Spectroelectrochemical determination of the redox potential of pheophytin a, the primary electron acceptor in photosystem II

Femtosecond optical studies of the primary charge separation reactions in far-red photosystem II from Synechococcus sp. PCC 7335

Primary processes of light energy conversion by Photosystem II (PSII) were studied using femtosecond broadband pump-probe absorption difference spectroscopy. Transient absorption changes of core complexes isolated from the cyanobacterium Synechococcus sp. PCC 7335 grown under far-red light (FRL-PSII) were compared with the canonical Chl a containing spinach PSII core complexes upon excitation into the red edge of the Qy band. Absorption changes of FRL-PSII were monitored at 278 K in the 400-800 nm spectral range on a timescale of 0.1-500 ps upon selective excitation at 740 nm of four chlorophyll (Chl) f molecules in the light harvesting antenna, or of one Chl d molecule at the ChlD1 position in the reaction center (RC) upon pumping at 710 nm. Numerical analysis of absorption changes and assessment of the energy levels of the presumed ion-radical states made it possible to identify PD1+ChlD1- as the predominant primary charge-separated radical pair, the formation of which upon selective excitation of Chl d has an apparent time of ∼1.6 ps. Electron transfer to the secondary acceptor pheophytin PheoD1 has an apparent time of ∼7 ps with a variety of excitation wavelengths. The energy redistribution between Chl a and Chl f in the antenna occurs within 1 ps, whereas the energy migration from Chl f to the RC occurs mostly with lifetimes of 60 and 400 ps. Potentiometric analysis suggests that in canonical PSII, PD1+ChlD1- can be partially formed from the excited (PD1ChlD1)* state.

Oxygenic Photosynthesis in Far-Red Light: Strategies and Mechanisms

Oxygenic photosynthesis, the process that converts light energy into chemical energy, is traditionally associated with the absorption of visible light by chlorophyll molecules. However, recent studies have revealed a growing number of organisms capable of using far-red light (700-800 nm) to drive oxygenic photosynthesis. This phenomenon challenges the conventional understanding of the limits of this process. In this review, we briefly introduce the organisms that exhibit far-red photosynthesis and explore the different strategies they employ to harvest far-red light. We discuss the modifications of photosynthetic complexes and their impact on the delivery of excitation energy to photochemical centers and on overall photochemical efficiency. Finally, we examine the solutions employed to drive electron transport and water oxidation using relatively low-energy photons. The findings discussed here not only expand our knowledge of the remarkable adaptation capacities of photosynthetic organisms but also offer insights into the potential for enhancing light capture in crops.

The effect of ultrafine WO3 nanoparticles on the organization of thylakoids enriched in photosystem II and energy transfer in photosystem II complexes

In this work, a new approach to construct self-assembled hybrid systems based on natural PSII-enriched thylakoid membranes (PSII BBY) is demonstrated. Superfine m-WO3 NPs (≈1-2 nm) are introduced into PSII BBY. Transmission electron microscopy (TEM) measurements showed that even the highest concentrations of NPs used did not degrade the PSII BBY membranes. Using atomic force microscopy (AFM), it is shown that the organization of PSII BBY depends strongly on the concentration of NPs applied. This proved that the superfine NPs can easily penetrate the thylakoid membrane and interact with its components. These changes are also related to the modified energy transfer between the external light-harvesting antennas and the PSII reaction center, shown by absorption and fluorescence experiments. The biohybrid system shows stability at pH 6.5, the native operating environment of PSII, so a high rate of O2 evolution is expected. In addition, the light-induced water-splitting process can be further stimulated by the direct interaction of superfine WO3 NPs with the donor and acceptor sides of PSII. The water-splitting activity and stability of this colloidal system are under investigation. RESEARCH HIGHLIGHTS: The phenomenon of the self-organization of a biohybrid system composed of thylakoid membranes enriched in photosystem II and superfine WO3 nanoparticles is studied using AFM and TEM. A strong dependence of the organization of PSII complexes within PSII BBY membranes on the concentration of NPs applied is observed. This observation turns out to be crucial to understand the complexity of the mechanism of the action of WO3 NPs on modifications of energy transfer from external antenna complexes to the PSII reaction center.

Energetics and proton release in photosystem II from Thermosynechococcus elongatus with a D1 protein encoded by either the psbA2 or psbA3 gene

In the cyanobacterium Thermosynechococcus elongatus, there are three psbA genes coding for the Photosystem II (PSII) D1 subunit that interacts with most of the main cofactors involved in the electron transfers. Recently, the 3D crystal structures of both PsbA2-PSII and PsbA3-PSII have been solved [Nakajima et al., J. Biol. Chem. 298 (2022) 102668.]. It was proposed that the loss of one hydrogen bond of Phe_D1 due to the D1-Y147F exchange in PsbA2-PSII resulted in a more negative E_m of Phe_D1 in PsbA2-PSII when compared to PsbA3-PSII. In addition, the loss of two water molecules in the Cl-1 channel was attributed to the D1-P173M substitution in PsbA2-PSII. This exchange, by narrowing the Cl-1 proton channel, could be at the origin of a slowing down of the proton release. Here, we have continued the characterization of PsbA2-PSII by measuring the thermoluminescence from the S₂Q_A^-/DCMU charge recombination and by measuring proton release kinetics using time-resolved absorption changes of the dye bromocresol purple. It was found that i) the E_m of Phe_D1^-•/Phe_D1 was decreased by ~30 mV in PsbA2-PSII when compared to PsbA3-PSII and ii) the kinetics of the proton release into the bulk was significantly slowed down in PsbA2-PSII in the S₂Tyr_Z^• to S₃Tyr_Z and S₃Tyr_Z^• → (S₃Tyr_Z^•)' transitions. This slowing down was partially reversed by the PsbA2/M173P mutation and induced by the PsbA3/P173M mutation thus confirming a role of the D1-173 residue in the egress of protons trough the Cl-1 channel.

The Mechanism of Non-Photochemical Quenching in Plants: Localization and Driving Forces

Non-photochemical chlorophyll fluorescence quenching (NPQ) remains one of the most studied topics of the 21st century in photosynthesis research. Over the past 30 years, profound knowledge has been obtained on the molecular mechanism of NPQ in higher plants. First, the largely overlooked significance of NPQ in protecting the reaction center of photosystem II (RCII) against damage, and the ways to assess its effectiveness are highlighted. Then, the key in vivo signals that can monitor the life of the major NPQ component, qE, are presented. Finally, recent knowledge on the site of qE and the possible molecular events that transmit ΔpH into the conformational change in the major LHCII [the major trimeric light harvesting complex of photosystem II (PSII)] antenna complex are discussed. Recently, number of reports on Arabidopsis mutants lacking various antenna components of PSII confirmed that the in vivo site of qE rests within the major trimeric LHCII complex. Experiments on biochemistry, spectroscopy, microscopy and molecular modeling suggest an interplay between thylakoid membrane geometry and the dynamics of LHCII, the PsbS (PSII subunit S) protein and thylakoid lipids. The molecular basis for the qE-related conformational change in the thylakoid membrane, including the possible onset of a hydrophobic mismatch between LHCII and lipids, potentiated by PsbS protein, begins to unfold.

Membrane Protein Modified Electrodes in Bioelectrocatalysis

Transmembrane proteins involved in metabolic redox reactions and photosynthesis catalyse a plethora of key energy-conversion processes and are thus of great interest for bioelectrocatalysis-based applications. The development of membrane protein modified electrodes has made it possible to efficiently exchange electrons between proteins and electrodes, allowing mechanistic studies and potentially applications in biofuels generation and energy conversion. Here, we summarise the most common electrode modification and their characterisation techniques for membrane proteins involved in biofuels conversion and semi-artificial photosynthesis. We discuss the challenges of applications of membrane protein modified electrodes for bioelectrocatalysis and comment on emerging methods and future directions, including recent advances in membrane protein reconstitution strategies and the development of microbial electrosynthesis and whole-cell semi-artificial photosynthesis.

Oxygen and ROS in Photosynthesis

Oxygen is a natural acceptor of electrons in the respiratory pathway of aerobic organisms and in many other biochemical reactions. Aerobic metabolism is always associated with the formation of reactive oxygen species (ROS). ROS may damage biomolecules but are also involved in regulatory functions of photosynthetic organisms. This review presents the main properties of ROS, the formation of ROS in the photosynthetic electron transport chain and in the stroma of chloroplasts, and ROS scavenging systems of thylakoid membrane and stroma. Effects of ROS on the photosynthetic apparatus and their roles in redox signaling are discussed.

Redox Potential of the Oxygen-Evolving Complex in the Electron Transfer Cascade of Photosystem II

In photosystem II (PSII), water oxidation occurs in the Mn₄CaO₅ cluster with the release of electrons via the redox-active tyrosine (TyrZ) to the reaction-center chlorophylls (P_D1/P_D2). Using a quantum mechanical/molecular mechanical approach, we report the redox potentials (E_m) of these cofactors in the PSII protein environment. The E_m values suggest that the Mn₄CaO₅ cluster, TyrZ, and P_D1/P_D2 form a downhill electron transfer pathway. E_m for the first oxidation step, E_m(S₀/S₁), is uniquely low (730 mV) and is ∼100 mV lower than that for the second oxidation step, E_m(S₁/S₂) (830 mV) only when the O4 site of the Mn₄CaO₅ cluster is protonated in S₀. The O4-water chain, which directly forms a low-barrier H-bond with the Mn₄CaO₅ cluster and mediates proton-coupled electron transfer in the S₀ to S₁ transition, explains why the second lowest oxidation state, S₁, is the most stable and S₀ is converted to S₁ even in the dark.

Evolution of Photochemical Reaction Centres: More Twists?

One of the earliest events in the molecular evolution of photosynthesis is the structural and functional specialisation of type I (ferredoxin-reducing) and type II (quinone-reducing) reaction centres. In this opinion article we point out that the homodimeric type I reaction centre of heliobacteria has a calcium-binding site with striking structural similarities to the Mn₄CaO₅ cluster of photosystem II. These similarities indicate that most of the structural elements required to evolve water oxidation chemistry were present in the earliest reaction centres. We suggest that the divergence of type I and type II reaction centres was made possible by a drastic structural shift linked to a change in redox properties that coincided with or facilitated the origin of photosynthetic water oxidation.

Energetics of the exchangeable quinone, QB, in Photosystem II

Photosystem II (PSII), the light-driven water/plastoquinone photooxidoreductase, is of central importance in the planetary energy cycle. The product of the reaction, plastohydroquinone (PQH₂), is released into the membrane from the Q_B site, where it is formed. A plastoquinone (PQ) from the membrane pool then binds into the Q_B site. Despite their functional importance, the thermodynamic properties of the PQ in the Q_B site, Q_B, in its different redox forms have received relatively little attention. Here we report the midpoint potentials (E_m ) of Q_B in PSII from Thermosynechococcus elongatus using electron paramagnetic resonance (EPR) spectroscopy: E_m Q_B/Q_B ^•- ≈ 90 mV, and E_m Q_B ^•-/Q_BH₂ ≈ 40 mV. These data allow the following conclusions: 1) The semiquinone, Q_B ^•-, is stabilized thermodynamically; 2) the resulting E_m Q_B/Q_BH₂ (∼65 mV) is lower than the E_m PQ/PQH₂ (∼117 mV), and the difference (ΔE ≈ 50 meV) represents the driving force for Q_BH₂ release into the pool; 3) PQ is ∼50× more tightly bound than PQH₂; and 4) the difference between the E_m Q_B/Q_B ^•- measured here and the E_m Q_A/Q_A ^•- from the literature is ∼234 meV, in principle corresponding to the driving force for electron transfer from Q_A ^•- to Q_B The pH dependence of the thermoluminescence associated with Q_B ^•- provided a functional estimate for this energy gap and gave a similar value (≥180 meV). These estimates are larger than the generally accepted value (∼70 meV), and this is discussed. The energetics of Q_B in PSII are comparable to those in the homologous purple bacterial reaction center.

Quantitative assessment of the high-light tolerance in plants with an impaired photosystem II donor side

Photoinhibition is the light-induced down-regulation of photosynthetic efficiency, the primary target of which is photosystem II (PSII). Currently, there is no clear consensus on the exact mechanism of this process. However, it is clear that inhibition can occur through limitations on both the acceptor- and donor side of PSII. The former mechanism is caused by electron transport limitations at the PSII acceptor side. Whilst, the latter mechanism relies on the disruption of the oxygen-evolving complex. Both of these mechanisms damage the PSII reaction centre (RC). Using a novel chlorophyll fluorescence methodology, RC photoinactivation can be sensitively measured and quantified alongside photoprotection in vivo This is achieved through estimation of the redox state of Q _A, using the parameter of photochemical quenching in the dark (qPd). This study shows that through the use of PSII donor-side inhibitors, such as UV-B and Cd²⁺, there is a steeper gradient of photoinactivation in the systems with a weakened donor side, independent of the level of NPQ attained. This is coupled with a concomitant decline in the light tolerance of PSII. The native light tolerance is partially restored upon the use of 1,5-diphenylcarbazide (DPC), a PSII electron donor, allowing for the balance between the inhibitory pathways to be sensitively quantified. Thus, this study confirms that the impact of donor-side inhibition can be detected alongside acceptor-side photoinhibition using the qPd parameter and confirms qPd as a valid, sensitive and unambiguous parameter to sensitively quantify the onset of photoinhibition through both acceptor- or donor-side mechanisms.

Photochemistry beyond the red limit in chlorophyll f-containing photosystems

Photosystems I and II convert solar energy into the chemical energy that powers life. Chlorophyll a photochemistry, using red light (680 to 700 nm), is near universal and is considered to define the energy "red limit" of oxygenic photosynthesis. We present biophysical studies on the photosystems from a cyanobacterium grown in far-red light (750 nm). The few long-wavelength chlorophylls present are well resolved from each other and from the majority pigment, chlorophyll a. Charge separation in photosystem I and II uses chlorophyll f at 745 nm and chlorophyll f (or d) at 727 nm, respectively. Each photosystem has a few even longer-wavelength chlorophylls f that collect light and pass excitation energy uphill to the photochemically active pigments. These photosystems function beyond the red limit using far-red pigments in only a few key positions.

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

We report E_m values of (bacterio-)chlorophylls for one-electron reduction in both electron-transfer branches of PbRC, PSI, and PSII.

We report redox potentials (E_m) for one-electron reduction for all chlorophylls in the two electron-transfer branches of water-oxidizing enzyme photosystem II (PSII), photosystem I (PSI), and purple bacterial photosynthetic reaction centers (PbRC). In PSI, E_m values for the accessory chlorophylls were similar in both electron-transfer branches. In PbRC, the corresponding E_m value was 170 mV less negative in the active L-branch (B_L) than in the inactive M-branch (B_M), favoring B_L˙^– formation. This contrasted with the corresponding chlorophylls, Chl_D1 and Chl_D2, in PSII, where E_m(Chl_D1) was 120 mV more negative than E_m(Chl_D2), implying that to rationalize electron transfer in the D1-branch, Chl_D1 would need to serve as the primary electron donor. Residues that contributed to E_m(Chl_D1) < E_m(Chl_D2) simultaneously played a key role in (i) releasing protons from the substrate water molecules and (ii) contributing to the larger cationic population on the chlorophyll closest to the Mn₄CaO₅ cluster (P_D1), favoring electron transfer from water molecules. These features seem to be the nature of PSII, which needs to possess the proton-exit pathway to use a protonated electron source—water molecules.

Recent advances in spectroelectrochemistry

The integration of two quite different techniques, conventional electrochemistry and spectroscopy, into spectroelectrochemistry (SEC) provides a complete description of chemically driven electron transfer processes and redox events for different kinds of molecules and nanoparticles. SEC possesses interdisciplinary advantages and can further expand the scopes in the fields of analysis and other applications, emphasizing the hot issues of analytical chemistry, materials science, biophysics, chemical biology, and so on. Considering the past and future development of SEC, a review on the recent progress of SEC is presented and selected examples involving surface-enhanced Raman scattering (SERS), ultraviolet-visible (UV-Vis), near-infrared (NIR), Fourier transform infrared (FTIR), fluorescence, as well as other SEC are summarized to fully demonstrate these techniques. In addition, the optically transparent electrodes and SEC cell design, and the typical applications of SEC in mechanism study, electrochromic device fabrication, sensing and protein study are fully introduced. Finally, the key issues, future perspectives and trends in the development of SEC are also discussed.

Effects of gold nanoparticles on the photophysical and photosynthetic parameters of leaves and chloroplasts

Interaction between light and plants containing gold nanoparticles: a deep insight into the physicochemical processes taking place.

Genetically introduced hydrogen bond interactions reveal an asymmetric charge distribution on the radical cation of the special-pair chlorophyll P680

The special-pair chlorophyll (Chl) P680 in photosystem II has an extremely high redox potential (E_m ) to enable water oxidation in photosynthesis. Significant positive-charge localization on one of the Chl constituents, P_D1 or P_D2, in P680⁺ has been proposed to contribute to this high E_m To identify the Chl molecule on which the charge is mainly localized, we genetically introduced a hydrogen bond to the 13¹-keto C=O group of P_D1 and P_D2 by changing the nearby D1-Val-157 and D2-Val-156 residues to His, respectively. Successful hydrogen bond formation at P_D1 and P_D2 in the obtained D1-V157H and D2-V156H mutants, respectively, was monitored by detecting 13¹-keto C=O vibrations in Fourier transfer infrared (FTIR) difference spectra upon oxidation of P680 and the symmetrically located redox-active tyrosines Y_Z and Y_D, and they were simulated by quantum-chemical calculations. Analysis of the P680⁺/P680 FTIR difference spectra of D1-V157H and D2-V156H showed that upon P680⁺ formation, the 13¹-keto C=O frequency upshifts by a much larger extent in P_D1 (23 cm^-1) than in P_D2 (<9 cm^-1). In addition, thermoluminescence measurements revealed that the D1-V157H mutation increased the E_m of P680 to a larger extent than did the D2-V156H mutation. These results, together with the previous results for the mutants of the His ligands of P_D1 and P_D2, lead to a definite conclusion that a charge is mainly localized to P_D1 in P680<sup/>.

Redox Tuning in Photosystem II

In photosynthesis, oxygen is liberated from water, not from CO₂; however, this model has been silent on why photosynthesis requires bicarbonate. Rutherford and colleagues solve this problem elegantly: bicarbonate tunes water-oxidising photosystem II to make onward electron transfer efficient; an absence of bicarbonate retunes, redirects, and safely shuts down energy flow.

Generating Electric Current by Bioartificial Photosynthesis

Abundant solar energy can be a sustainable source of energy. This chapter highlights recent advancements, challenges, and future scenarios in bioartificial photosynthesis, which is a new subset of bioelectrochemical systems (BESs) and technologies. BES technologies exploit the catalytic interactions between biological moieties and electrodes. At the nexus of BES and photovoltaics, this review focuses on light-harvesting technologies based on bioartificial photosynthesis. Such technologies are promising because electrical energy is generated from sunlight and water without the need for additional organic feedstock. This review focuses on photosynthetic electron generation and transfer and compares the current status of bioartificial photosynthesis with other artificial systems that mimic the chemistry of photosynthetic energy transformation.The fundamental principles and the operation of functional units of bioartificial photosynthesis are addressed. Selected photobioelectrochemical systems employed to obtain light-driven electric currents from photosynthetic organisms are presented. The achievable current output and theoretical maxima are revisited by conceptualizing operational and process window techniques. Factors affecting overall photocurrent efficiency, performance limitations, and scaleup bottlenecks are highlighted in view of enhancing the energy conversion efficiency of photobioelectrochemical systems. To finish, the challenges associated with bioartificial photosynthetic technologies are outlined. Graphical Abstract Operational window for (bio-)artificial photosynthesis. Green circle in the upper right corner: development objective for research and engineering efforts.

Photocurrents from photosystem II in a metal oxide hybrid system: Electron transfer pathways

We have investigated the nature of the photocurrent generated by Photosystem II (PSII), the water oxidizing enzyme, isolated from Thermosynechococcus elongatus, when immobilized on nanostructured titanium dioxide on an indium tin oxide electrode (TiO2/ITO). We investigated the properties of the photocurrent from PSII when immobilized as a monolayer versus multilayers, in the presence and absence of an inhibitor that binds to the site of the exchangeable quinone (QB) and in the presence and absence of exogenous mobile electron carriers (mediators). The findings indicate that electron transfer occurs from the first quinone (QA) directly to the electrode surface but that the electron transfer through the nanostructured metal oxide is the rate-limiting step. Redox mediators enhance the photocurrent by taking electrons from the nanostructured semiconductor surface to the ITO electrode surface not from PSII. This is demonstrated by photocurrent enhancement using a mediator incapable of accepting electrons from PSII. This model for electron transfer also explains anomalies reported in the literature using similar and related systems. The slow rate of the electron transfer step in the TiO2 is due to the energy level of electron injection into the semiconducting material being below the conduction band. This limits the usefulness of the present hybrid electrode. Strategies to overcome this kinetic limitation are discussed.

Natural isoforms of the Photosystem II D1 subunit differ in photoassembly efficiency of the water-oxidizing complex

Oxygenic photosynthesis efficiency at increasing solar flux is limited by light-induced damage (photoinhibition) of Photosystem II (PSII), primarily targeting the D1 reaction center subunit. Some cyanobacteria contain two natural isoforms of D1 that function better under low light (D1:1) or high light (D1:2). Herein, rates and yields of photoassembly of the Mn4CaO5 water-oxidizing complex (WOC) from the free inorganic cofactors (Mn(2+), Ca(2+), water, electron acceptor) and apo-WOC-PSII are shown to differ significantly: D1:1 apo-WOC-PSII exhibits a 2.3-fold faster rate-limiting step of photoassembly and up to seven-fold faster rate to the first light-stable Mn(3+) intermediate, IM1*, but with a much higher rate of photoinhibition than D1:2. Conversely, D1:2 apo-WOC-PSII assembles slower but has up to seven-fold higher yield, achieved by a higher quantum yield of charge separation and slower photoinhibition rate. These results confirm and extend previous observations of the two holoenzymes: D1:2-PSII has a greater quantum yield of primary charge separation, faster [P680 (+) Q A (-) ] charge recombination and less photoinhibition that results in a slower rate and higher yield of photoassembly of its apo-WOC-PSII complex. In contrast, D1:1-PSII has a lower quantum yield of primary charge separation, a slower [P680 (+) Q A (-) ] charge recombination rate, and faster photoinhibition that together result in higher rate but lower yield of photoassembly at higher light intensities. Cyanobacterial PSII reaction centers that contain the high- and low-light D1 isoforms can tailor performance to optimize photosynthesis at varying light conditions, with similar consequences on their photoassembly kinetics and yield. These different efficiencies of photoassembly versus photoinhibition impose differential costs for biosynthesis as a function of light intensity.

Redox potential of the terminal quinone electron acceptor QB in photosystem II reveals the mechanism of electron transfer regulation

Photosystem II (PSII) extracts electrons from water at a Mn4CaO5 cluster using light energy and then transfers them to two plastoquinones, the primary quinone electron acceptor QA and the secondary quinone electron acceptor QB. This forward electron transfer is an essential process in light energy conversion. Meanwhile, backward electron transfer is also significant in photoprotection of PSII proteins. Modulation of the redox potential (Em) gap of QA and QB mainly regulates the forward and backward electron transfers in PSII. However, the full scheme of electron transfer regulation remains unresolved due to the unknown Em value of QB. Here, for the first time (to our knowledge), the Em value of QB reduction was measured directly using spectroelectrochemistry in combination with light-induced Fourier transform infrared difference spectroscopy. The Em(QB (-)/QB) was determined to be approximately +90 mV and was virtually unaffected by depletion of the Mn4CaO5 cluster. This insensitivity of Em(QB (-)/QB), in combination with the known large upshift of Em(QA (-)/QA), explains the mechanism of PSII photoprotection with an impaired Mn4CaO5 cluster, in which a large decrease in the Em gap between QA and QB promotes rapid charge recombination via QA (-).

Modified molecular interactions of the pheophytin and plastoquinone electron acceptors in photosystem II of chlorophyll D-containing Acaryochloris marina as revealed by FTIR spectroscopy

Acaryochloris marina is a unique cyanobacterium that contains chlorophyll (Chl) d as a major pigment. Because Chl d has smaller excitation energy than Chl a used in ordinary photosynthetic organisms, the energetics of the photosystems of A. marina have been the subject of interest. It was previously shown that the redox potentials (E m's) of the redox-active pheophytin a (Pheo) and the primary plastoquinone electron acceptor (QA) in photosystem II (PSII) of A. marina are higher than those in Chl a-containing PSII, to compensate for the smaller excitation energy of Chl d (Allakhverdiev et al., Proc Natl Acad Sci USA 107: 3924-3929, 2010; ibid. 108: 8054-8058, 2011). To clarify the mechanisms of these E m increases, in this study, we have investigated the molecular interactions of Pheo and QA in PSII core complexes from A. marina using Fourier transform infrared (FTIR) spectroscopy. Light-induced FTIR difference spectra upon single reduction of Pheo and QA showed that spectral features in the regions of the keto and ester C=O stretches and the chlorin ring vibrations of Pheo and in the CO/CC stretching region of the Q A (-) semiquinone anion in A. marina are significantly different from those of the corresponding spectra in Chl a-containing cyanobacteria. These observations indicate that the molecular interactions, including the hydrogen bond interactions at the C=O groups, of these cofactors are modified in their binding sites of PSII proteins. From these results, along with the sequence information of the D1 and D2 proteins, it is suggested that A. marina tunes the E m's of Pheo and QA by altering nearby hydrogen bond networks to modify the structures of the binding pockets of these cofactors.

Evidence for the involvement of loosely bound plastosemiquinones in superoxide anion radical production in photosystem II

Recent evidence has indicated the presence of novel plastoquinone-binding sites, Q_C and Q_D, in photosystem II (PSII). Here, we investigated the potential involvement of loosely bound plastosemiquinones in superoxide anion radical (O₂^•−) formation in spinach PSII membranes using electron paramagnetic resonance (EPR) spin-trapping spectroscopy. Illumination of PSII membranes in the presence of the spin trap EMPO (5-(ethoxycarbonyl)-5-methyl-1-pyrroline N-oxide) resulted in the formation of O₂^•−, which was monitored by the appearance of EMPO-OOH adduct EPR signal. Addition of exogenous short-chain plastoquinone to PSII membranes markedly enhanced the EMPO-OOH adduct EPR signal. Both in the unsupplemented and plastoquinone-supplemented PSII membranes, the EMPO-OOH adduct EPR signal was suppressed by 50% when the urea-type herbicide DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) was bound at the Q_B site. However, the EMPO-OOH adduct EPR signal was enhanced by binding of the phenolic-type herbicide dinoseb (2,4-dinitro-6-sec-butylphenol) at the Q_D site. Both in the unsupplemented and plastoquinone-supplemented PSII membranes, DCMU and dinoseb inhibited photoreduction of the high-potential form of cytochrome b₅₅₉ (cyt b₅₅₉). Based on these results, we propose that O₂^•− is formed via the reduction of molecular oxygen by plastosemiquinones formed through one-electron reduction of plastoquinone at the Q_B site and one-electron oxidation of plastoquinol by cyt b₅₅₉ at the Q_C site. On the contrary, the involvement of a plastosemiquinone formed via the one-electron oxidation of plastoquinol by cyt b₅₅₉ at the Q_D site seems to be ambiguous. In spite of the fact that the existence of Q_C and Q_D sites is not generally accepted yet, the present study provided more spectroscopic data on the potential functional role of these new plastoquinone-binding sites.

Ideal osmotic spaces for chlorobionts or cyanobionts are differentially realized by lichenized fungi

Lichens result from symbioses between a fungus and either a green alga or a cyanobacterium. They are known to exhibit extreme desiccation tolerance. We investigated the mechanism that makes photobionts biologically active under severe desiccation using green algal lichens (chlorolichens), cyanobacterial lichens (cyanolichens), a cephalodia-possessing lichen composed of green algal and cyanobacterial parts within the same thallus, a green algal photobiont, an aerial green alga, and a terrestrial cyanobacterium. The photosynthetic response to dehydration by the cyanolichen was almost the same as that of the terrestrial cyanobacterium but was more sensitive than that of the chlorolichen or the chlorobiont. Different responses to dehydration were closely related to cellular osmolarity; osmolarity was comparable between the cyanolichen and a cyanobacterium as well as between a chlorolichen and a green alga. In the cephalodium-possessing lichen, osmolarity and the effect of dehydration on cephalodia were similar to those exhibited by cyanolichens. The green algal part response was similar to those exhibited by chlorolichens. Through the analysis of cellular osmolarity, it was clearly shown that photobionts retain their original properties as free-living organisms even after lichenization.

Protein film voltammetry and co-factor electron transfer dynamics in spinach photosystem II core complex

Direct protein film voltammetry (PFV) was used to investigate the redox properties of the photosystem II (PSII) core complex from spinach. The complex was isolated using an improved protocol not used previously for PFV. The PSII core complex had high oxygen-evolving capacity and was incorporated into thin lipid and polyion films. Three well-defined reversible pairs of reduction and oxidation voltammetry peaks were observed at 4 °C in the dark. Results were similar in both types of films, indicating that the environment of the PSII-bound cofactors was not influenced by film type. Based on comparison with various control samples including Mn-depleted PSII, peaks were assigned to chlorophyll a (Chl a) (Em = -0.47 V, all vs. NHE, at pH 6), quinones (-0.12 V), and the manganese (Mn) cluster (Em = 0.18 V). PFV of purified iron heme protein cytochrome b-559 (Cyt b-559), a component of PSII, gave a partly reversible peak pair at 0.004 V that did not have a potential similar to any peaks observed from the intact PSII core complex. The closest peak in PSII to 0.004 V is the 0.18 V peak that was found to be associated with a two-electron process, and thus is inconsistent with iron heme protein voltammetry. The -0.47 V peak had a peak potential and peak potential-pH dependence similar to that found for purified Chl a incorporated into DMPC films. The midpoint potentials reported here may differ to various extents from previously reported redox titration data due to the influence of electrode double-layer effects. Heterogeneous electron transfer (hET) rate constants were estimated by theoretical fitting and digital simulations for the -0.47 and 0.18 V peaks. Data for the Chl a peaks were best fit to a one-electron model, while the peak assigned to the Mn cluster was best fit by a two-electron/one-proton model.

Engineered Photosystem II reaction centers optimize photochemistry versus photoprotection at different solar intensities

The D1 protein of Photosystem II (PSII) provides most of the ligating amino acid residues for the Mn4CaO5 water-oxidizing complex (WOC) and half of the reaction center cofactors, and it is present as two isoforms in the cyanobacterium Synechococcus elongatus PCC 7942. These isoforms, D1:1 and D1:2, confer functional advantages for photosynthetic growth at low and high light intensities, respectively. D1:1, D1:2, and seven point mutations in the D1:2 background that are native to D1:1 were expressed in the green alga Chlamydomonas reinhardtii. We used these nine strains to show that those strains that confer a higher yield of PSII charge separation under light-limiting conditions (where charge recombination is significant) have less efficient photochemical turnover, measured in terms of both a lower WOC turnover probability and a longer WOC cycle period. Conversely, these same strains under light saturation (where charge recombination does not compete) confer a correspondingly faster O2 evolution rate and greater protection against photoinhibition. Taken together, the data clearly establish that PSII primary charge separation is a trade-off between photochemical productivity (water oxidation and plastoquinone reduction) and charge recombination (photoprotection). These trade-offs add up to a significant growth advantage for the two natural isoforms. These insights provide fundamental design principles for engineering of PSII reaction centers with optimal photochemical efficiencies for growth at low versus high light intensities.

Some Photosystem II properties depending on the D1 protein variants in Thermosynechococcus elongatus

Cyanobacteria have multiple psbA genes encoding PsbA, the D1 reaction center protein of the Photosystem II complex which bears together with PsbD, the D2 protein, most of the cofactors involved in electron transfer reactions. The thermophilic cyanobacterium Thermosynechococcus elongatus has three psbA genes differently expressed depending on the environmental conditions. Among the 344 residues constituting each of the 3 possible PsbA variants there are 21 substitutions between PsbA1 and PsbA3, 31 between PsbA1 and PsbA2 and 27 between PsbA2 and PsbA3. In this review, we summarize the changes already identified in the properties of the redox cofactors depending on the D1 variant constituting Photosystem II in T. elongatus. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Structural, functional and auxiliary proteins of photosystem II

Photosystem II (PSII) is the water-splitting enzyme complex of photosynthesis and consists of a large number of protein subunits. Most of these proteins have been structurally and functionally characterized, although there are differences between PSII of plants, algae and cyanobacteria. Here we catalogue all known PSII proteins giving a brief description, where possible of their genetic origin, physical properties, structural relationships and functions. We have also included details of auxiliary proteins known at present to be involved in the in vivo assembly, maintenance and turnover of PSII and which transiently bind to the reaction centre core complex. Finally, we briefly give details of the proteins which form the outer light-harvesting systems of PSII in different types of organisms.

Natural variants of photosystem II subunit D1 tune photochemical fitness to solar intensity

Photosystem II (PSII) is composed of six core polypeptides that make up the minimal unit capable of performing the primary photochemistry of light-driven charge separation and water oxidation in all oxygenic phototrophs. The D1 subunit of this complex contains most of the ligating amino acid residues for the Mn(4)CaO(5) core of the water-oxidizing complex (WOC). Most cyanobacteria have 3-5 copies of the psbA gene coding for at least two isoforms of D1, whereas algae and plants have only one isoform. Synechococcus elongatus PCC 7942 contains two D1 isoforms; D1:1 is expressed under low light conditions, and D1:2 is up-regulated in high light or stress conditions. Using a heterologous psbA expression system in the green alga Chlamydomonas reinhardtii, we have measured growth rate, WOC cycle efficiency, and O(2) yield as a function of D1:1, D1:2, or the native algal D1 isoform. D1:1-PSII cells outcompete D1:2-PSII cells and accumulate more biomass in light-limiting conditions. However, D1:2-PSII cells easily outcompete D1:1-PSII cells at high light intensities. The native C. reinhardtii-PSII WOC cycles less efficiently at all light intensities and produces less O(2) than either cyanobacterial D1 isoform. D1:2-PSII makes more O(2) per saturating flash than D1:1-PSII, but it exhibits lower WOC cycling efficiency at low light intensities due to a 40% faster charge recombination rate in the S(3) state. These functional advantages of D1:1-PSII and D1:2-PSII at low and high light regimes, respectively, can be explained by differences in predicted redox potentials of PSII electron acceptors that control kinetic performance.

Environment of TyrZ in photosystem II from Thermosynechococcus elongatus in which PsbA2 is the D1 protein

The main cofactors that determine the photosystem II (PSII) oxygen evolution activity are borne by the D1 and D2 subunits. In the cyanobacterium Thermosynechococcus elongatus, there are three psbA genes coding for D1. Among the 344 residues constituting D1, there are 21 substitutions between PsbA1 and PsbA3, 31 between PsbA1 and PsbA2, and 27 between PsbA2 and PsbA3. Here, we present the first study of PsbA2-PSII. Using EPR and UV-visible time-resolved absorption spectroscopy, we show that: (i) the time-resolved EPR spectrum of Tyr(Z)(•) in the (S(3)Tyr(Z)(•))' is slightly modified; (ii) the split EPR signal arising from Tyr(Z)(•) in the (S(2)Tyr(Z)(•))' state induced by near-infrared illumination at 4.2 K of the S(3)Tyr(Z) state is significantly modified; and (iii) the slow phases of P(680)(+) reduction by Tyr(Z) are slowed down from the hundreds of μs time range to the ms time range, whereas both the S(1)Tyr(Z)(•) → S(2)Tyr(Z) and the S(3)Tyr(Z)(•) → S(0)Tyr(Z) + O(2) transition kinetics remained similar to those in PsbA(1/3)-PSII. These results show that the geometry of the Tyr(Z) phenol and its environment, likely the Tyr-O···H···Nε-His bonding, are modified in PsbA2-PSII when compared with PsbA(1/3)-PSII. They also point to the dynamics of the proton-coupled electron transfer processes associated with the oxidation of Tyr(Z) being affected. From sequence comparison, we propose that the C144P and P173M substitutions in PsbA2-PSII versus PsbA(1/3)-PSII, respectively located upstream of the α-helix bearing Tyr(Z) and between the two α-helices bearing Tyr(Z) and its hydrogen-bonded partner, His-190, are responsible for these changes.

Deactivation processes in PsbA1-Photosystem II and PsbA3-Photosystem II under photoinhibitory conditions in the cyanobacterium Thermosynechococcus elongatus

The sensitivity to high light conditions of Photosystem II with either PsbA1 (WT*1) or PsbA3 (WT*3) as the D1 protein was studied in whole cells of the thermophilic cyanobacterium Thermosynechococcus elongatus. When the cells are cultivated under high light conditions the following results were found: (i) The O(2) evolution activity decreases faster in WT*1 cells than in WT*3 cells both in the absence and in the presence of lincomycin, a protein synthesis inhibitor; (ii) In WT*1 cells, the rate constant for the decrease of the O(2) evolution activity is comparable in the presence and in the absence of lincomycin; (iii) The D1 content revealed by western blot analysis decays similarly in both WT*1 and WT*3 cells and much slowly than O(2) evolution; (iv) The faster decrease in O(2) evolution in WT*1 than in WT*3 cells correlates with a much faster inhibition of the S(2)-state formation; (v) The shape of the WT*1 cells is altered. All these results are in agreement with a photo-inhibition process resulting in the loss of the O(2) activity much faster than the D1 turnover in PsbA1-PSII and likely to a greater production of reactive oxygen species under high light conditions in WT*1 than in WT*3. This latter result is discussed in view of the known effects of the PsbA1 to PsbA3 substitution on the redox properties of the Photosystem II cofactors. The observation that under low light conditions WT*3 cells are able to express the psbA(3) gene, whereas under similar conditions wild type cells are expressing mainly the psbA(1) gene is also discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Cationic state distribution over the chlorophyll d-containing P(D1)/P(D2) pair in photosystem II

Most of the chlorophyll (Chl) cofactors in photosystem II (PSII) from Acaryochloris marina are Chld, although a few Chla molecules are also present. To evaluate the possibility that Chla may participate in the P(D1)/P(D2) Chl pair in PSII from A. marina, the P(D1)(•+)/P(D2)(•+) charge ratio was investigated using the PSII crystal structure analyzed at 1.9-Å resolution, while considering all possibilities for the Chld-containing P(D1)/P(D2) pair, i.e., Chld/Chld, Chla/Chld, and Chld/Chla pairs. Chld/Chld and Chla/Chld pairs resulted in a large P(D1)(•+) population relative to P(D2)(•+), as identified in Chla/Chla homodimer pairs in PSII from other species, e.g., Thermosynechococcus elongatus PSII. However, the Chld/Chla pair possessed a P(D1)(•+)/P(D2)(•+) ratio of approximately 50/50, which is in contrast to previous spectroscopic studies on A. marina PSII. The present results strongly exclude the possibility that the Chld/Chla pair serves as P(D1)/P(D2) in A. marina PSII. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Charge separation in photosystem II: a comparative and evolutionary overview

Our current understanding of the PSII reaction centre owes a great deal to comparisons to the simpler and better understood, purple bacterial reaction centre. Here we provide an overview of the similarities with a focus on charge separation and the electron acceptors. We go on to discuss some of the main differences between the two kinds of reaction centres that have been highlighted by the improving knowledge of PSII. We attempt to relate these differences to functional requirements of water splitting. Some are directly associated with that function, e.g. high oxidation potentials, while others are associated with regulation and protection against photodamage. The protective and regulatory functions are associated with the harsh chemistry performed during its normal function but also with requirements of the enzyme while it is undergoing assembly and repair. Key aspects of PSII reaction centre evolution are also addressed. This article is part of a Special Issue entitled: Photosystem II.

Redox potentials of primary electron acceptor quinone molecule (QA)- and conserved energetics of photosystem II in cyanobacteria with chlorophyll a and chlorophyll d

In a previous study, we measured the redox potential of the primary electron acceptor pheophytin (Phe) a of photosystem (PS) II in the chlorophyll d-dominated cyanobacterium Acaryochloris marina and a chlorophyll a-containing cyanobacterium, Synechocystis. We obtained the midpoint redox potential (E(m)) values of -478 mV for A. marina and -536 mV for Synechocystis. In this study, we measured the redox potentials of the primary electron acceptor quinone molecule (Q(A)), i.e., E(m)(Q(A)/Q(A)(-)), of PS II and the energy difference between [P680·Phe a(-)·Q(A)] and [P680·Phe a·Q(A)(-)], i.e., ΔG(PhQ). The E(m)(Q(A)/Q(A)(-)) of A. marina was determined to be +64 mV without the Mn cluster and was estimated to be -66 to -86 mV with a Mn-depletion shift (130-150 mV), as observed with other organisms. The E(m)(Phe a/Phe a(-)) in Synechocystis was measured to be -525 mV with the Mn cluster, which is consistent with our previous report. The Mn-depleted downshift of the potential was measured to be approximately -77 mV in Synechocystis, and this value was applied to A. marina (-478 mV); the E(m)(Phe a/Phe a(-)) was estimated to be approximately -401 mV. These values gave rise to a ΔG(PhQ) of -325 mV for A. marina and -383 mV for Synechocystis. In the two cyanobacteria, the energetics in PS II were conserved, even though the potentials of Q(A)(-) and Phe a(-) were relatively shifted depending on the special pair, indicating a common strategy for electron transfer in oxygenic photosynthetic organisms.

Constitution and energetics of photosystem I and photosystem II in the chlorophyll d-dominated cyanobacterium Acaryochloris marina

This mini review presents current topics of discussion about photosystem (PS) I and PS II of photosynthesis in the Acaryochloris marina. A. marina is a photosynthetic cyanobacterium in which chlorophyll (Chl) d is the major antenna pigment (>95%). However, Chl a is always present in a few percent. Chl d absorbs light with a wavelength up to 30 nm red-shifted from Chl a. Therefore, the chlorophyll species of the special pair in PS II has been a matter of debate because if Chl d was the special pair component, the overall energetics must be different in A. marina. The history of this field indicates that a purified sample is necessary for the reliable identification and characterization of the special pair. In view of the spectroscopic data and the redox potential of pheophytin, we discuss the nature of special pair constituents and the localization of the enigmatic Chl a.