Three-dimensional structure of cyanobacterial photosystem I at 2.5 A resolution

Isomerization of carotenoids in photosynthesis and metabolic adaptation

In nature, carotenoids are present as trans- and cis-isomers. Various physical and chemical factors like light, heat, acids, catalytic agents, and photosensitizers can contribute to the isomerization of carotenoids. Living organisms in the process of evolution have developed different mechanisms of adaptation to light stress, which can also involve isomeric forms of carotenoids. Particularly, light stress conditions can enhance isomerization processes. The purpose of this work is to review the recent studies on cis/trans isomerization of carotenoids as well as the role of carotenoid isomers for the light capture, energy transfer, photoprotection in light-harvesting complexes, and reaction centers of the photosynthetic apparatus of plants and other photosynthetic organisms. The review also presents recent studies of carotenoid isomers for the biomedical aspects, showing cis- and trans-isomers differ in bioavailability, antioxidant activity and biological activity, which can be used for therapeutic and prophylactic purposes.

Time-resolved FTIR difference spectroscopy for the study of photosystem I with high potential naphthoquinones incorporated into the A1 binding site 2: Identification of neutral state quinone bands

Time-resolved step-scan FTIR difference spectroscopy at 77 K has been used to study photosystem I (PSI) from Synechocystis sp. PCC 6803 with four high-potential, 1,4-naphthoquinones (NQs) incorporated into the A1 binding site. The incorporated quinones are 2-chloro-NQ (2ClNQ), 2-bromo-NQ (2BrNQ), 2,3-dichloro-NQ (Cl2NQ), and 2,3-dibromo-NQ (Br2NQ). For completeness 2-methyl-NQ (2MNQ) was also incorporated and studied. Previously, PSI with the same quinones incorporated were studied in the, so-called, anion spectral region between 1550 and 1400 cm-1 (Agarwala et al. in Biochim Biophys Acta 1864(1):148918, 2023). Here we focus on spectra in the previously unexplored 1400-1200 cm-1 spectral region. In this region several bands are identified and assigned to the neutral state of the incorporated quinones. This is important as identification of neutral state quinone bands in the regular 1700-1600 cm-1 region has proven difficult in the past. For neutral PhQ in PSI a broad, intense band appears at ~ 1300 cm-1. For the symmetric di-substituted NQs (Cl2NQ/Br2NQ) a single intense neutral state band is found at ~ 1280/1269 cm-1, respectively. For both mono-substituted NQs, 2ClNQ and 2BrNQ, however, two neutral state bands are observed at ~ 1280 and ~ 1250 cm-1, respectively. These observations from time-resolved spectra agree well with conclusions drawn from absorption spectra of the NQs in THF, which are also presented here. Density functional theory based vibrational frequency calculations were undertaken allowing an identification of the normal modes associated with the neutral state quinone bands.

Energy transfer and trapping in photosystem I with and without chlorophyll-f

We establish a general kinetic scheme for energy transfer and trapping in the photosystem I (PSI) of cyanobacteria grown under white light (WL) or far-red light (FRL) conditions. With the help of simultaneous target analysis of all emission and transient absorption datasets measured in five cyanobacterial strains, we resolved the spectral and kinetic properties of the different species present in PSI. WL-PSI can be described by Bulk Chl a, two Red Chl a, and a reaction center compartment (WL-RC). The FRL-PSI contains two additional Chl f compartments. The lowest excited state of the FRL-RC is downshifted by ≈ 29 nm. The rate of charge separation drops from ≈900 ns-1 in WL-RC to ≈300 ns-1 in FRL-RC. The delayed trapping in the FRL-PSI (≈130 ps) is explained by uphill energy transfer from the Chl f compartments with Gibbs free energies of ≈kBT below that of the FRL-RC.

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.

Lipidome of the Brown Macroalga Undaria pinnatifida: Influence of Season and Endophytic Infection

An analysis of the lipidome of the brown alga Undaria pinnatifida (Laminariales) was performed' more than 900 molecular species were identified in 12 polar lipids and 1 neutral lipid using HPLC/MS-MS. The seasonal changes of U. pinnatifida lipidome were determined. It was shown that acclimatization to winter and spring was accompanied by an increase in the unsaturation of both polar and neutral lipids. In autumn and summer, on the contrary, the contents of more saturated molecular species of all lipid classes increased. Based on the data obtained, a scheme for the polar and neutral lipid synthesis in brown algae was proposed. In addition, the influence of infection with the brown filamentous endophyte Laminariocolax aecidioides (Ectocarpales) on U. pinnatifida lipidome was studied. It was found that infection has the most noticeable effect on the molecular species composition of triacylglycerides, phosphatidylglycerol, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylhydroxyethylglycine of the host macrophyte. In infected samples of algae, changes in the composition of triacylglycerides were revealed both in areas with the presence of an endophyte and in adjacent intact tissues, which may indicate the occurrence of a secondary infection.

The Ycf48 accessory factor occupies the site of the oxygen-evolving manganese cluster during photosystem II biogenesis

Robust oxygenic photosynthesis requires a suite of accessory factors to ensure efficient assembly and repair of the oxygen-evolving photosystem two (PSII) complex. The highly conserved Ycf48 assembly factor binds to the newly synthesized D1 reaction center polypeptide and promotes the initial steps of PSII assembly, but its binding site is unclear. Here we use cryo-electron microscopy to determine the structure of a cyanobacterial PSII D1/D2 reaction center assembly complex with Ycf48 attached. Ycf48, a 7-bladed beta propeller, binds to the amino-acid residues of D1 that ultimately ligate the water-oxidising Mn4CaO5 cluster, thereby preventing the premature binding of Mn2+ and Ca2+ ions and protecting the site from damage. Interactions with D2 help explain how Ycf48 promotes assembly of the D1/D2 complex. Overall, our work provides valuable insights into the early stages of PSII assembly and the structural changes that create the binding site for the Mn4CaO5 cluster.

Diurnally dynamic iron allocation promotes N2 fixation in marine dominant diazotroph Trichodesmium

Trichodesmium is the dominant photoautotrophic dinitrogen (N₂) fixer (diazotroph) in the ocean. Iron is an important factor limiting growth of marine diazotrophs including Trichodesmium mainly because of high iron content of its N₂-fixing enzyme, nitrogenase. However, it still lacks a quantitative understanding of how dynamic iron allocation among physiological processes acts to regulate growth and N₂ fixation in Trichodesmium. Here, we constructed a model of Trichodesmium trichome in which intracellular iron could be dynamically re-allocated in photosystems and nitrogenase during the daytime. The results demonstrate that the dynamic iron allocation enhances modeled N₂ fixation and growth rates of Trichodesmium, especially in iron-limited conditions, albeit having a marginal impact under high iron concentrations. Although the reuse of iron during a day is an apparent cause that dynamic iron allocation can benefit Trichodesmium under iron limitation, our model reveals two important mechanisms. First, the release of iron from photosystems downregulates the intracellular oxygen (O₂) production and reduces the demand of respiratory protection, a process that Trichodesmium wastefully respires carbohydrates to create a lower O₂ window for N₂ fixation. Hence, more carbohydrates can be used in growth. Second, lower allocation of iron to nitrogenase during early daytime, a period when photosynthesis is active and intracellular O₂ is high, reduces the amount of iron that is trapped in the inactivated nitrogenase induced by O₂. This mechanism further increases the iron use efficiency in Trichodesmium. Overall, our study provides mechanistic and quantitative insight into the diurnal iron allocation that can alleviate iron limitation to Trichodesmium.

Calculated vibrational properties of pigments in protein binding sites 2: Semiquinones in photosynthetic proteins

[Q_A^- - Q_A] Fourier transform infrared difference spectra have previously been obtained using purple bacterial reaction centers from Rhodobacter sphaeroides with unlabeled, ¹⁸O and ¹³C isotope labeled phylloquinone (PhQ, also known as vitamin K₁) incorporated into the Q_A protein binding site (Breton, (1997), Proc. Natl. Acad. Sci. USA94 11318-11323). The nature of the bands in these spectra and the isotope induced band shifts are poorly understood, especially for the phyllosemiquinone anion (PhQ^-) state. To aid in the interpretation of the bands in these experimental spectra, ONIOM type QM/MM vibrational frequency calculations were undertaken. Calculations were also undertaken for PhQ^- in solution. Surprisingly, both sets of calculated spectra are similar and agree well with the experimental spectra. This similarity suggests pigment-protein interactions do not perturb the electronic structure of the semiquinone in the Q_A binding site. This is not found to be the case for the neutral PhQ species in the same protein binding site. PhQ also occupies the A₁ protein binding site in photosystem I, and the vibrational properties of PhQ^- in the Q_A and A₁ binding sites are compared and shown to exhibit considerable differences. These differences probably arise because of changes in the degree of asymmetry of hydrogen bonding of PhQ^- in the A₁ and Q_A binding sites.

Redox Potentials of Iron-Sulfur Clusters in Type I Photosynthetic Reaction Centers

The electron transfer pathways in type I photosynthetic reaction centers, such as photosystem I (PSI) and reaction centers from green sulfur bacteria (GsbRC), are terminated by two Fe4S4 clusters, FA and FB. The protein structures are the basis of understanding how the protein electrostatic environment interacts with the Fe4S4 clusters and facilitates electron transfer. Using the protein structures, we calculated the redox potential (Em) values for FA and FB in PSI and GsbRC, solving the linear Poisson-Boltzmann equation. The FA-to-FB electron transfer is energetically downhill in the cyanobacterial PSI structure, while it is isoenergetic in the plant PSI structure. The discrepancy arises from differences in the electrostatic influences of conserved residues, including PsaC-Lys51 and PsaC-Arg52, located near FA. The FA-to-FB electron transfer is slightly downhill in the GsbRC structure. Em(FA) and Em(FB) exhibit similar levels upon isolation of the membrane-extrinsic PsaC and PscB subunits from the PSI and GsbRC reaction centers, respectively. The binding of the membrane-extrinsic subunit at the heterodimeric/homodimeric reaction center plays a key role in tuning Em(FA) and Em(FB).

Simulating optical linear absorption for mesoscale molecular aggregates: An adaptive hierarchy of pure states approach

In this paper, we present dyadic adaptive HOPS (DadHOPS), a new method for calculating linear absorption spectra for large molecular aggregates. This method combines the adaptive HOPS (adHOPS) framework, which uses locality to improve computational scaling, with the dyadic HOPS method previously developed to calculate linear and nonlinear spectroscopic signals. To construct a local representation of dyadic HOPS, we introduce an initial state decomposition that reconstructs the linear absorption spectra from a sum over locally excited initial conditions. We demonstrate the sum over initial conditions can be efficiently Monte Carlo sampled and that the corresponding calculations achieve size-invariant [i.e., O(1)] scaling for sufficiently large aggregates while trivially incorporating static disorder in the Hamiltonian. We present calculations on the photosystem I core complex to explore the behavior of the initial state decomposition in complex molecular aggregates as well as proof-of-concept DadHOPS calculations on an artificial molecular aggregate inspired by perylene bis-imide to demonstrate the size-invariance of the method.

Structural insights into a unique PSI-LHCI-LHCII-Lhcb9 supercomplex from moss Physcomitrium patens

Photosystem I (PSI) possesses a variable supramolecular organization among different photosynthetic organisms to adapt to different light environments. Mosses are evolutionary intermediates that diverged from aquatic green algae and evolved into land plants. The moss Physcomitrium patens (P. patens) has a light-harvesting complex (LHC) superfamily more diverse than those of green algae and higher plants. Here, we solved the structure of a PSI-LHCI-LHCII-Lhcb9 supercomplex from P. patens at 2.68 Å resolution using cryo-electron microscopy. This supercomplex contains one PSI-LHCI, one phosphorylated LHCII trimer, one moss-specific LHC protein, Lhcb9, and one additional LHCI belt with four Lhca subunits. The complete structure of PsaO was observed in the PSI core. One Lhcbm2 in the LHCII trimer interacts with PSI core through its phosphorylated N terminus, and Lhcb9 mediates assembly of the whole supercomplex. The complicated pigment arrangement provided important information for possible energy-transfer pathways from the peripheral antennae to the PSI core.

Electron transport in cyanobacterial thylakoid membranes: Are cyanobacteria simple models for photosynthetic organisms?

Cyanobacteria are structurally the simplest oxygenic phototrophs, which makes it difficult to understand the regulation of photosynthesis because the photosynthetic and respiratory processes share the same thylakoid membranes and cytosolic space. This review aimed to summarise the molecular mechanisms and in vivo activities of electron transport in cyanobacterial thylakoid membranes based on the latest progress in photosynthesis research in cyanobacteria. Photosynthetic linear electron transport for CO2 assimilation has the dominant electron flux in the thylakoid membranes. The capacity of O2 photoreduction in cyanobacteria is comparable to the photosynthetic CO2 assimilation, which is mediated by flavodiiron proteins. Additionally, cyanobacterial thylakoid membranes harbour the significant electron flux of respiratory electron transport through a homologue of respiratory complex I, which is also recognized as the part of cyclic electron transport chain if it is coupled with photosystem I in the light. Further, O2-independent alternative electron transports through hydrogenase and nitrate reductase function with reduced ferredoxin as the electron donor. Whereas all these electron transports are recently being understood one by one, the complexity as the whole regulatory system remains to be uncovered in near future.

Chiral-phase HPLC separation of (divinyl-)protochlorophyllide-a enantiomers as key precursors in chlorophyll biosynthesis from their 132-stereoisomeric prime forms

Protochlorophyllide(PChlide)-a and its 8-vinylated analog, divinyl(DV)-PChlide-a, are common and essential intermediates in the biosynthesis of all naturally occurring chlorophyll (Chl) pigments. These porphyrinoid-type pigments have a single optically active (asymmetric) carbon atom at the 13²-position, so their stereoisomers are (13²R)- and (13²S)-enantiomers. The former and latter are called (DV-)PChlide-a and (DV-)PChlide-a', respectively. In this study, chiral-phase HPLC separation of enantiomeric (DV-)PChlides-a/a' was demonstrated. The (13²R)-enantiomeric PChlide-a was eluted more slowly than the corresponding (13²S)-enantiomeric PChlide-a' under the present HPLC conditions. On the other hand, the elution order of (13²R)-DV-PChlide-a and (13²S)-DV-PChlide-a' was reverse to that of PChlides-a/a'. After the separation of each enantiomer by the chiral-phase HPLC, the stereoisomeric configuration at the 13²-position was characterized by means of circular dichroism spectroscopy. The present chiral-phase HPLC method enables us to evaluate optical purities of (DV-)PChlide-a species. For example, PChlide-a and/or DV-PChlide-a extracted from the spent medium and harvested cells of cultured purple photosynthetic bacterial mutants, the former of which has been often used as the source of (DV-)PChlide-a substrates for enzymatic reactions, were revealed to be mostly racemized, giving enantiomeric mixtures of (DV-)PChlides-a/a'.

Mass spectrometry and spectroscopic characterization of a tetrameric photosystem I supercomplex from Leptolyngbya ohadii, a desiccation-tolerant cyanobacterium

Cyanobacteria inhabiting desert biological soil crusts face the harsh conditions of the desert. They evolved a suite of strategies toward desiccation-hydration cycles mixed with high light irradiations, etc. In this study we purified and characterized the structure and function of Photosystem I (PSI) from Leptolyngbya ohadii, a desiccation-tolerant desert cyanobacterium. We discovered that PSI forms tetrameric (PSI-Tet) aggregate. We investigated it by using sucrose density gradient centrifugation, clear native PAGE, high performance liquid chromatography, mass spectrometry (MS), time-resolved fluorescence (TRF) and time-resolved transient absorption (TA) spectroscopy. MS analysis identified the presence of two PsaB and two PsaL proteins in PSI-Tet and uniquely revealed that PsaLs are N-terminally acetylated in contrast to non-modified PsaL in the trimeric PSI from Synechocystis sp. PCC 6803. Chlorophyll (Chl) a fluorescence decay profiles of the PSI-Tet performed at 77 K revealed two emission bands at ∼690 nm and 725 nm with the former appearing only at early delay time. The main fluorescence emission peak, associated with emission from the low energy Chls a, decays within a few nanoseconds. TA studies demonstrated that the 725 nm emission band is associated with low energy Chls a with absorption band clearly resolved at ∼710 nm at 77 K. In summary, our work suggests that the heterogenous composition of PsaBs and PsaL in PSI-Tet is related with the adaptation mechanisms needed to cope with stressful conditions under which this bacterium naturally grows.

Photosynthesis re-wired on the pico-second timescale

Photosystems II and I (PSII, PSI) are the reaction centre-containing complexes driving the light reactions of photosynthesis; PSII performs light-driven water oxidation and PSI further photo-energizes harvested electrons. The impressive efficiencies of the photosystems have motivated extensive biological, artificial and biohybrid approaches to 're-wire' photosynthesis for higher biomass-conversion efficiencies and new reaction pathways, such as H₂ evolution or CO₂ fixation^1,2. Previous approaches focused on charge extraction at terminal electron acceptors of the photosystems³. Electron extraction at earlier steps, perhaps immediately from photoexcited reaction centres, would enable greater thermodynamic gains; however, this was believed impossible with reaction centres buried at least 4 nm within the photosystems^4,5. Here, we demonstrate, using in vivo ultrafast transient absorption (TA) spectroscopy, extraction of electrons directly from photoexcited PSI and PSII at early points (several picoseconds post-photo-excitation) with live cyanobacterial cells or isolated photosystems, and exogenous electron mediators such as 2,6-dichloro-1,4-benzoquinone (DCBQ) and methyl viologen. We postulate that these mediators oxidize peripheral chlorophyll pigments participating in highly delocalized charge-transfer states after initial photo-excitation. Our results challenge previous models that the photoexcited reaction centres are insulated within the photosystem protein scaffold, opening new avenues to study and re-wire photosynthesis for biotechnologies and semi-artificial photosynthesis.

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.

Absolute quantification of cellular levels of photosynthesis-related proteins in Synechocystis sp. PCC 6803

Quantifying cellular components is a basic and important step for understanding how a cell works, how it responds to environmental changes, and for re-engineering cells to produce valuable metabolites and increased biomass. We quantified proteins in the model cyanobacterium Synechocystis sp. PCC 6803 given the general importance of cyanobacteria for global photosynthesis, for synthetic biology and biotechnology research, and their ancestral relationship to the chloroplasts of plants. Four mass spectrometry methods were used to quantify cellular components involved in the biosynthesis of chlorophyll, carotenoid and bilin pigments, membrane assembly, the light reactions of photosynthesis, fixation of carbon dioxide and nitrogen, and hydrogen and sulfur metabolism. Components of biosynthetic pathways, such as those for chlorophyll or for photosystem II assembly, range between 1000 and 10,000 copies per cell, but can be tenfold higher for CO₂ fixation enzymes. The most abundant subunits are those for photosystem I, with around 100,000 copies per cell, approximately 2 to fivefold higher than for photosystem II and ATP synthase, and 5-20 fold more than for the cytochrome b₆f complex. Disparities between numbers of pathway enzymes, between components of electron transfer chains, and between subunits within complexes indicate possible control points for biosynthetic processes, bioenergetic reactions and for the assembly of multisubunit complexes.

Energetic Diversity in the Electron-Transfer Pathways of Type I Photosynthetic Reaction Centers

Photosynthetic reaction centers from heliobacteria (HbRC) and green sulfur bacteria (GsbRC) are homodimeric proteins and share a common ancestor with photosystem I (PSI), classified as type I reaction centers. Using the HbRC crystal structure, we calculated the redox potential (Em) values in the electron-transfer branches, solving the linear Poisson-Boltzmann equation and considering the protonation states of all titratable sites in the entire protein-pigment complex. Em(A-1) for bacteriochlorophyll g at the secondary site in HbRC (-1157 mV) is as low as Em(A-1) for chlorophyll a in PSI (-1173 mV). Em(A0/HbRC) is at the same level as Em(A0/GsbRC) and is 200 mV higher than Em(A0/PSI) due to the replacement of PsaA-Trp697/PsaB-Trp677 in PSI with PshA-Arg554 in HbRC. In contrast, Em(FX) for the Fe4S4 cluster in HbRC (-420 mV) is significantly higher than Em(FX) in GsbRC (-719 mV) and PSI (-705 mV) due to the absence of acidic residues that correspond to PscA-Asp634 in GsbRC and PsaB-Asp575 in PSI. It seems likely that type I reaction centers have evolved, adopting (bacterio)chlorophylls suitable for their light environments while maintaining electron-transfer cascades.

Structure of a monomeric photosystem I core associated with iron-stress-induced-A proteins from Anabaena sp. PCC 7120

Iron-stress-induced-A proteins (IsiAs) are expressed in cyanobacteria under iron-deficient conditions. The cyanobacterium Anabaena sp. PCC 7120 has four isiA genes; however, their binding property and functional roles in PSI are still missing. We analyzed a cryo-electron microscopy structure of a PSI-IsiA supercomplex isolated from Anabaena grown under an iron-deficient condition. The PSI-IsiA structure contains six IsiA subunits associated with the PsaA side of a PSI core monomer. Three of the six IsiA subunits were identified as IsiA1 and IsiA2. The PSI-IsiA structure lacks a PsaL subunit; instead, a C-terminal domain of IsiA2 occupies the position of PsaL, which inhibits the oligomerization of PSI, leading to the formation of a PSI monomer. Furthermore, excitation-energy transfer from IsiAs to PSI appeared with a time constant of 55 ps. These findings provide insights into both the molecular assembly of the Anabaena IsiA family and the functional roles of IsiAs.

Quantum mechanical analysis of excitation energy transfer couplings in photosystem II

We evaluated excitation energy transfer (EET) coupling (J) between all pairs of chlorophylls (Chls) and pheophytins (Pheos) in the protein environment of photosystem II based on the time-dependent density functional theory with a quantum mechanical/molecular mechanics approach. In the reaction center, the EET coupling between Chls PD1 and PD2 is weaker (|J(PD1/PD2)| = 79 cm-1), irrespective of a short edge-to-edge distance of 3.6 Å (Mg-to-Mg distance of 8.1 Å), than the couplings between PD1 and the accessory ChlD1 (|J(PD1/ChlD2)| = 104 cm-1) and between PD2 and ChlD2 (|J(PD2/ChlD1)| = 101 cm-1), suggesting that PD1 and PD2 are two monomeric Chls rather than a "special pair". There exist strongly coupled Chl pairs (|J| > ∼100 cm-1) in the CP47 and CP43 core antennas, which may be candidates for the red-shifted Chls observed in spectroscopic studies. In CP47 and CP43, Chls ligated to CP47-His26 and CP43-His56, which are located in the middle layer of the thylakoid membrane, play a role in the "hub" that mediates the EET from the lumenal to stromal layers. In the stromal layer, Chls ligated to CP47-His466, CP43-His441, and CP43-His444 mediate the EET from CP47 to ChlD2/PheoD2 and from CP43 to ChlD1/PheoD1 in the reaction center. Thus, the excitation energy from both CP47 and CP43 can always be utilized for the charge-separation reaction in the reaction center.

Remodeling of algal photosystem I through phosphorylation

Photosystem I (PSI) with its associated light-harvesting system is the most important generator of reducing power in photosynthesis. The PSI core complex is highly conserved, whereas peripheral subunits as well as light-harvesting proteins (LHCI) reveal a dynamic plasticity. Moreover, in green alga, PSI-LHCI complexes are found as monomers, dimers, and state transition complexes, where two LHCII trimers are associated. Herein, we show light-dependent phosphorylation of PSI subunits PsaG and PsaH as well as Lhca6. Potential consequences of the dynamic phosphorylation of PsaG and PsaH are structurally analyzed and discussed in regard to the formation of the monomeric, dimeric, and LHCII-associated PSI-LHCI complexes.

Control of spin coupling through a redox-active bridge in a dinickel(II) porphyrin dimer: step-wise oxidations enable isolations of a chlorin-porphyrin heterodimer and a dication diradical with a singlet ground state

A dinickel(II)porphyrin dimer has been used here in which the redox-active pyrrole-moiety, similar to the tryptophan residue in diheme enzymes such as MauG and bCcP, has been placed between two Ni(II)porphyrin centers connected via a flexible, but unconjugated methylene bridge. This arrangement provides a large physical separation between the two metal centers and thus displays almost no communication between them through the bridge. Upon treatment with DDQ as an oxidant, the dinickel(II) porphyrin dimer slowly gets converted into an indolizinium-fused chlorin-porphyrin heterodimer. However, oxidations of the dinickel(II) porphyrin dimer up to two oxidizing equivalents using oxidants such as AgSbF₆ and FeCl₃ resulted in the formation of a dication diradical complex. Interestingly, in order to stabilize such a highly oxidized dication diradical, two non-conjugated methylene spacers undergo facile 2e^-/-2H⁺ oxidation to make the bridge fully π-conjugated for promoting through-bond communication. Through the oxidized and conjugated bridge, two porphyrin π-cation radicals display considerable communications leading to an efficient intramolecular spin coupling to form a singlet state. Interestingly, the redox-active nature of the bridge controls the electronic communication just by simple oxidation or reduction, and thereby, acts as a molecular switch for efficient magnetic relay.

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.

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.

Time-resolved FTIR difference spectroscopy for the study of photosystem I with high potential naphthoquinones incorporated into the A1 binding site

Time-resolved step-scan Fourier transform infrared difference spectroscopy has been used to study cyanobacterial photosystem I photosynthetic reaction centers from Synechocystis sp. PCC 6803 (S6803) with four high-potential, 1,4-naphthoquinones incorporated into the A₁ binding site. The high-potential naphthoquinones are 2-chloro-, 2-bromo-, 2,3-dichloro- and 2,3-dibromo-1,4-naphthoquinone. "Foreign minus native" double difference spectra (DDS) were constructed by subtracting difference spectra for native photosystem I (with phylloquinone in the A₁ binding site) from corresponding spectra obtained using photosystem I with the different quinones incorporated. To help assess and assign bands in the difference and double difference spectra, density functional theory based vibrational frequency calculations for the different quinones in solvent, or in the presence of a single asymmetric H- bond to either a water molecule or a peptide backbone NH group, were undertaken. Calculated and experimental spectra agree best for the peptide backbone asymmetrically H- bonded system. By comparing multiple sets of double difference spectra, several new bands for the native quinone (phylloquinone) are identified. By comparing calculated and experimental spectra we conclude that the mono-substituted halogenated NQs can occupy the binding site in either of two different orientations, with the chlorine or bromine atom being either ortho or meta to the H- bonded CO group.

Spectral diversity of photosystem I from flowering plants

Photosystem I and II (PSI and PSII) work together to convert solar energy into chemical energy. Whilst a lot of research has been done to unravel variability of PSII fluorescence in response to biotic and abiotic factors, the contribution of PSI to in vivo fluorescence measurements has often been neglected or considered to be constant. Furthermore, little is known about how the absorption and emission properties of PSI from different plant species differ. In this study, we have isolated PSI from five plant species and compared their characteristics using a combination of optical and biochemical techniques. Differences have been identified in the fluorescence emission spectra and at the protein level, whereas the absorption spectra were virtually the same in all cases. In addition, the emission spectrum of PSI depends on temperature over a physiologically relevant range from 280 to 298 K. Combined, our data show a critical comparison of the absorption and emission properties of PSI from various plant species.

Cryo-electron microscopy structure of the intact photosynthetic light-harvesting antenna-reaction center complex from a green sulfur bacterium

The photosynthetic reaction center complex (RCC) of green sulfur bacteria (GSB) consists of the membrane-imbedded RC core and the peripheric energy transmitting proteins called Fenna-Matthews-Olson (FMO). Functionally, FMO transfers the absorbed energy from a huge peripheral light-harvesting antenna named chlorosome to the RC core where charge separation occurs. In vivo, one RC was found to bind two FMOs, however, the intact structure of RCC as well as the energy transfer mechanism within RCC remain to be clarified. Here we report a structure of intact RCC which contains a RC core and two FMO trimers from a thermophilic green sulfur bacterium Chlorobaculum tepidum at 2.9 Å resolution by cryo-electron microscopy. The second FMO trimer is attached at the cytoplasmic side asymmetrically relative to the first FMO trimer reported previously. We also observed two new subunits (PscE and PscF) and the N-terminal transmembrane domain of a cytochrome-containing subunit (PscC) in the structure. These two novel subunits possibly function to facilitate the binding of FMOs to RC core and to stabilize the whole complex. A new bacteriochlorophyll (numbered as 816) was identified at the interspace between PscF and PscA-1, causing an asymmetrical energy transfer from the two FMO trimers to RC core. Based on the structure, we propose an energy transfer network within this photosynthetic apparatus.

Role of Thylakoid Lipids in Protochlorophyllide Oxidoreductase Activation: Allosteric Mechanism Elucidated by a Computational Study

Light-dependent protochlorophyllide oxidoreductase (LPOR) is a chlorophyll synthetase that catalyzes the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide) with indispensable roles in regulating photosynthesis processes. A recent study confirmed that thylakoid lipids (TL) were able to allosterically enhance modulator-induced LPOR activation. However, the allosteric modulation mechanism of LPOR by these compounds remains unclear. Herein, we integrated multiple computational approaches to explore the potential cavities in the Arabidopsis thaliana LPOR and an allosteric site around the helix-G region where high affinity for phosphatidyl glycerol (PG) was identified. Adopting accelerated molecular dynamics simulation for different LPOR states, we rigorously analyzed binary LPOR/PG and ternary LPOR/NADPH/PG complexes in terms of their dynamics, energetics, and attainable allosteric regulation. Our findings clarify the experimental observation of increased NADPH binding affinity for LPOR with PGs. Moreover, the simulations indicated that allosteric regulators targeting LPOR favor a mechanism involving lid opening upon binding to an allosteric hinge pocket mechanism. This understanding paves the way for designing novel LPOR activators and expanding the applications of LPOR.

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.

Structure of the Acidobacteria homodimeric reaction center bound with cytochrome c

Photosynthesis converts light energy to chemical energy to fuel life on earth. Light energy is harvested by antenna pigments and transferred to reaction centers (RCs) to drive the electron transfer (ET) reactions. Here, we present cryo-electron microscopy (cryo-EM) structures of two forms of the RC from the microaerophilic Chloracidobacterium thermophilum (CabRC): one containing 10 subunits, including two different cytochromes; and the other possessing two additional subunits, PscB and PscZ. The larger form contained 2 Zn-bacteriochlorophylls, 16 bacteriochlorophylls, 10 chlorophylls, 2 lycopenes, 2 hemes, 3 Fe₄S₄ clusters, 12 lipids, 2 Ca²⁺ ions and 6 water molecules, revealing a type I RC with an ET chain involving two hemes and a hybrid antenna containing bacteriochlorophylls and chlorophylls. Our results provide a structural basis for understanding the excitation energy and ET within the CabRC and offer evolutionary insights into the origin and adaptation of photosynthetic RCs.

Resolving the Impact of Hydrogen Bonding on the Phylloquinone Cofactor through Two-Dimensional Infrared Spectroscopy

Two-dimensional infrared spectroscopy (2DIR) was applied to phylloquinone (PhQ), an important biological cofactor, to elucidate the impact of hydrogen bonding on the ultrafast dynamics and energetics of the carbonyl stretching modes. 2DIR measurements were performed on PhQ dissolved in hexanol, which served as the hydrogen bonding solvent, and hexane, which served as a non-hydrogen bonding control. Molecular dynamics simulations and quantum chemical calculations were performed to aid in spectral assignment and interpretation. From the position of the peaks in the 2DIR spectra, we extracted the transition frequencies for the fundamental, overtone, and combination bands of hydrogen bonded and non-hydrogen bonded carbonyl groups of PhQ in the 1635-1680 cm^-1 region. We find that hydrogen bonding to a single carbonyl group acts to decouple the two carbonyl units of PhQ. Through analysis of the time-resolved 2DIR data, we find that hydrogen bonding leads to faster vibrational relaxation as well as an increase in the inhomogeneous broadening of the carbonyl groups. Overall, this work demonstrates how hydrogen bonding to the carbonyl groups of PhQ presents in the 2DIR spectra, laying the groundwork to use PhQ as a 2DIR probe to characterize the ultrafast fluctuations in the local environment of natural photosynthetic complexes.

Energetics of the Electron Transfer Pathways in the Homodimeric Photosynthetic Reaction Center

Photosynthetic reaction centers from a green sulfur bacterium (GsbRC), the PscA/PscA proteins, and photosystem I (PSI), PsaA/PsaB proteins, share structural similarities. Here, we report the redox potential (Em) values of GsbRC by solving the linear Poisson-Boltzmann equation and considering the protonation states of all titratable sites in the entire GsbRC protein and identify the factors that shift the Em values with respect to PSI. The Em values for one-electron reduction of the accessory (A-1) and adjacent (A0) chlorophylls in GsbRC are 100-250 mV higher than those in PSI, whereas the Em values for the Fe4S4 cluster (FX) are at the same level. The PsaA-Trp697/PsaB-Trp677 pair in PSI, which forms the A1-quinone binding site, is replaced with PscA-Arg638 in GsbRC. PsaB-Asp575 in PSI, which is responsible for the Em difference between A1A and A1B quinones in PSI, is absent in GsbRC. These discrepancies also contribute to the upshift in Em(A-1) and Em(A0) in GsbRC with respect to PSI. It seems likely that the upshifted Em for chlorophylls in GsbRC ultimately originates from the characteristics of the electrostatic environment that corresponds to the A1 site of PSI.

Modular Nucleic Acid Scaffolds for Synthesizing Monodisperse and Sequence-Encoded Antibody Oligomers

Synthesizing protein oligomers that contain exact numbers of multiple different proteins in defined architectures is challenging. DNA-DNA interactions can be used to program protein assembly into oligomers; however, existing methods require changes to DNA design to achieve different numbers and oligomeric sequences of proteins. Herein, we develop a modular DNA scaffold that uses only six synthetic oligonucleotides to organize proteins into defined oligomers. As a proof-of-concept, model proteins (antibodies) are oligomerized into dimers and trimers, where antibody function is retained. Illustrating the modularity of this technique, dimer and trimer building blocks are then assembled into pentamers containing three different antibodies in an exact stoichiometry and oligomeric sequence. In sum, this report describes a generalizable method for organizing proteins into monodisperse, sequence-encoded oligomers using DNA. This advance will enable studies into how oligomeric protein sequences affect material properties in areas spanning pharmaceutical development, cascade catalysis, synthetic photosynthesis, and membrane transport.

Phosphate limitation intensifies negative effects of ocean acidification on globally important nitrogen fixing cyanobacterium

Growth of the prominent nitrogen-fixing cyanobacterium Trichodesmium is often limited by phosphorus availability in the ocean. How nitrogen fixation by phosphorus-limited Trichodesmium may respond to ocean acidification remains poorly understood. Here, we use phosphate-limited chemostat experiments to show that acidification enhanced phosphorus demands and decreased phosphorus-specific nitrogen fixation rates in Trichodesmium. The increased phosphorus requirements were attributed primarily to elevated cellular polyphosphate contents, likely for maintaining cytosolic pH homeostasis in response to acidification. Alongside the accumulation of polyphosphate, decreased NADP(H):NAD(H) ratios and impaired chlorophyll synthesis and energy production were observed under acidified conditions. Consequently, the negative effects of acidification were amplified compared to those demonstrated previously under phosphorus sufficiency. Estimating the potential implications of this finding, using outputs from the Community Earth System Model, predicts that acidification and dissolved inorganic and organic phosphorus stress could synergistically cause an appreciable decrease in global Trichodesmium nitrogen fixation by 2100.

Native architecture and acclimation of photosynthetic membranes in a fast-growing cyanobacterium

Efficient solar energy conversion is ensured by the organization, physical association, and physiological coordination of various protein complexes in photosynthetic membranes. Here, we visualize the native architecture and interactions of photosynthetic complexes within the thylakoid membranes from a fast-growing cyanobacterium Synechococcus elongatus UTEX 2973 (Syn2973) using high-resolution atomic force microscopy. In the Syn2973 thylakoid membranes, both photosystem I (PSI)-enriched domains and crystalline photosystem II (PSII) dimer arrays were observed, providing favorable membrane environments for photosynthetic electron transport. The high light (HL)-adapted thylakoid membranes accommodated a large amount of PSI complexes, without the incorporation of iron-stress-induced protein A (IsiA) assemblies and formation of IsiA-PSI supercomplexes. In the iron deficiency (Fe-)-treated thylakoid membranes, in contrast, IsiA proteins densely associated with PSI, forming the IsiA-PSI supercomplexes with varying assembly structures. Moreover, type-I NADH dehydrogenase-like complexes (NDH-1) were upregulated under the HL and Fe- conditions and established close association with PSI complexes to facilitate cyclic electron transport. Our study provides insight into the structural heterogeneity and plasticity of the photosynthetic apparatus in the context of their native membranes in Syn2973 under environmental stress. Advanced understanding of the photosynthetic membrane organization and adaptation will provide a framework for uncovering the molecular mechanisms of efficient light harvesting and energy conversion.

Molecular asymmetry of a photosynthetic supercomplex from green sulfur bacteria

The photochemical reaction center (RC) features a dimeric architecture for charge separation across the membrane. In green sulfur bacteria (GSB), the trimeric Fenna-Matthews-Olson (FMO) complex mediates the transfer of light energy from the chlorosome antenna complex to the RC. Here we determine the structure of the photosynthetic supercomplex from the GSB Chlorobaculum tepidum using single-particle cryogenic electron microscopy (cryo-EM) and identify the cytochrome c subunit (PscC), two accessory protein subunits (PscE and PscF), a second FMO trimeric complex, and a linker pigment between FMO and the RC core. The protein subunits that are assembled with the symmetric RC core generate an asymmetric photosynthetic supercomplex. One linker bacteriochlorophyll (BChl) is located in one of the two FMO-PscA interfaces, leading to differential efficiencies of the two energy transfer branches. The two FMO trimeric complexes establish two different binding interfaces with the RC cytoplasmic surface, driven by the associated accessory subunits. This structure of the GSB photosynthetic supercomplex provides mechanistic insight into the light excitation energy transfer routes and a possible evolutionary transition intermediate of the bacterial photosynthetic supercomplex from the primitive homodimeric RC.

Algal photosystem I dimer and high-resolution model of PSI-plastocyanin complex

Photosystem I (PSI) enables photo-electron transfer and regulates photosynthesis in the bioenergetic membranes of cyanobacteria and chloroplasts. Being a multi-subunit complex, its macromolecular organization affects the dynamics of photosynthetic membranes. Here we reveal a chloroplast PSI from the green alga Chlamydomonas reinhardtii that is organized as a homodimer, comprising 40 protein subunits with 118 transmembrane helices that provide scaffold for 568 pigments. Cryogenic electron microscopy identified that the absence of PsaH and Lhca2 gives rise to a head-to-head relative orientation of the PSI-light-harvesting complex I monomers in a way that is essentially different from the oligomer formation in cyanobacteria. The light-harvesting protein Lhca9 is the key element for mediating this dimerization. The interface between the monomers is lacking PsaH and thus partially overlaps with the surface area that would bind one of the light-harvesting complex II complexes in state transitions. We also define the most accurate available PSI-light-harvesting complex I model at 2.3 Å resolution, including a flexibly bound electron donor plastocyanin, and assign correct identities and orientations to all the pigments, as well as 621 water molecules that affect energy transfer pathways.

Small angle neutron scattering and lipidomic analysis of a native, trimeric PSI-SMALP from a thermophilic cyanobacteria

The use of styrene-maleic acid copolymers (SMAs) to produce membrane protein-containing nanodiscs without the initial detergent isolation has gained significant interest over the last decade. We have previously shown that a Photosystem I SMALP from the thermophilic cyanobacterium, Thermosynechococcus elongatus (PSI-SMALP), has much more rapid energy transfer and charge separation in vitro than detergent isolated PSI complexes. In this study, we have utilized small-angle neutron scattering (SANS) to better understand the geometry of these SMALPs. These techniques allow us to investigate the size and shape of these particles in their fully solvated state. Further, the particle's proteolipid core and detergent shell or copolymer belt can be interrogated separately using contrast variation, a capability unique to SANS. Here we report the dimensions of the Thermosynechococcus elongatus PSI-SMALP containing a PSI trimer. At ~1.5 MDa, PSI-SMALP is the largest SMALP to be isolated; our lipidomic analysis indicates it contains ~1300 lipids/per trimeric particle, >40-fold more than the PSI-DDM particle and > 100 fold more than identified in the 1JB0 crystal structure. Interestingly, the lipid composition to the PSI trimer in the PSI-SMALP differs significantly from bulk thylakoid composition, being enriched ~50 % in the anionic sulfolipid, SQDG. Finally, utilizing the contrast match point for the SMA 1440 copolymer, we also can observe the ~1 nm SMA copolymer belt surrounding this SMALP for the first time, consistent with most models of SMA organization.

Spatial Regulation of Acceptor Units in Olefin-Linked COFs toward Highly Efficient Photocatalytic H2 Evolution

Covalent organic frameworks (COFs)-based photocatalysts have received growing attention for photocatalytic hydrogen (H₂ ) production. One of the big challenges in the field is to find ways to promote energy/electron transfer and exciton dissociation. Addressing this challenge, herein, a series of olefin-linked 2D COFs is fabricated with high crystallinity, porosity, and robustness using a melt polymerization method without adding volatile organic solvents. It is found that regulation of the spatial distances between the acceptor units (triazine and 2, 2'-bipyridine) of COFs to match the charge carrier diffusion length can dramatically promote the exciton dissociation, hence leading to outstanding photocatalytic H₂ evolution performance. The COF with the appropriate acceptor distance achieves exceptional photocatalytic H₂ evolution with an apparent quantum yield of 56.2% at 475 nm, the second highest value among all COF photocatalysts and 70 times higher than the well-studied polymer carbon nitride. Various experimental and computation studies are then conducted to in-depth unveil the mechanism behind the enhanced performance. This study will provide important guidance for the design of highly efficient organic semiconductor photocatalysts.

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.

Rational design of photosynthetic reaction center protein maquettes

New technologies for efficient solar-to-fuel energy conversion will help facilitate a global shift from dependence on fossil fuels to renewable energy. Nature uses photosynthetic reaction centers to convert photon energy into a cascade of electron-transfer reactions that eventually produce chemical fuel. The design of new reaction centers de novo deepens our understanding of photosynthetic charge separation and may one day allow production of biofuels with higher thermodynamic efficiency than natural photosystems. Recently, we described the multi-step electron-transfer activity of a designed reaction center maquette protein (the RC maquette), which can assemble metal ions, tyrosine, a Zn tetrapyrrole, and heme into an electron-transport chain. Here, we detail our modular strategy for rational protein design and show that the intended RC maquette design agrees with crystal structures in various states of assembly. A flexible, dynamic apo-state collapses by design into a more ordered holo-state upon cofactor binding. Crystal structures illustrate the structural transitions upon binding of different cofactors. Spectroscopic assays demonstrate that the RC maquette binds various electron donors, pigments, and electron acceptors with high affinity. We close with a critique of the present RC maquette design and use electron-tunneling theory to envision a path toward a designed RC with a substantially higher thermodynamic efficiency than natural photosystems.

Structure of cyanobacterial photosystem I complexed with ferredoxin at 1.97 Å resolution

Photosystem I (PSI) is a light driven electron pump transferring electrons from Cytochrome c₆ (Cyt c₆) to Ferredoxin (Fd). An understanding of this electron transfer process is hampered by a paucity of structural detail concerning PSI:Fd interface and the possible binding sites of Cyt c₆. Here we describe the high resolution cryo-EM structure of Thermosynechococcus elongatus BP-1 PSI in complex with Fd and a loosely bound Cyt c₆. Side chain interactions at the PSI:Fd interface including bridging water molecules are visualized in detail. The structure explains the properties of mutants of PsaE and PsaC that affect kinetics of Fd binding and suggests a molecular switch for the dissociation of Fd upon reduction. Calorimetry-based thermodynamic analyses confirms a single binding site for Fd and demonstrates that PSI:Fd complexation is purely driven by entropy. A possible reaction cycle for the efficient transfer of electrons from Cyt c₆ to Fd via PSI is proposed.

In order to aid the understanding of the electron transfer process within the cyanobacterial photosystem I, its structure - when complexed with Ferredoxin - is determined at 1.97 Å resolution.

Serial femtosecond crystallography

With the advent of X-ray Free Electron Lasers (XFELs), new, high-throughput serial crystallography techniques for macromolecular structure determination have emerged. Serial femtosecond crystallography (SFX) and related methods provide possibilities beyond canonical, single-crystal rotation crystallography by mitigating radiation damage and allowing time-resolved studies with unprecedented temporal resolution. This primer aims to assist structural biology groups with little or no experience in serial crystallography planning and carrying out a successful SFX experiment. It discusses the background of serial crystallography and its possibilities. Microcrystal growth and characterization methods are discussed, alongside techniques for sample delivery and data processing. Moreover, it gives practical tips for preparing an experiment, what to consider and do during a beamtime and how to conduct the final data analysis. Finally, the Primer looks at various applications of SFX, including structure determination of membrane proteins, investigation of radiation damage-prone systems and time-resolved studies.

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.

Observation of anomalous carotenoid and blind chlorophyll activations in photosystem I under synthetic membrane confinements

The role of natural thylakoid membrane confinements in architecting the robust structural and electrochemical properties of PSI is not fully understood. Most PSI studies till date extract the proteins from their natural confinements that can lead to non-native conformations. Recently our group had successfully reconstituted PSI in synthetic lipid membranes using detergent-mediated liposome solubilizations. In this study, we investigate the alterations in chlorophylls and carotenoids interactions and reorganization in PSI based on spectral property changes induced by its confinement in anionic DPhPG and zwitterionic DPhPC phospholipid membranes. To this end, we employ a combination of absorption, fluorescence, and circular dichroism (CD) spectroscopic measurements. Our results indicate unique activation and alteration of photoresponses from the PSI carotenoid (Car) bands in PSI-DPhPG proteoliposomes that can tune the Excitation Energy Transfer (EET), otherwise absent in PSI at non-native environments. Specifically, we observe broadband light harvesting via enhanced absorption in the otherwise non-absorptive green region (500-580 nm) of the Chlorophylls (Chl) along with ~64% increase in the full-width half maximum of the Qy band (650-720 nm). The CD results indicate enhanced Chl-Chl and Chl-Car interactions along with conformational changes in protein secondary structures. Such distinct changes in the Car and Chl bands are not observed in PSI confined in DPhPC. The fundamental insights into membrane microenvironments tailoring PSI subunits reorganization and interactions provide novel strategies for tuning photoexcitation processes and rational designing of biotic-abiotic interfaces in PSI-based photoelectrochemical energy conversion systems.

Structures of Multisubunit Membrane Complexes With the CRYO ARM 200

Progress in structural membrane biology has been significantly accelerated by the ongoing ‘Resolution Revolution’ in cryo-electron microscopy (cryo-EM). In particular, structure determination by single-particle analysis has evolved into the most powerful method for atomic model building of multisubunit membrane protein complexes. This has created an ever-increasing demand in cryo-EM machine time, which to satisfy is in need of new and affordable cryo-electron microscopes. Here, we review our experience in using the JEOL CRYO ARM 200 prototype for the structure determination by single-particle analysis of three different multisubunit membrane complexes: the Thermus thermophilus V-type ATPase V_O complex, the Thermosynechococcus elongatus photosystem I monomer and the flagellar motor lipopolysaccharide peptidoglycan ring (LP ring) from Salmonella enterica.

Fatty acid desaturases (FADs) modulate multiple lipid metabolism pathways to improve plant resistance

BACKGROUND

Biological and abiotic stresses such as salt, extreme temperatures, and pests and diseases place major constraints on plant growth and crop yields. Fatty acids (FAs) and FA- derivatives are unique biologically active substance that show a wide range of functions in biological systems. They are not only participated in the regulation of energy storage substances and cell membrane plasm composition, but also extensively participate in the regulation of plant basic immunity, effector induced resistance and systemic resistance and other defense pathways, thereby improving plant resistance to adversity stress. Fatty acid desaturases (FADs) is involved in the desaturation of fatty acids, where desaturated fatty acids can be used as substrates for FA-derivatives.

OBJECTIVE

In this paper, the role of omega-FADs (ω-3 FADs and ω-6 FADs) in the prokaryotic and eukaryotic pathways of fatty acid biosynthesis in plant defense against stress (biological and abiotic stress) and the latest research progress were summarized. Moreover' the existing problems in related research and future research directions were also discussed.

RESULTS

Fatty acid desaturases are involved in various responses of plants during biotic and abiotic stress. For example, it is involved in regulating the stability and fluidity of cell membranes, reactive oxygen species signaling pathways, etc. In this review, we have collected several experimental studies to represent the differential effects of fatty acid desaturases on biotic and abiotic species.

CONCLUSION

Fatty acid desaturases play an important role in regulating biotic and abiotic stresses.

A kaleidoscope of photosynthetic antenna proteins and their emerging roles

Photosynthetic light-harvesting antennae are pigment-binding proteins that perform one of the most fundamental tasks on Earth, capturing light and transferring energy that enables life in our biosphere. Adaptation to different light environments led to the evolution of an astonishing diversity of light-harvesting systems. At the same time, several strategies have been developed to optimize the light energy input into photosynthetic membranes in response to fluctuating conditions. The basic feature of these prompt responses is the dynamic nature of antenna complexes, whose function readily adapts to the light available. High-resolution microscopy and spectroscopic studies on membrane dynamics demonstrate the crosstalk between antennae and other thylakoid membrane components. With the increased understanding of light-harvesting mechanisms and their regulation, efforts are focusing on the development of sustainable processes for effective conversion of sunlight into functional bio-products. The major challenge in this approach lies in the application of fundamental discoveries in light-harvesting systems for the improvement of plant or algal photosynthesis. Here, we underline some of the latest fundamental discoveries on the molecular mechanisms and regulation of light harvesting that can potentially be exploited for the optimization of photosynthesis.