations. The model is then used to investigate the influence of exciton hopping on triplet state transient EPR spectra. Using the triplet state of the primary donor of Photosystem I as an example, it is shown that the influence of unequal hopping rates becomes more pronounced in the spectrum at longer delay times after the laser flash.

Changes in the Electron Transfer Symmetry in the Photosystem I Reaction Centers upon Removal of Iron-Sulfur Clusters

In photosynthetic reaction centers of intact photosystem I (PSI) complexes from cyanobacteria, electron transfer at room temperature occurs along two symmetrical branches of redox cofactors A and B at a ratio of ~3 : 1 in favor of branch A. Previously, this has been indirectly demonstrated using pulsed absorption spectroscopy and more directly by measuring the decay modulation frequencies of electron spin echo signals (electron spin echo envelope modulation, ESEEM), which allows to determine the distance between the separated charges of the primary electron donor P₇₀₀⁺ and phylloquinone acceptors A_1A^- and A_1B^- in the symmetric redox cofactors branches A and B. In the present work, these distances were determined using ESEEM in PSI complexes lacking three 4Fe-4S clusters, F_X, F_A, and F_B, and the PsaC protein subunit (the so-called P₇₀₀-A₁ core), in which phylloquinone molecules A_1A and A_1B serve as the terminal electron acceptors. It was shown that in the P₇₀₀-A₁ core preparations, the average distance between the centers of the P₇₀₀⁺A₁^- ion-radical pair at a temperature of 150 K in an aqueous glycerol solution and in a dried trehalose matrix, as well as in a trehalose matrix at 280 K, is ~25.5 Å, which corresponds to the symmetrical electron transfer along the A and B branches of redox cofactors at a ratio of 1 : 1. Possible reasons for the change in the electron transfer symmetry in PSI upon removal of the PsaC subunit and 4Fe-4S clusters F_X, F_A, and F_B are discussed.

Current state of the primary charge separation mechanism in photosystem I of cyanobacteria

This review analyzes new data on the mechanism of ultrafast reactions of primary charge separation in photosystem I (PS I) of cyanobacteria obtained in the last decade by methods of femtosecond absorption spectroscopy. Cyanobacterial PS I from many species harbours 96 chlorophyll a (Chl a) molecules, including six specialized Chls denoted Chl1A/Chl1B (dimer P700, or PAPB), Chl2A/Chl2B, and Chl3A/Chl3B arranged in two branches, which participate in electron transfer reactions. The current data indicate that the primary charge separation occurs in a symmetric exciplex, where the special pair P700 is electronically coupled to the symmetrically located monomers Chl2A and Chl2B, which can be considered together as a symmetric exciplex Chl2APAPBChl2B with the mixed excited (Chl2APAPBChl2B)* and two charge-transfer states P700 +Chl2A - and P700 +Chl2B -. The redistribution of electrons between the branches in favor of the A-branch occurs after reduction of the Chl2A and Chl2B monomers. The formation of charge-transfer states and the symmetry breaking mechanisms were clarified by measuring the electrochromic Stark shift of β-carotene and the absorption dynamics of PS I complexes with the genetically altered Chl 2B or Chl 2A monomers. The review gives a brief description of the main methods for analyzing data obtained using femtosecond absorption spectroscopy. The energy levels of excited and charge-transfer intermediates arising in the cyanobacterial PS I are critically analyzed.

Cofactor Dynamics Couples the Protein Surface to the Heme in Cytochrome c, Facilitating Electron Transfer

Electron transport through biomolecules and in biological transport networks is of great importance to bioenergetics and biocatalysis. More generally, it is of crucial importance to understand how the pathways that connect buried metallocofactors to other cofactors, and to protein surfaces, affect the biological chemistry of metalloproteins. In terms of electron transfer (ET), the strongest coupling pathways usually comprise covalent and hydrogen bonded networks, with a limited number of through-space contacts. Herein, we set out to determine the relative roles of hydrogen bonds involved in ET via an established heme-to-surface tunneling pathway in cytochrome (cyt) c (i.e., heme-W59-D60-E61-N62). A series of cyt c variants were produced where a ruthenium tris(diimine) photooxidant was placed at position 62 via covalent modification of the N62C residue. Surprisingly, variants where the H-bonding residues W59 and D60 were replaced (i.e., W59F and D60A) showed no change in ET rate from the ferrous heme to Ru(III). In contrast, changing the composition of an alternative tunneling pathway (i.e., heme-M64-N63-C62) with the M64L substitution shows a factor of 2 decrease in the rate of heme-to-Ru ET. This pathway involves a through-space tunneling step between the heme and M64 residue, and such steps are usually disfavored. To rationalize why the heme-M64-N63-C62 is preferred, molecular dynamics (MD) simulations and Pathways analysis were employed. These simulations show that the change in heme-Ru ET rates is attributed to different conformations with compressed donor-acceptor distances, by ∼2 Å in pathway distance, in the M64-containing protein as compared to the M64L protein. The change in distance is correlated with changes in the electronic coupling that are in accord with the experimentally observed heme-Ru ET rates. Remarkably, the M64L variation at the core of the protein translates to changes in cofactor dynamics at the protein surface. The surface changes identified by MD simulations include dynamic anion-π and dipole-dipole interactions. These interactions influence the strength of tunneling pathways and ET rates by facilitating decreases in through-space tunneling distances in key coupling pathways.

Electron Transfer in a Bio-Photoelectrode Based on Photosystem I Multilayer Immobilized on the Conducting Glass

A film of ~40 layers of partially oriented photosystem I (PSI) complexes isolated from the red alga Cyanidioschyzon merolae formed on the conducting glass through electrodeposition was investigated by time-resolved absorption spectroscopy and chronoamperometry. The experiments were performed at a range of electric potentials applied to the film and at different compositions of electrolyte solution being in contact with the film. The amount of immobilized proteins supporting light-induced charge separation (active PSI) ranged from ~10%, in the absence of any reducing agents (redox compounds or low potential), to ~20% when ascorbate and 2,6-dichlorophenolindophenol were added, and to ~35% when the high negative potential was additionally applied. The origin of the large fraction of permanently inactive PSI (65–90%) was unclear. Both reducing agents increased the subpopulation of active PSI complexes, with the neutral P700 primary electron donor, by reducing significant fractions of the photo-oxidized P700 species. The efficiencies of light-induced charge separation in the PSI film (10–35%) did not translate into an equally effective generation of photocurrent, whose internal quantum efficiency reached the maximal value of 0.47% at the lowest potentials. This mismatch indicates that the vast majority of the charge-separated states in multilayered PSI complexes underwent charge recombination.

Mechanism of Mixed-Valence Fe2.5+···Fe2.5+ Formation in Fe4S4 Clusters in the Ferredoxin Binding Motif

Most low-potential Fe₄S₄ clusters exist in the conserved binding sequence CxxCxxC (CⁿCⁿ⁺³Cⁿ⁺⁶). Fe(II) and Fe(III) at the first (Cⁿ) and third (Cⁿ⁺⁶) cysteine ligand sites form a mixed-valence Fe^2.5+···Fe^2.5+ pair in the reduced Fe(II)₃Fe(III) cluster. Here, we investigate the mechanism of how the conserved protein environment induces mixed-valence pair formation in the Fe₄S₄ clusters, F_X, F_A, and F_B in photosystem I, using a quantum mechanical/molecular mechanical approach. Exchange coupling between Fe sites is predominantly determined by the shape of the Fe₄S₄ cluster, which is stabilized by the preorganized protein electrostatic environment. The backbone NH and CO groups in the conserved CxxCxxC and adjacent helix regions orient along the Fe^Cn···Fe^C(n+6) axis, generating an electric field and stabilizing the Fe^Cn(II)Fe^C(n+6)(III) state in F_A and F_B. The overlap of the d orbitals via -S- (superexchange) is observed for the single Fe^Cn(II)···Fe^C(n+6)(III) pair, leading to the formation of the mixed-valence Fe^2.5+···Fe^2.5+ pair. In contrast, several superexchange Fe(II)···Fe(III) pairs are observed in F_X due to the highly symmetric pair of the CDGPGRGGTC sequences. This is likely the origin of F_X serving as an electron acceptor in the two electron transfer branches.

Structural basis for the absence of low-energy chlorophylls in a photosystem I trimer from Gloeobacter violaceus

Photosystem I (PSI) is a multi-subunit pigment-protein complex that functions in light-harvesting and photochemical charge-separation reactions, followed by reduction of NADP to NADPH required for CO₂ fixation in photosynthetic organisms. PSI from different photosynthetic organisms has a variety of chlorophylls (Chls), some of which are at lower-energy levels than its reaction center P700, a special pair of Chls, and are called low-energy Chls. However, the sites of low-energy Chls are still under debate. Here, we solved a 2.04-Å resolution structure of a PSI trimer by cryo-electron microscopy from a primordial cyanobacterium Gloeobacter violaceus PCC 7421, which has no low-energy Chls. The structure shows the absence of some subunits commonly found in other cyanobacteria, confirming the primordial nature of this cyanobacterium. Comparison with the known structures of PSI from other cyanobacteria and eukaryotic organisms reveals that one dimeric and one trimeric Chls are lacking in the Gloeobacter PSI. The dimeric and trimeric Chls are named Low1 and Low2, respectively. Low2 is missing in some cyanobacterial and eukaryotic PSIs, whereas Low1 is absent only in Gloeobacter. These findings provide insights into not only the identity of low-energy Chls in PSI, but also the evolutionary changes of low-energy Chls in oxyphototrophs.

Computational Approach for Probing Redox Potential for Iron-Sulfur Clusters in Photosystem I

Photosystem I is a light-driven electron transfer device. Available X-ray crystal structure from Thermosynechococcus elongatus showed that electron transfer pathways consist of two nearly symmetric branches of cofactors converging at the first iron-sulfur cluster F_X, which is followed by two terminal iron-sulfur clusters F_A and F_B. Experiments have shown that F_X has lower oxidation potential than F_A and F_B, which facilitates the electron transfer reaction. Here, we use density functional theory and Multi-Conformer Continuum Electrostatics to explain the differences in the midpoint Em potentials of the F_X, F_A and F_B clusters. Our calculations show that F_X has the lowest oxidation potential compared to F_A and F_B due to strong pairwise electrostatic interactions with surrounding residues. These interactions are shown to be dominated by the bridging sulfurs and cysteine ligands, which may be attributed to the shorter average bond distances between the oxidized Fe ion and ligating sulfurs for F_X compared to F_A and F_B. Moreover, the electrostatic repulsion between the 4Fe-4S clusters and the positive potential of the backbone atoms is lowest for F_X compared to both F_A and F_B. These results agree with the experimental measurements from the redox titrations of low-temperature EPR signals and of room temperature recombination kinetics.

Competition between intra-protein charge recombination and electron transfer outside photosystem I complexes used for photovoltaic applications

Photosystem I (PSI) complexes isolated from three different species were electrodeposited on FTO conducting glass, forming a photoactive multilayer of the photo-electrode, for investigation of intricate electron transfer (ET) properties in such green hybrid nanosystems. The internal quantum efficiency of photo-electrochemical cells (PEC) containing the PSI-based photo-electrodes did not exceed ~ 0.5%. To reveal the reason for such a low efficiency of photocurrent generation, the temporal evolution of the transient concentration of the photo-oxidized primary electron donor, P⁺, was studied in aqueous suspensions of the PSI complexes by time-resolved absorption spectroscopy. The results of these measurements provided the information on: (1) completeness of charge separation in PSI reaction centers (RCs), (2) dynamics of internal charge recombination, and (3) efficiency of electron transfer from PSI to the electrolyte, which is the reaction competing with the internal charge recombination in the PSI RC. The efficiency of the full charge separation in the PSI complexes used for functionalization of the electrodes was ~ 90%, indicating that incomplete charge separation was not the main reason for the small yield of photocurrents. For the PSI particles isolated from a green alga Chlamydomonas reinhardtii, the probability of ET outside PSI was ~ 30-40%, whereas for their counterparts isolated from a cyanobacterium Synechocystis sp. PCC 6803 and a red alga Cyanidioschyzon merolae, it represented a mere ~ 4%. We conclude from the transient absorption data for the PSI biocatalysts in solution that the observed small photocurrent efficiency of ~ 0.5% for all the PECs analyzed in this study is likely due to: (1) limited efficiency of ET outside PSI, particularly in the case of PECs based on PSI from Synechocystis and C. merolae, and (2) the electrolyte-mediated electric short-circuiting in PSI particles forming the photoactive layer, particularly in the case of the C. reinhardtii PEC.

Structure of a photosystem I-ferredoxin complex from a marine cyanobacterium provides insights into far-red light photoacclimation

Far-red light photoacclimation exhibited by some cyanobacteria allows these organisms to use the far-red region of the solar spectrum (700-800 nm) for photosynthesis. Part of this process includes the replacement of six photosystem I (PSI) subunits with isoforms that confer the binding of chlorophyll (Chl) f molecules that absorb far-red light (FRL). However, the exact sites at which Chl f molecules are bound are still challenging to determine. To aid in the identification of Chl f-binding sites, we solved the cryo-EM structure of PSI from far-red light-acclimated cells of the cyanobacterium Synechococcus sp. PCC 7335. We identified six sites that bind Chl f with high specificity and three additional sites that are likely to bind Chl f at lower specificity. All of these binding sites are in the core-antenna regions of PSI, and Chl f was not observed among the electron transfer cofactors. This structural analysis also reveals both conserved and nonconserved Chl f-binding sites, the latter of which exemplify the diversity in FRL-PSI among species. We found that the FRL-PSI structure also contains a bound soluble ferredoxin, PetF1, at low occupancy, which suggests that ferredoxin binds less transiently than expected according to the canonical view of ferredoxin-binding to facilitate electron transfer. We suggest that this may result from structural changes in FRL-PSI that occur specifically during FRL photoacclimation.

Excitation-energy transfer in heterocysts isolated from the cyanobacterium Anabaena sp. PCC 7120 as studied by time-resolved fluorescence spectroscopy

Heterocysts are formed in filamentous heterocystous cyanobacteria under nitrogen-starvation conditions, and possess a very low amount of photosystem II (PSII) complexes than vegetative cells. Molecular, morphological, and biochemical characterizations of heterocysts have been investigated; however, excitation-energy dynamics in heterocysts are still unknown. In this study, we examined excitation-energy-relaxation processes of pigment-protein complexes in heterocysts isolated from the cyanobacterium Anabaena sp. PCC 7120. Thylakoid membranes from the heterocysts showed no oxygen-evolving activity under our experimental conditions and no thermoluminescence-glow curve originating from charge recombination of S₂Q_A^-. Two dimensional blue-native/SDS-PAGE analysis exhibits tetrameric, dimeric, and monomeric photosystem I (PSI) complexes but almost no dimeric and monomeric PSII complexes in the heterocyst thylakoids. The steady-state fluorescence spectrum of the heterocyst thylakoids at 77 K displays both characteristic PSI fluorescence and unusual PSII fluorescence different from the fluorescence of PSII dimer and monomer complexes. Time-resolved fluorescence spectra at 77 K, followed by fluorescence decay-associated spectra, showed different PSII and PSI fluorescence bands between heterocysts and vegetative thylakoids. Based on these findings, we discuss excitation-energy-transfer mechanisms in the heterocysts.

Dynamics of photoconversion processes: the energetic cost of lifetime gain in photosynthetic and photovoltaic systems

The continued development of solar energy conversion technologies relies on an improved understanding of their limitations. In this review, we focus on a comparison of the charge carrier dynamics underlying the function of photovoltaic devices with those of both natural and artificial photosynthetic systems. The solar energy conversion efficiency is determined by the product of the rate of generation of high energy species (charges for solar cells, chemical fuels for photosynthesis) and the energy contained in these species. It is known that the underlying kinetics of the photophysical and charge transfer processes affect the production yield of high energy species. Comparatively little attention has been paid to how these kinetics are linked to the energy contained in the high energy species or the energy lost in driving the forward reactions. Here we review the operational parameters of both photovoltaic and photosynthetic systems to highlight the energy cost of extending the lifetime of charge carriers to levels that enable function. We show a strong correlation between the energy lost within the device and the necessary lifetime gain, even when considering natural photosynthesis alongside artificial systems. From consideration of experimental data across all these systems, the emprical energetic cost of each 10-fold increase in lifetime is 87 meV. This energetic cost of lifetime gain is approx. 50% greater than the 59 meV predicted from a simple kinetic model. Broadly speaking, photovoltaic devices show smaller energy losses compared to photosynthetic devices due to the smaller lifetime gains needed. This is because of faster charge extraction processes in photovoltaic devices compared to the complex multi-electron, multi-proton redox reactions that produce fuels in photosynthetic devices. The result is that in photosynthetic systems, larger energetic costs are paid to overcome unfavorable kinetic competition between the excited state lifetime and the rate of interfacial reactions. We apply this framework to leading examples of photovoltaic and photosynthetic devices to identify kinetic sources of energy loss and identify possible strategies to reduce this energy loss. The kinetic and energetic analyses undertaken are applicable to both photovoltaic and photosynthetic systems allowing for a holistic comparison of both types of solar energy conversion approaches.

Light as a substrate: migration of LHCII antennas in extended Michaelis-Menten model for PSI kinetics

We extended, for the first time, the Michaelis-Menten (M-M) model to describe the kinetics of photosystem I (PSI) complexes using light as a substrate. Our work is novel as it can be useful for studying the phenomenon of "state transitions" because it quantifies the affinity of light to PSI reaction centers depending on the associated light harvesting complex II (LHCII) antennas. We verified our models by measuring the PSI activity as a function of light intensity using an oxygen electrode for chloroplast from plants grown in low light conditions and treated with far red light. We determined the kinetics constant K_M for: PSI-LHCI, PSI-LHCI-LHCII and PSI-PSII megacomplexes and have shown that K_M for PSI located in the megacomplexes was smaller in magnitude than PSI-LHCI, thus demonstrating that LHCII antennas are functionally associated with PSI. The parameter [S]_1/2used in our models is the equivalent of M-M constant. Far red light increases [S]_1/2, which indicates that transition from state 1 to state 2 leads to an energy gain while reaching the PSI reaction centers. We also observed that redistribution of the absorbed excitation energy is realized not only by LHCII migration but also by association of the photosystems in the megacomplexes.

Fullerenes Enhance Self-Assembly and Electron Injection of Photosystem I in Biophotovoltaic Devices

This paper describes the fabrication of microfluidic devices with a focus on controlling the orientation of photosystem I (PSI) complexes, which directly affects the performance of biophotovoltaic devices by maximizing the efficiency of the extraction of electron/hole pairs from the complexes. The surface chemistry of the electrode on which the complexes assemble plays a critical role in their orientation. We compared the degree of orientation on self-assembled monolayers of phenyl-C61-butyric acid and a custom peptide on nanostructured gold electrodes. Biophotovoltaic devices fabricated with the C61 fulleroid exhibit significantly improved performance and reproducibility compared to those utilizing the peptide, yielding a 1.6-fold increase in efficiency. In addition, the C61-based devices were more stable under continuous illumination. Our findings show that fulleroids, which are well-known acceptor materials in organic photovoltaic devices, facilitate the extraction of electrons from PSI complexes without sacrificing control over the orientation of the complexes, highlighting this combination of traditional organic semiconductors with biomolecules as a viable approach to coopting natural photosynthetic systems for use in solar cells.

A unique photosystem I reaction center from a chlorophyll d-containing cyanobacterium Acaryochloris marina

Photosystem I (PSI) is a large protein supercomplex that catalyzes the light-dependent oxidation of plastocyanin (or cytochrome c₆ ) and the reduction of ferredoxin. This catalytic reaction is realized by a transmembrane electron transfer chain consisting of primary electron donor (a special chlorophyll (Chl) pair) and electron acceptors A₀ , A₁ , and three Fe₄ S₄ clusters, F_X , F_A , and F_B . Here we report the PSI structure from a Chl d-dominated cyanobacterium Acaryochloris marina at 3.3 Å resolution obtained by single-particle cryo-electron microscopy. The A. marina PSI exists as a trimer with three identical monomers. Surprisingly, the structure reveals a unique composition of electron transfer chain in which the primary electron acceptor A₀ is composed of two pheophytin a rather than Chl a found in any other well-known PSI structures. A novel subunit Psa27 is observed in the A. marina PSI structure. In addition, 77 Chls, 13 α-carotenes, two phylloquinones, three Fe-S clusters, two phosphatidyl glycerols, and one monogalactosyl-diglyceride were identified in each PSI monomer. Our results provide a structural basis for deciphering the mechanism of photosynthesis in a PSI complex with Chl d as the dominating pigments and absorbing far-red light.

High-light modification of excitation-energy-relaxation processes in the green flagellate Euglena gracilis

Photosynthetic organisms finely tune their photosynthetic machinery including pigment compositions and antenna systems to adapt to various light environments. However, it is poorly understood how the photosynthetic machinery in the green flagellate Euglena gracilis is modified under high-light conditions. In this study, we examined high-light modification of excitation-energy-relaxation processes in Euglena cells. Oxygen-evolving activity in the cells incubated at 300 µmol photons m^-2 s^-1 (HL cells) cannot be detected, reflecting severe photodamage to photosystem II (PSII) in vivo. Pigment compositions in the HL cells showed relative increases in 9'-cis-neoxanthin, diadinoxanthin, and chlorophyll b compared with the cells incubated at 30 µmol photons m^-2 s^-1 (LL cells). Absolute fluorescence spectra at 77 K exhibit smaller intensities of the PSII and photosystem I (PSI) fluorescence in the HL cells than in the LL cells. Absolute fluorescence decay-associated spectra at 77 K of the HL cells indicate suppression of excitation-energy transfer from light-harvesting complexes (LHCs) to both PSI and PSII with the time constant of 40 ps. Rapid energy quenching in LHCs and PSII in the HL cells is distinctly observed by averaged Chl-fluorescence lifetimes. These findings suggest that Euglena modifies excitation-energy-relaxation processes in addition to pigment compositions to deal with excess energy. These results provide insights into the photoprotection strategies of this alga under high-light conditions.

Electron Acceptor-Donor Iron Sites in the Iron-Sulfur Cluster of Photosynthetic Electron-Transfer Pathways

In photosystem I, two electron-transfer pathways via quinones (A_1A and A_1B) are merged at the iron-sulfur Fe₄S₄ cluster F_X into a single pathway toward the other two Fe₄S₄ clusters F_A and F_B. Using a quantum mechanical/molecular mechanical approach, we identify the redox-active Fe sites in the clusters. In F_A and F_B, the Fe site, which does not belong to the CxxCxxCxxxCP motif, serves as an electron acceptor/donor. F_X has two independent electron acceptor Fe sites for A- and B-branch electron transfers, depending on the Asp-B575 protonation state, which causes the A_1A-to-F_X electron transfer to be uphill and the A_1B-to-F_X electron transfer to be downhill. The two asymmetric electron-transfer pathways from A₁ to F_X and the separation of the electron acceptor and donor Fe sites are likely associated with the specific role of F_X in merging the two electron transfer pathways into the single pathway.

Development of a Novel Nanoarchitecture of the Robust Photosystem I from a Volcanic Microalga Cyanidioschyzon merolae on Single Layer Graphene for Improved Photocurrent Generation

Here, we report the development of a novel photoactive biomolecular nanoarchitecture based on the genetically engineered extremophilic photosystem I (PSI) biophotocatalyst interfaced with a single layer graphene via pyrene-nitrilotriacetic acid self-assembled monolayer (SAM). For the oriented and stable immobilization of the PSI biophotocatalyst, an His6-tag was genetically engineered at the N-terminus of the stromal PsaD subunit of PSI, allowing for the preferential binding of this photoactive complex with its reducing side towards the graphene monolayer. This approach yielded a novel robust and ordered nanoarchitecture designed to generate an efficient direct electron transfer pathway between graphene, the metal redox center in the organic SAM and the photo-oxidized PSI biocatalyst. The nanosystem yielded an overall current output of 16.5 µA·cm-2 for the nickel- and 17.3 µA·cm-2 for the cobalt-based nanoassemblies, and was stable for at least 1 h of continuous standard illumination. The novel green nanosystem described in this work carries the high potential for future applications due to its robustness, highly ordered and simple architecture characterized by the high biophotocatalyst loading as well as simplicity of manufacturing.

Phylloquinone is the principal Mehler reaction site within photosystem I in high light

Photosynthesis is a vital process, responsible for fixing carbon dioxide, and producing most of the organic matter on the planet. However, photosynthesis has some inherent limitations in utilizing solar energy, and a part of the energy absorbed is lost in the reduction of O2 to produce the superoxide radical (O2•-) via the Mehler reaction, which occurs principally within photosystem I (PSI). For decades, O2 reduction within PSI was assumed to take place solely in the distal iron-sulfur clusters rather than within the two asymmetrical cofactor branches. Here, we demonstrate that under high irradiance, O2 photoreduction by PSI primarily takes place at the phylloquinone of one of the branches (the A-branch). This conclusion derives from the light dependency of the O2 photoreduction rate constant in fully mature wild-type PSI from Chlamydomonas reinhardtii, complexes lacking iron-sulfur clusters, and a mutant PSI, in which phyllosemiquinone at the A-branch has a significantly longer lifetime. We suggest that the Mehler reaction at the phylloquinone site serves as a release valve under conditions where both the iron-sulfur clusters of PSI and the mobile ferredoxin pool are highly reduced.

Photosynthesis: a multiscopic view

A recurring analogy for photosynthesis research is the fable of the blind men and the elephant. Photosynthesis has many complex working parts, which has driven the need to study each of them individually, with an inherent understanding that a more complete picture will require systematic integration of these views. However, unlike the blind men, who are limited to using their hands, researchers have developed over the past decades a repertoire of methods for studying these components, many of which capitalize on unique features intrinsic to each. More recent concerns about food security and clean, renewable energy have increased support for applied photosynthesis research, with the idea of either improving photosynthetic performance as a desired trait in select species or using photosynthetic measurements as a phenotyping tool in breeding efforts or for high precision crop management. In this review, we spotlight the migration of approaches for studying photosynthesis from the laboratory into field environments, highlight some recent advances and speculate on areas where further development would be fruitful, with an eye towards how applied photosynthesis research can have impacts at local and global scales.

A dimeric chlorophyll electron acceptor differentiates type I from type II photosynthetic reaction centers

This research addresses one of the most compelling issues in the field of photosynthesis, namely, the role of the accessory chlorophyll molecules in primary charge separation. Using a combination of empirical and computational methods, we demonstrate that the primary acceptor of photosystem (PS) I is a dimer of accessory and secondary chlorophyll molecules, Chl_2A and Chl_3A, with an asymmetric electron charge density distribution. The incorporation of highly coupled donors and acceptors in PS I allows for extensive delocalization that prolongs the lifetime of the charge-separated state, providing for high quantum efficiency. The discovery of this motif has widespread implications ranging from the evolution of naturally occurring reaction centers to the development of a new generation of highly efficient artificial photosynthetic systems.

Video abstract

Conserved residue PsaB-Trp673 is essential for high-efficiency electron transfer between the phylloquinones and the iron-sulfur clusters in Photosystem I

Despite the high level of symmetry between the PsaA and PsaB polypeptides in Photosystem I, some amino acids pairs are strikingly different, such as PsaA-Gly693 and PsaB-Trp673, which are located near a cluster of 11 water molecules between the A_1A and A_1B quinones and the F_X iron-sulfur cluster. In this work, we changed PsaB-Trp673 to PsaB-Phe673 in Synechocystis sp. PCC 6803. The variant contains ~ 85% of wild-type (WT) levels of Photosystem I but is unable to grow photoautotrophically. Both time-resolved and steady-state optical measurements show that in the PsaB-W673F variant less than 50% of the electrons reach the terminal iron-sulfur clusters F_A and F_B; the majority of the electrons recombine from A_1A^- and A_1B^-. However, in those reaction centers which pass electrons forward the transfer is heterogeneous: a minor population shows electron transfer rates from A_1A^- and A_1B^- to F_X slightly slower than that of the WT, whereas a major population shows forward electron transfer rates to F_X slowed to the ~ 10 µs time range. Competition between relatively similar forward and backward rates of electron transfer from the quinones to the F_X cluster account for the relatively low yield of long-lived charge separation in the PsaB-W673F variant. A higher water content and its increased mobility observed in MD simulations in the interquinone cavity of the PsaB-W673F variant shifts the pK of PsaB-Asp575 and allows its deprotonation in situ. The heterogeneity found may be rooted in protonation state of PsaB-Asp575, which controls whether electron transfer can proceed beyond the phylloquinone cofactors.

Chlorophyll-Based Organic Heterojunction on Ti3 C2 Tx MXene Nanosheets for Efficient Hydrogen Production

The Z-scheme process is a photoinduced electron-transfer pathway in natural oxygenic photosynthesis involving electron transport from photosystem II (PSII) to photosystem I (PSI). Inspired by the interesting Z-scheme process, herein a photocatalytic hydrogen evolution reaction (HER) employing chlorophyll (Chl) derivatives, Chl-1 and Chl-2, on the surface of Ti₃ C₂ T_x MXene with two-dimensional accordion-like morphology, forming Chl-1@Chl-2@Ti₃ C₂ T_x composite, is demonstrated. Due to the frontier molecular orbital energy alignments of Chl-1 and Chl-2, sublayer Chl-1 is a simulation of PSI, whereas upper layer Chl-2 is equivalent to PSII, and the resultant electron transport can take place from Chl-2 to Chl-1. Under the illumination of visible light (>420 nm), the HER performance of Chl-1@Chl-2@Ti₃ C₂ T_x photocatalyst was found to be as high as 143 μmol h^-1  g_cat ^-1 , which was substantially higher than that of photocatalysts of either Chl-1@Ti₃ C₂ T_x (20 μmol h^-1  g^-1 ) or Chl-2@Ti₃ C₂ T_x (15 μmol h^-1  g^-1 ).

Fast Photo-Chrono-Amperometry of Photosynthetic Complexes for Biosensors and Electron Transport Studies

Photosynthetic reactions in plants, algae, and cyanobacteria are driven by photosystem I and photosystem II complexes, which specifically reduce or oxidize partner redox biomolecules. Photosynthetic complexes can also bind synthetic organic molecules, which inhibit their photoactivity and can be used both to study the electron transport chain and as herbicides and algicides. Thus, their development, characterization, and sensing bears fundamental and applied interest. Substantial efforts have been devoted to developing photosensors based on photosystem II to detect compounds that bind to the plastoquinone sites of this complex. In comparison, photosystem I based sensors have received less attention and could be used to identify novel substances displaying phytotoxic effects, including those obtained from natural product extracts. We have developed a robust procedure to functionalize gold electrodes with photo- and redox-active photosystem I complexes based on transparent gold and a thiolate self-assembled monolayer, and we have obtained reproducible electrochemical photoresponses. Chronoamperometric recordings have allowed us to measure photocurrents in the presence of the viologen derivative paraquat at concentrations below 100 nM under lock-in operation and a sensor dynamic range spanning six orders of magnitude up to 100 mM. We have modeled their time course to identify the main electrochemical processes and limiting steps in the electron transport chain. Our results allow us to isolate the contributions from photosystem I and the redox mediator, and evaluate photocurrent features (spectral and power dependence, fast transient kinetics) that could be used as a sensing signal to detect other inhibitors and modulators of photosystem I activity.

Primary charge separation within the structurally symmetric tetrameric Chl2APAPBChl2B chlorophyll exciplex in photosystem I

In Photosystem I (PS I), the role of the accessory chlorophyll (Chl) molecules, Chl_2A and Chl_2B (also termed A_-1A and A_-1B), which are directly adjacent to the special pair P₇₀₀ and fork into the A- and B-branches of electron carriers, is incompletely understood. In this work, the Chl_2A and Chl_2B transient absorption ΔA₀(λ) at a time delay of 100 fs was identified by ultrafast pump-probe spectroscopy in three pairs of PS I complexes from Synechocystis sp. PCC 6803 with residues PsaA-N600 or PsaB-N582 (which ligate Chl_2B or Chl_2A through a H₂O molecule) substituted by Met, His, and Leu. The ΔA₀(λ) spectra were quantified using principal component analysis, the main component of which was interpreted as a mutation-induced shift of the equilibrium between the excited state of primary donor P₇₀₀^⁎ and the primary charge-separated state P₇₀₀⁺Chl₂^-. This equilibrium is shifted to the charge-separated state in wild-type PS I and to the excited P₇₀₀ in the PS I complexes with the substituted ligands to the Chl_2A and Chl_2B monomers. The results can be rationalized within the framework of an adiabatic model in which the P₇₀₀ is electronically coupled with the symmetrically arranged monomers Chl_2A and Chl_2B; such a structure can be considered a symmetric tetrameric exciplex Chl_2AP_AP_BChl_2B, in which the excited state (Chl_2AP_AP_BChl_2B)* is mixed with two charge-transfer states P₇₀₀⁺Chl_2A^- and P₇₀₀⁺Chl_2B^-. The electron redistribution between the two branches in favor of the A-branch apparently takes place in the picosecond time scale after reduction of the Chl_2A and Chl_2B monomers.

Fabrication of Electronic Junctions between Oriented Multilayers of Photosystem I and the Electrodes of Optoelectronic Solid-State Devices

The efficient optoelectronic properties of photosynthetic proteins were explored in the quest for the fabrication of novel solid biohybrid devices. As most optoelectronic devices function in a dry environment, an attempt was made to fabricate an efficient electronic junction by covalent binding of photosynthetic reaction center proteins to metals, transparent semiconductor polymers, and solid semiconductors that function in a dry environment. The primary stages of photosynthesis take place in nanometric-size protein-chlorophyll complexes. Such is photosystem I (PSI), which generates a photovoltage of 1 V. The isolated PSI generates an absorbed light-energy conversion efficiency of ∼47% (∼23% solar energy) and internal quantum efficiency of ∼100%. The robust cyanobacterial PSI was used in the fabrication of solid-state optoelectronic devices by forming oriented multilayers from genetically engineered cysteine mutants between metal and transparent conducting semiconductor electrodes. Current-voltage measurements of the cells generated diode- and photodiode-like responses in the dark and the light, respectively. The cells were stable for many months in a dry environment. The generation of photocurrent and V_oc indicated the formation of good electronic coupling between PSI and the electrodes, which can serve in the fabrication of solid-state biohybrid optoelectronic devices.