Inhibitor "double occupancy" in the Q(o) pocket of the chloroplast cytochrome b6f complex

Effects of Novel Photosynthetic Inhibitor [CuL2]Br2 Complex on Photosystem II Activity in Spinach

The effects of the novel [CuL₂]Br₂ complex (L = bis{4H-1,3,5-triazino [2,1-b]benzothiazole-2-amine,4-(2-imidazole)}copper(II) bromide complex) on the photosystem II (PSII) activity of PSII membranes isolated from spinach were studied. The absence of photosynthetic oxygen evolution by PSII membranes without artificial electron acceptors, but in the presence of [CuL₂]Br₂, has shown that it is not able to act as a PSII electron acceptor. In the presence of artificial electron acceptors, [CuL₂]Br₂ inhibits photosynthetic oxygen evolution. [CuL₂]Br₂ also suppresses the photoinduced changes of the PSII chlorophyll fluorescence yield (F_V) related to the photoreduction of the primary quinone electron acceptor, Q_A. The inhibition of both characteristic PSII reactions depends on [CuL₂]Br₂ concentration. At all studied concentrations of [CuL₂]Br₂, the decrease in the F_M level occurs exclusively due to a decrease in Fv. [CuL₂]Br₂ causes neither changes in the F₀ level nor the retardation of the photoinduced rise in F_M, which characterizes the efficiency of the electron supply from the donor-side components to Q_A through the PSII reaction center (RC). Artificial electron donors (sodium ascorbate, DPC, Mn²⁺) do not cancel the inhibitory effect of [CuL₂]Br₂. The dependences of the inhibitory efficiency of the studied reactions of PSII on [CuL₂]Br₂ complex concentration practically coincide. The inhibition constant Ki is about 16 µM, and logKi is 4.8. As [CuL₂]Br₂ does not change the aromatic amino acids’ intrinsic fluorescence of the PSII protein components, it can be proposed that [CuL₂]Br₂ has no significant effect on the native state of PSII proteins. The results obtained in the present study are compared to the literature data concerning the inhibitory effects of PSII Cu(II) aqua ions and Cu(II)-organic complexes.

Transcriptional regulation of photoprotection in dark-to-light transition-More than just a matter of excess light energy

In nature, photosynthetic organisms are exposed to different light spectra and intensities depending on the time of day and atmospheric and environmental conditions. When photosynthetic cells absorb excess light, they induce nonphotochemical quenching to avoid photodamage and trigger expression of "photoprotective" genes. In this work, we used the green alga Chlamydomonas reinhardtii to assess the impact of light intensity, light quality, photosynthetic electron transport, and carbon dioxide on induction of the photoprotective genes (LHCSR1, LHCSR3, and PSBS) during dark-to-light transitions. Induction (mRNA accumulation) occurred at very low light intensity and was independently modulated by blue and ultraviolet B radiation through specific photoreceptors; only LHCSR3 was strongly controlled by carbon dioxide levels through a putative enhancer function of CIA5, a transcription factor that controls genes of the carbon concentrating mechanism. We propose a model that integrates inputs of independent signaling pathways and how they may help the cells anticipate diel conditions and survive in a dynamic light environment.

Catalytic Reactions and Energy Conservation in the Cytochrome bc1 and b6f Complexes of Energy-Transducing Membranes

This review focuses on key components of respiratory and photosynthetic energy-transduction systems: the cytochrome bc1 and b6f (Cytbc1/b6f) membranous multisubunit homodimeric complexes. These remarkable molecular machines catalyze electron transfer from membranous quinones to water-soluble electron carriers (such as cytochromes c or plastocyanin), coupling electron flow to proton translocation across the energy-transducing membrane and contributing to the generation of a transmembrane electrochemical potential gradient, which powers cellular metabolism in the majority of living organisms. Cytsbc1/b6f share many similarities but also have significant differences. While decades of research have provided extensive knowledge on these enzymes, several important aspects of their molecular mechanisms remain to be elucidated. We summarize a broad range of structural, mechanistic, and physiological aspects required for function of Cytbc1/b6f, combining textbook fundamentals with new intriguing concepts that have emerged from more recent studies. The discussion covers but is not limited to (i) mechanisms of energy-conserving bifurcation of electron pathway and energy-wasting superoxide generation at the quinol oxidation site, (ii) the mechanism by which semiquinone is stabilized at the quinone reduction site, (iii) interactions with substrates and specific inhibitors, (iv) intermonomer electron transfer and the role of a dimeric complex, and (v) higher levels of organization and regulation that involve Cytsbc1/b6f. In addressing these topics, we point out existing uncertainties and controversies, which, as suggested, will drive further research in this field.

A. A Commonly Used Photosynthetic Inhibitor Fails to Block Electron Flow to Photosystem I in Intact Systems

In plant science, 2,4-dinitrophenylether of iodonitrothymol (DNP-INT) is frequently used as an alternative to 2,5-dibromo-6-isopropyl-3-methyl-1,4-benzoquinone (DBMIB) to examine the capacity of plastoquinol and semiquinone to reduce O₂. DNP-INT is considered to be an effective inhibitor of the photosynthetic electron transfer chain (PETC) through its binding at the Q₀ site of Cyt-b6f. The binding and action of DNP-INT has been previously characterized spectroscopically in purified Cyt-b6f complex reconstituted with Plastocyanin, PSII membranes and plastoquinone, as well as in isolated thylakoids based on its property to block MV-mediated O₂ consumption. Contrary to the conclusions obtained from these experiments, we observed clear reduction of P700⁺ in samples incubated with DNP-INT during our recent investigation into the sites of oxygen consumption in isolated thylakoids. Therefore, we carried out an extensive investigation of DNP-INT's chemical efficacy in isolated thylakoids and intact leaves. This included examination of its capacity to block the PETC before PSI, and therefore its inhibition of CO₂ fixation. P700 redox kinetics were measured using Dual-PAM whilst Membrane Inlet Mass Spectrometry (MIMS) was used for simultaneous determination of the rates of O₂ evolution and O₂ consumption in isolated thylakoids and CO₂ fixation in intact leaves, using two stable isotopes of oxygen (¹⁶O₂, ¹⁸O₂) and CO₂ (¹²C, ¹³C), respectively. Based on these investigations we confirmed that DNP-INT is unable to completely block the PETC and CO₂ fixation, therefore its use may produce artifacts if applied to isolated thylakoids or intact cells, especially when determining the locations of reactive oxygen species formation in the photosynthetic apparatus.

Flavodiiron proteins 1-to-4 function in versatile combinations in O2 photoreduction in cyanobacteria

Flavodiiron proteins (FDPs) constitute a group of modular enzymes widespread in Bacteria, Archaea and Eukarya. Synechocystis sp. PCC 6803 has four FDPs (Flv1-4), which are essential for the photoprotection of photosynthesis. A direct comparison of light-induced O2 reduction (Mehler-like reaction) under high (3% CO2, HC) and low (air level CO2, LC) inorganic carbon conditions demonstrated that the Flv1/Flv3 heterodimer is solely responsible for an efficient steady-state O2 photoreduction under HC, with flv2 and flv4 expression strongly down-regulated. Conversely, under LC conditions, Flv1/Flv3 acts only as a transient electron sink, due to the competing withdrawal of electrons by the highly induced NDH-1 complex. Further, in vivo evidence is provided indicating that Flv2/Flv4 contributes to the Mehler-like reaction when naturally expressed under LC conditions, or, when artificially overexpressed under HC. The O2 photoreduction driven by Flv2/Flv4 occurs down-stream of PSI in a coordinated manner with Flv1/Flv3 and supports slow and steady-state O2 photoreduction.

Light-adaptive state transitions in the Ross Sea haptophyte Phaeocystis antarctica and in dinoflagellate cells hosting kleptoplasts derived from it

Light state transitions (STs) is a reversible physiological process that oxygenic photosynthetic organisms use in order to minimize imbalances in the electronic excitation delivery to the reaction centers of Photosystems I and II, and thus to optimize photosynthesis. STs have been studied extensively in plants, green algae, red algae and cyanobacteria, but sparsely in algae with secondary red algal plastids, such as diatoms and haptophytes, despite their immense ecological significance. In the present work, we examine whether the haptophyte alga Phaeocystis antarctica, and dinoflagellate cells that host kleptoplasts derived from P. antarctica, both endemic in the Ross Sea, Antarctica, are capable of light adaptive STs. In these organisms, Chl a fluorescence can be excited either by direct light absorption, or indirectly by electronic excitation (EE) transfer from ultraviolet light absorbing mycosporine-like amino acids (MAAs) to Chl a (Stamatakis et al., Biochim. Biophys. Acta 1858 [2017] 189-195). Here we show that, on adaptation to PS II-selective light, dark-adapted P. antarctica cells shift from light state 1 (ST1; more EE ending up in PS II) to light state 2 (ST2; more EE ending up in PS I), as revealed by the spectral distribution of directly-excited Chl a fluorescence and by changes in the macro-organization of pigment-protein complexes evidenced by circular dichroism (CD) spectroscopy. In contrast, no STs are clearly detected in the case of the kleptoplast-hosting dinoflagellate cells, and in the case of indirectly excited Chls a, via MAAs, in P. antarctica cells.

The redox potential of the plastoquinone pool of the cyanobacterium Synechocystis species strain PCC 6803 is under strict homeostatic control

A method is presented for rapid extraction of the total plastoquinone (PQ) pool from Synechocystis sp. strain PCC 6803 cells that preserves the in vivo plastoquinol (PQH2) to -PQ ratio. Cells were rapidly transferred into ice-cold organic solvent for instantaneous extraction of the cellular PQ plus PQH2 content. After high-performance liquid chromatography fractionation of the organic phase extract, the PQH2 content was quantitatively determined via its fluorescence emission at 330 nm. The in-cell PQH2-PQ ratio then followed from comparison of the PQH2 signal in samples as collected and in an identical sample after complete reduction with sodium borohydride. Prior to PQH2 extraction, cells from steady-state chemostat cultures were exposed to a wide range of physiological conditions, including high/low availability of inorganic carbon, and various actinic illumination conditions. Well-characterized electron-transfer inhibitors were used to generate a reduced or an oxidized PQ pool for reference. The in vivo redox state of the PQ pool was correlated with the results of pulse-amplitude modulation-based chlorophyll a fluorescence emission measurements, oxygen exchange rates, and 77 K fluorescence emission spectra. Our results show that the redox state of the PQ pool of Synechocystis sp. strain PCC 6803 is subject to strict homeostatic control (i.e. regulated between narrow limits), in contrast to the more dynamic chlorophyll a fluorescence signal.

NMR reveals double occupancy of quinone-type ligands in the catalytic quinone binding site of the Na+-translocating NADH:Quinone oxidoreductase from Vibrio cholerae

The sodium ion-translocating NADH:quinone oxidoreductase (Na(+)-NQR) from the pathogen Vibrio cholerae exploits the free energy liberated during oxidation of NADH with ubiquinone to pump sodium ions across the cytoplasmic membrane. The Na(+)-NQR consists of four membrane-bound subunits NqrBCDE and the peripheral NqrF and NqrA subunits. NqrA binds ubiquinone-8 as well as quinones with shorter prenyl chains (ubiquinone-1 and ubiquinone-2). Here we show that the quinone derivative 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), a known inhibitor of the bc1 and b6f complexes found in mitochondria and chloroplasts, also inhibits quinone reduction by the Na(+)-NQR in a mixed inhibition mode. Tryptophan fluorescence quenching and saturation transfer difference NMR experiments in the presence of Na(+)-NQR inhibitor (DBMIB or 2-n-heptyl-4-hydroxyquinoline N-oxide) indicate that two quinone analog ligands are bound simultaneously by the NqrA subunit with very similar interaction constants as observed with the holoenzyme complex. We conclude that the catalytic site of quinone reduction is located on NqrA. The two ligands bind to an extended binding pocket in direct vicinity to each other as demonstrated by interligand Overhauser effects between ubiquinone-1 and DBMIB or 2-n-heptyl-4-hydroxyquinoline N-oxide, respectively. We propose that a similar spatially close arrangement of the native quinone substrates is also operational in vivo, enhancing the catalytic efficiency during the final electron transfer steps in the Na(+)-NQR.

Expression of the minor isoform pea ferredoxin in tobacco alters photosynthetic electron partitioning and enhances cyclic electron flow

Ferredoxins (Fds) are ferrosulfoproteins that function as low-potential electron carriers in plants. The Fd family is composed of several isoforms that share high sequence homology but differ in functional characteristics. In leaves, at least two isoforms conduct linear and cyclic photosynthetic electron transport around photosystem I, and mounting evidence suggests the existence of at least partial division of duties between these isoforms. To evaluate the contribution of different kinds of Fds to the control of electron fluxes along the photosynthetic electron transport chain, we overexpressed a minor pea (Pisum sativum) Fd isoform (PsFd1) in tobacco (Nicotiana tabacum) plants. The transplastomic OeFd1 plants exhibited variegated leaves and retarded growth and developmental rates. Photosynthetic studies of these plants indicated a reduction in carbon dioxide assimilation rates, photosystem II photochemistry, and linear electron flow. However, the plants showed an increase in nonphotochemical quenching, better control of excitation pressure at photosystem II, and no evidence of photoinhibition, implying a better dynamic regulation to remove excess energy from the photosynthetic electron transport chain. Finally, analysis of P700 redox status during illumination confirmed that the minor pea Fd isoform promotes enhanced cyclic flow around photosystem I. The two novel features of this work are: (1) that Fd levels achieved in transplastomic plants promote an alternative electron partitioning even under greenhouse light growth conditions, a situation that is exacerbated at higher light intensity measurements; and (2) that an alternative, minor Fd isoform has been overexpressed in plants, giving new evidence of labor division among Fd isoforms.

Activation of violaxanthin cycle in darkness is a common response to different abiotic stresses: a case study in Pelvetia canaliculata

BACKGROUND

In the violaxanthin (V) cycle, V is de-epoxidized to zeaxanthin (Z) when strong light or light combined with other stressors lead to an overexcitation of photosystems. However, plants can also suffer stress in darkness and recent reports have shown that dehydration triggers V-de-epoxidation in the absence of light. In this study, we used the highly stress-tolerant brown alga Pelvetia canaliculata as a model organism, due to its lack of lutein and its non-photochemical quenching independent of the transthylakoidal-ΔpH, to study the triggering of the V-cycle in darkness induced by abiotic stressors.

RESULTS

We have shown that besides desiccation, other factors such as immersion, anoxia and high temperature also induced V-de-epoxidation in darkness. This process was reversible once the treatments had ceased (with the exception of heat, which caused lethal damage). Irrespective of the stressor applied, the resulting de-epoxidised xanthophylls correlated with a decrease in Fv/Fm, suggesting a common function in the down-regulation of photosynthetical efficiency. The implication of the redox-state of the plastoquinone-pool and of the differential activity of V-cycle enzymes on V-de-epoxidation in darkness was also examined. Current results suggest that both violaxanthin de-epoxidase (VDE) and zeaxanthin-epoxidase (ZE) have a basal constitutive activity even in darkness, being ZE inhibited under stress. This inhibition leads to Z accumulation.

CONCLUSION

This study demonstrates that V-cycle activity is triggered by several abiotic stressors even when they occur in an absolute absence of light, leading to a decrease in Fv/Fm. This finding provides new insights into an understanding of the regulation mechanism of the V-cycle and of its ecophysiological roles.

The Q cycle of cytochrome bc complexes: a structure perspective

Aspects of the crystal structures of the hetero-oligomeric cytochrome bc(1) and b(6)f ("bc") complexes relevant to their electron/proton transfer function and the associated redox reactions of the lipophilic quinones are discussed. Differences between the b(6)f and bc(1) complexes are emphasized. The cytochrome bc(1) and b(6)f dimeric complexes diverge in structure from a core of subunits that coordinate redox groups consisting of two bis-histidine coordinated hemes, a heme b(n) and b(p) on the electrochemically negative (n) and positive (p) sides of the complex, the high potential [2Fe-2S] cluster and c-type heme at the p-side aqueous interface and aqueous phase, respectively, and quinone/quinol binding sites on the n- and p-sides of the complex. The bc(1) and b(6)f complexes diverge in subunit composition and structure away from this core. b(6)f Also contains additional prosthetic groups including a c-type heme c(n) on the n-side, and a chlorophyll a and β-carotene. Common structure aspects; functions of the symmetric dimer. (I) Quinone exchange with the bilayer. An inter-monomer protein-free cavity of approximately 30Å along the membrane normal×25Å (central inter-monomer distance)×15Å (depth in the center), is common to both bc(1) and b(6)f complexes, providing a niche in which the lipophilic quinone/quinol (Q/QH(2)) can be exchanged with the membrane bilayer. (II) Electron transfer. The dimeric structure and the proximity of the two hemes b(p) on the electrochemically positive side of the complex in the two monomer units allow the possibility of two alternate routes of electron transfer across the complex from heme b(p) to b(n): intra-monomer and inter-monomer involving electron cross-over between the two hemes b(p). A structure-based summary of inter-heme distances in seven bc complexes, representing mitochondrial, chromatophore, cyanobacterial, and algal sources, indicates that, based on the distance parameter, the intra-monomer pathway would be favored kinetically. (III) Separation of quinone binding sites. A consequence of the dimer structure and the position of the Q/QH(2) binding sites is that the p-side QH(2) oxidation and n-side Q reduction sites are each well separated. Therefore, in the event of an overlap in residence time by QH(2) or Q molecules at the two oxidation or reduction sites, their spatial separation would result in minimal steric interference between extended Q or QH(2) isoprenoid chains. (IV) Trans-membrane QH(2)/Q transfer. (i) n/p-side QH(2)/Q transfer may be hindered by lipid acyl chains; (ii) the shorter less hindered inter-monomer pathway across the complex would not pass through the center of the cavity, as inferred from the n-side antimycin site on one monomer and the p-side stigmatellin site on the other residing on the same surface of the complex. (V) Narrow p-side portal for QH(2)/Q passage. The [2Fe-2S] cluster that serves as oxidant, and whose histidine ligand serves as a H(+) acceptor in the oxidation of QH(2), is connected to the inter-monomer cavity by a narrow extended portal, which is also occupied in the b(6)f complex by the 20 carbon phytyl chain of the bound chlorophyll.

Transmembrane traffic in the cytochrome b6f complex

Crystal structures and their implications for function are described for the energy transducing hetero-oligomeric dimeric cytochrome b6f complex of oxygenic photosynthesis from the thermophilic cyanobacterium, Mastigocladus laminosus, and the green alga, Chlamydomonas reinhardtii. The complex has a cytochrome b core and a central quinone exchange cavity, defined by the two monomers that are very similar to those in the respiratory cytochrome bc1 complex. The pathway of quinol/quinone (Q/QH2) transfer emphasizes the labyrinthine internal structure of the complex, including an 11x12 A portal through which Q/QH2, containing a 45-carbon isoprenoid chain, must pass. Three prosthetic groups are present in the b6f complex that are not found in the related bc1 complex: a chlorophyll (Chl) a, a beta-carotene, and a structurally unique covalently bound heme that does not possess amino acid side chains as axial ligands. It is hypothesized that this heme, exposed to the cavity and a neighboring plastoquinone and close to the positive surface potential of the complex, can function in cyclic electron transport via anionic ferredoxin.

Intraprotein transfer of the quinone analogue inhibitor 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone in the cytochrome b6f complex

Details are presented of the structural analysis of the cytochrome b(6)f complex from the thermophilic cyanobacterium, Mastigocladus laminosus, in the presence of the electrochemically positive (p)-side quinone analogue inhibitor, 2,5-dibromo-3-methyl-6-isopropylbenzoquinone (DBMIB). One DBMIB binding site was found. This site is peripheral to the quinone binding space defined by the binding sites of other p-side inhibitors previously resolved in cytochrome bc(1)/b(6)f complexes. This high-affinity site resides in a p-side interfacial niche bounded by cytochrome f, subunit IV, and cytochrome b(6), is close (8 A) to the p-side heme b, but distant (19 A) from the [2Fe-2S] cluster. No significant electron density associated with the DBMIB was found elsewhere in the structure. However, the site at which DBMIB can inhibit light-induced redox turnover is within a few A of the [2Fe-2S] cluster, as shown by the absence of inhibition in mutants of Synechococcus sp. PCC 7002 at iron sulfur protein-Leu-111 near the cluster. The ability of a minimum amount of initially oxidized DBMIB to inhibit turnover of WT complex after a second light flash implies that there is a light-activated movement of DBMIB from the distal peripheral site to an inhibitory site proximal to the [2Fe-2S] cluster. Together with the necessary passage of quinone/quinol through the small Q(p) portal in the complex, it is seen that transmembrane traffic of quinone-like molecules through the core of cytochrome bc complexes can be labyrinthine.

Ubiquinol oxidation in the cytochrome bc1 complex: Reaction mechanism and prevention of short-circuiting

This review is focused on the mechanism of ubiquinol oxidation by the cytochrome bc1 complex (bc1). This integral membrane complex serves as a "hub" in the vast majority of electron transfer chains. The bc1 oxidizes a ubiquinol molecule to ubiquinone by a unique "bifurcated" reaction where the two released electrons go to different acceptors: one is accepted by the mobile redox active domain of the [2Fe-2S] iron-sulfur Rieske protein (FeS protein) and the other goes to cytochrome b. The nature of intermediates in this reaction remains unclear. It is also debatable how the enzyme prevents short-circuiting that could happen if both electrons escape to the FeS protein. Here, I consider a reaction mechanism that (i) agrees with the available experimental data, (ii) entails three traits preventing the short-circuiting in bc1, and (iii) exploits the evident structural similarity of the ubiquinone binding sites in the bc1 and the bacterial photosynthetic reaction center (RC). Based on the latter congruence, it is suggested that the reaction route of ubiquinol oxidation by bc1 is a reversal of that leading to the ubiquinol formation in the RC. The rate-limiting step of ubiquinol oxidation is then the re-location of a ubiquinol molecule from its stand-by site within cytochrome b into a catalytic site, which is formed only transiently, after docking of the mobile redox domain of the FeS protein to cytochrome b. In the catalytic site, the quinone ring is stabilized by Glu-272 of cytochrome b and His-161 of the FeS protein. The short circuiting is prevented as long as: (i) the formed semiquinone anion remains bound to the reduced FeS domain and impedes its undocking, so that the second electron is forced to go to cytochrome b; (ii) even after ubiquinol is fully oxidized, the reduced FeS domain remains docked to cytochrome b until electron(s) pass through cytochrome b; (iii) if cytochrome b becomes (over)reduced, the binding and oxidation of further ubiquinol molecules is hampered; the reason is that the Glu-272 residue is turned towards the reduced hemes of cytochrome b and is protonated to stabilize the surplus negative charge; in this state, this residue cannot participate in the binding/stabilization of a ubiquinol molecule.

Salt-induced redox-independent phosphorylation of light harvesting chlorophyll a/b proteins in Dunaliella salina thylakoid membranes

This study investigated the regulation of the major light harvesting chlorophyll a/b protein (LHCII) phosphorylation in Dunaliella salina thylakoid membranes. We found that both light and NaCl could induce LHCII phosphorylation in D. salina thylakoid membranes. Treatments with oxidants (ferredoxin and NADP) or photosynthetic electron flow inhibitors (DCMU, DBMIB, and stigmatellin) inhibited LHCII phosphorylation induced by light but not that induced by NaCl. Furthermore, neither addition of CuCl(2), an inhibitor of cytochrome b(6)f complex reduction, nor oxidizing treatment with ferricyanide inhibited light- or NaCl-induced LHCII phosphorylation, and both salts even induced LHCII phosphorylation in dark-adapted D. salina thylakoid membranes as other salts did. Together, these results indicate that the redox state of the cytochrome b(6)f complex is likely involved in light- but not salt-induced LHCII phosphorylation in D. salina thylakoid membranes.

Structure of the cytochrome b6f complex of oxygenic photosynthesis: tuning the cavity

The cytochrome b6f complex provides the electronic connection between the photosystem I and photosystem II reaction centers of oxygenic photosynthesis and generates a transmembrane electrochemical proton gradient for adenosine triphosphate synthesis. A 3.0 angstrom crystal structure of the dimeric b6f complex from the thermophilic cyanobacterium Mastigocladus laminosus reveals a large quinone exchange cavity, stabilized by lipid, in which plastoquinone, a quinone-analog inhibitor, and a novel heme are bound. The core of the b6f complex is similar to the analogous respiratory cytochrome bc1 complex, but the domain arrangement outside the core and the complement of prosthetic groups are strikingly different. The motion of the Rieske iron-sulfur protein extrinsic domain, essential for electron transfer, must also be different in the b6f complex.