Electron Transport In Thylakoid Membranes Research Articles

Acclimation of green algae to sulfur deficiency: underlying mechanisms and application for hydrogen production

Hydrogen is definitely one of the most acceptable fuels in the future. Some photosynthetic microorganisms, such as green algae and cyanobacteria, can produce hydrogen gas from water by using solar energy. In green algae, hydrogen evolution is coupled to the photosynthetic electron transport in thylakoid membranes via reaction catalyzed by the specific enzyme, (FeFe)-hydrogenase. However, this enzyme is highly sensitive to oxygen and can be quickly inhibited when water splitting is active. A problem of incompatibility between the water splitting and hydrogenase reaction can be overcome by depletion of algal cells of sulfur which is essential element for life. In this review the mechanisms underlying sustained hydrogen photoproduction in sulfur deprived C. reinhardtii and the recent achievements in studying of this process are discussed. The attention is focused on the biophysical and physiological aspects of photosynthetic response to sulfur deficiency in green algae.

Chloroplast stromal proteins, CRR6 and CRR7, are required for assembly of the NAD(P)H dehydrogenase subcomplex A in Arabidopsis.

In higher plants, the chloroplast NAD(P)H dehydrogenase (NDH) complex mediates chlororespiration and photosystem I (PSI) cyclic electron transport in thylakoid membranes. Because of its low abundance and fragility, our knowledge on the assembly of chloroplast NDH is very limited, and some nuclear-encoded factors may be involved in this process. We show here that two Arabidopsis proteins, CHLORORESPIRATORY REDUCTION 6 (CRR6) and CRR7, which were previously identified in mutants specifically defective in NDH accumulation, are present in the stroma, and their stability is independent of the NDH complex, suggesting that they are unlikely to be NDH subunits. Blue native PAGE analysis showed that the accumulation of NDH subcomplex A, which is a core part of NDH that is conserved in divergent species, was specifically impaired in the crr6 and crr7-1 mutants. However, the expression of plastid-encoded genes encoding the subcomplex A subunits was not affected, suggesting that CRR6 and CRR7 are involved in post-translational steps during the biogenesis of subcomplex A. We also discovered that a substantial quantity of NdhH is present in several protein complexes in the chloroplast stroma, possibly as early assembly intermediates of subcomplex A. Although the accumulation of these stromal complexes was not affected in crr6 or crr7-1, CRR6 was co-purified with NdhH, implying that CRR6 functions in the later step of subcomplex-A biogenesis. Accumulation of CRR7 was independent of that of CRR6; we propose that CRR7 functions in a different step in subcomplex-A biogenesis from CRR6.

A pronounced light-induced zeaxanthin formation accompanied by an unusually slight increase in non-photochemical quenching: A study with barley leaves treated with methyl viologen at moderate light

Light-induced deepoxidation of violaxanthin to antheraxanthin and zeaxanthin in plants is associated with the induction of pronounced xanthophyll-dependent non-photochemical quenching (NPQ). To date, a misbalance between a high amount of zeaxanthin in thylakoid membranes and low NPQ has been explained by an absence of lumen acidification (e.g. when NPQ is measured in the dark after high light stress). In this study, we report that this misbalance can also be observed under moderate light. We found this result (deepoxidation state, DEPS, above 55% and NPQ approximately 0.9) in barley leaves treated with 10 microM methyl viologen (MV) under white light (100 micromol photonsm(-2)s(-1), photosynthetically active radiation (PAR), growth irradiance). The addition of MV at this moderate light did not accelerate electron transport in thylakoid membranes, and induced only slight oxidative stress (no lipid peroxidation, almost unchanged maximum yield of photosystem II photochemistry, a decrease in activity of ascorbate peroxidase, and an increase in that of glutathion reductase). We suggest that, in leaves treated under the conditions used here, the lumen acidification induced by light-limited electron transport in thylakoid membranes was high enough to activate violaxanthin deepoxidase, but not sufficiently high to form the expected number of zeaxanthin-dependent quenching centers in photosystem II antennae.

Biological and polymeric self-assembled hybrid systems: Structure and properties of thylakoid/polyelectrolyte complexes

A novel hybrid system composed of biological components and synthetic polymer, thylakoid/polycation complex, has been formed and studied. Effects of complex formation on the structure, electrostatics and functioning of thylakoid membranes have been examined. Thylakoids from bean leaves were used to form complexes with polycation polyallylamine hydrochloride (PAAH) in two systems: (i) thylakoid/polycation complexes formed in an aqueous bulk phase, and (ii) immobilized thylakoid/polycation planar complexes. Immobilized on a solid substrate surface, thylakoid/polycation complexes were prepared using layer-by-layer stepwise alternate adsorption technique, i.e., via the sequential alternate adsorption of thylakoids and polycation molecules. The morphology of built up structures was investigated by scanning electron microscopy. Light-induced electron transport in chloroplasts was studied by the electron paramagnetic resonance (EPR) method. Spin probe technique was employed to study the structural and electrostatic characteristics of thylakoid membranes. We have found that efficiency of light-induced electron transport in thylakoid membranes and membrane structure were not changed noticeably by PAAH binding to thylakoids in a wide range of PAAH concentrations. The data obtained indicate the physiologically-soft character of polycation interactions with thylakoid membranes and demonstrate effectiveness of interfacial self-assembly approach to fabrication of complex planar functional nanostructures from biological components and synthetic polymers.

NADPH Dehydrogenase-mediated Respiratory Electron Transport in Thylakoid Membranes of the Cyanobacterium Synechocystis sp. PCC 6803 Is Inactive in the Light

NADPH Dehydrogenase-mediated Respiratory Electron Transport in Thylakoid Membranes of the Cyanobacterium Synechocystis sp. PCC 6803 Is Inactive in the Light

Electron transport routes in whole cells of Synechocystis sp. strain PCC 6803: the role of the cytochrome bd-type oxidase.

The plastoquinone pool is the central switching point of both respiratory and photosynthetic electron transport in cyanobacteria. Its redox state can be monitored noninvasively in whole cells using chlorophyll fluorescence induction, avoiding possible artifacts associated with thylakoid membrane preparations. This method was applied to cells of Synechocystis sp. PCC 6803 to study respiratory reactions involving the plastoquinone pool. The role of the respiratory oxidases known from the genomic sequence of Synechocystis sp. PCC 6803 was investigated by a combined strategy using inhibitors and deletion strains that lack one or more of these oxidases. The putative quinol oxidase of the cytochrome bd-type was shown to participate in electron transport in thylakoid membranes. The activity of this enzyme in thylakoids was strongly dependent on culture conditions; it was increased under conditions where the activity of the cytochrome b(6)f complex alone may be insufficient for preventing over-reduction of the PQ pool. In contrast, no indication of quinol oxidase activity in thylakoids was found for a second alternative oxidase encoded by the ctaII genes.

The Function of Tocopherols and Tocotrienols in Plants

Referee: Dr. Kozi Asada, Department of Biotechnology, Faculty of Engineering, Fukuyama University, Gakuencho 1, Fukuyama 729-0292, JapanTocopherols and tocotrienols, which differ only in the degree of saturation of their hydrophobic prenyl side chains, are lipid-soluble molecules that have a number of functions in plants. Synthesized from homogentisic acid and isopentenyl diphosphate in the plastid envelope, tocopherols and tocotrienols are essential to maintain membrane integrity. α-Tocopherol is the major form found in green parts of plants, while tocotrienols are mostly found in seeds. These compounds are antioxidants, thus they protect the plant from oxygen toxicity. Tocopherols and tocotrienols scavenge lipid peroxy radicals, thereby preventing the propagation of lipid peroxidation in membranes, and the ensuing products tocopheroxyl and tocotrienoxyl radicals, respectively, are recycled back to tocopherols and tocotrienols by the concerted action of other antioxidants. Furthermore, tocopherols and tocotrienols protect lipids and other membrane components by physically quenching and reacting chemically with singlet oxygen. The scavenging of singlet oxygen by α-tocopherol in chloroplasts results in the formation of, among other products, α -tocopherol quinone, a known contributor to cyclic electron transport in thylakoid membranes, therefore providing photoprotection for chloroplasts. Moreover, given that α-tocopherol increases membrane rigidity, its concentration, together with that of the other membrane components, might be regulated to afford adequate fluidity for membrane function. Furthermore, α-tocopherol may affect intracellular signaling in plant cells. The effects of this compound in intracellular signaling may be either direct, by interacting with key components of the signaling cascade, or indirect, through the prevention of lipid peroxidation or the scavenging of singlet oxygen. In the latter case, α-tocopherol may regulate the concentration of reactive oxygen species and plant hormones, such as jasmonic acid, within the cell, which control both the growth and development of plants, and also plant response to stress.

Effects of Carbonic Anhydrase Inhibitors on Proton Exchange and Photosynthesis in Pea Protoplasts

At concentrations of 100–200 μM, ethoxyzolamide, a lipophilic inhibitor of carbonic anhydrase, considerably (by 60%) inhibited light-induced CO2-dependent oxygen evolution in pea protoplasts at the optimum concentration of inorganic carbon (100 μM CO2) in the medium. At the same concentrations of the inhibitor, electron transport in isolated pea thylakoids was inhibited only by 6–9%. Acetazolamide, a water-soluble inhibitor of carbonic anhydrase, affected neither the rate of CO2-dependent O2evolution in protoplasts nor electron transport in thylakoid membranes. A light-dependent proton uptake by protoplasts was demonstrated. At pH 7.2, the induction kinetics and the rate of proton uptake were similar to those for CO2-dependent O2evolution. The rate of proton uptake was decreased twofold by 1 mM acetazolamide. This fact agrees with the notion that a membrane-bound carbonic anhydrase is operative in the plasma membrane of higher plant cells. A mechanism of its functioning is suggested. Possible functions of carbonic anhydrases in the cells of C3-plants are discussed.

PH‐Dependent Extraction of Ca 2+ from Photosystem II Membranes and Thylakoid Membranes: Indication of a Ca 2+‐Sensitive Site on the Acceptor Side of Photosystem II

AbstractThe action of low pH treatment (pH 3.6) known to release Ca2+ from the oxygen‐evolving complex in photosystem II (PSII) membranes and to induce Ca2+‐revers‐ible inhibition of electron transport at the acceptor side of PSII in thylakoid membranes (TM) was compared in PSII membranes and TM. The rate of the inactivation of electron transport by low pH was four times higher in TM than in PSII membranes. Ferricyanide accelerated the inactivation of PSII membranes but decreased it in the case of TM. Low pH treatment also greatly modified the fluorescence induction kinetics in both preparations, but significant differences have been found in the fluorescence induction kinetics of treated TM and PSII membranes. Calcium restored the electron transport activity and fluorescence induction kinetics in PSII membranes and TM, whereas diphenylcarbazide restored these functions only in PSII membranes. The reactivation of Ca‐depleted PSII membranes was more effective in the dark, whereas the reactivation of TM required weak light. In the case of PSII membranes subjected to low pH citrate buffer, maximal reactivation was observed at 60 mM Ca2+ but for TM about 10 mM Ca2+ was required and 60 mM fully inhibited electron transport in TM during reactivation. These results indicate that the Ca‐dependent inactivation of the acceptor side of PSII in TM after low pH treatment cannot be explained by the extraction of Ca2+ from the oxygen‐evolving complex. It is rather suggested that the Ca2+ involved in this inhibition is bound to the acceptor side of the photosystem near to the QA‐non‐heme iron binding site and may participate in the binding of a polypeptide of the PSII light antenna complex to the PSII reaction center.

Photosynthetic electron transport in thylakoids from cotton cultivars (Gossypium sp.) differing in tolerance to prometryn

A method to determine photosynthetic electron transport in thylakoid membranes is described for Gossypium barbadense (cv. Pima S-7) and G. hirsutum (cv. DP 5415). These cultivars differed markedly in tolerance to prometryn, a PS II inhibitor. The rates of photosynthetic electron transport obtained were 245 μmole oxygen mg(-1) chl h(1). Plant age and leaf size influenced the activity of the thylakoid preparations. Thylakoids from leaves of plants 24 to 37 d and 50-70 mm in diameter had the highest activities; thylakoids from cotyledons, fully expanded leaves and young leaves had low activity. Thylakoids from both species had similar photosynthetic activities and I50's for prometryn, atrazine and diuron. Thus, tolerance to prometryn was not due to differential binding at D1 protein.

The thylakoid translocation of subunit 3 of photosystem I, the psaF gene product, depends on a bipartite transit peptide and proceeds along an azide-sensitive pathway

Subunit 3 of photosystem I (PSI-3), the product of the nuclear psaF gene, is the docking protein for plastocyanin during photosynthetic electron transport in thylakoid membranes and is synthesized in the cytosol with a transit peptide that resembles structurally the bipartite targeting signals of hydrophilic, lumenal components such as plastocyanin. In organello import experiments performed with the authentic PSI-3 precursor and chimeric polypeptides consisting of residue-correct fusions of transit peptides and mature proteins derived from different plastid proteins demonstrate that the PSI-3 transit peptide is indeed capable of translocating proteins into the thylakoid lumen and that, conversely, mature PSI-3 depends on a bipartite transit peptide for its thylakoid transfer. Of the three recently described translocation/integration pathways for nucleus-encoded proteins carrying bipartite transit peptides that are distinct in their physiological requirements and strictly protein-specific, PSI-3, like plastocyanin and the 33-kDa protein of the oxygen-evolving complex, is translocated by a pathway that involves stromal factors but no proton gradient across the membrane. It is not affected by saturating amounts of the precursor for the 23-kDa protein of the oxygen-evolving complex that follows the latter route. Thylakoid translocation of PSI-3 is, however, impaired in the presence of sodium azide, which indicates that a homolog to the bacterial SecA protein might be involved in this process suggesting, thus, a prokaryote-like translocation pathway. The azide-sensitive factor appears to interact predominantly with the transit peptide of a precursor protein, since chimeras consisting of a presequence from an azide-resistant precursor and a mature part of an azide-sensitive polypeptide are still translocated in the presence of the inhibitor.

Photosystem II Inhibition by Pyran-enamine Derivatives

Abstract Through the studies on structure-activity relationships of 5-acyl-3-(1-aminoalkylidene)-4-hydroxy-2 H-pyran-2,6(3 H)-dione derivatives in photosystem II (PS II) inhibition, overall lipophilicity of the molecule was found to be a major determinant for the activity. In the substituted N -benzyl derivatives, not only the lipophilicity but also the electronic and steric characters of the substituents greatly affected the activity. Their mode of PS II inhibition seemed to be similar to that of DCMU , whereas pyran-enamine derivatives needed to be highly lipophilic to block the electron transport in thylakoid membranes, which in turn diminished the permeability through biomembranes.

Changes in the activity and the polypeptide composition of the oxygen-evolving complex in photosystem II of tobacco leaves infected with cucumber mosaic virus strain Y.

The amount of 22- and 23-kDa polypeptides in a 23-kDa protein family, which plays a regulatory role in photosynthetic oxygen evolution, decreased significantly during the progress of appearance of chlorotic spots in cucumber mosaic virus strain Y (CMV[Y])-inoculated tobacco (Nicotiana tabacum 'Ky57') leaves (Plant Mol. Biol. 16:689-698, 1991). The present study was conducted to determine if the amount of other polypeptides of the oxygen-evolving complex and the oxygen-evolving activity also decreased in CMV(Y)-inoculated tobacco leaves. The amount of a 33-kDa polypeptide, which is essential to oxygen evolution, did not decrease in CMV(Y)-inoculated leaves showing early symptoms, although the amount of 22- and 23-kDa polypeptides began to decrease. However, comparative analysis of electron transport in thylakoid membranes indicated that the oxygen-evolving activity in CMV(Y)-inoculated tobacco leaves was only partly reduced, compared with the activity in CMV(O)-inoculated tobacco leaves which did not show clear symptoms. Partial inhibition of the oxygen-evolving activity, by a differential decrease in the amount of polypeptides of the oxygen-evolving complex, seems to be associated with the primary molecular process of symptom expression in CMV(Y)-inoculated tobacco leaves.

Effect of growth temperature on membrane dynamics in a thermophilic cyanobacterium: a spin label study

A strain of Synechococcus sp. was grown at its optimal growth temperature (58° C) and at 38° C, in order to investigate possible adaptations of membrane-related properties to growth temperature. Light-induced electron transport in thylakoid membranes from both types of cells showed linear Arrhenius plots with the same activation energy (48 kJ/mol). Membranes from cells grown at 58° C had a higher temperature optimum (53° C) than those from cells grown at 38°C (41°C). Growth at 38°C caused an increase in the proportion of unsaturated fatty acids compared to growth at 58°C. The fluidity of the membranes was investigated by measuring the temperature dependence of the parameters derived from electron spin resonance spectra of the spin-labels 5-doxyldecane, 5-doxylstearate and 16-doxylstearate. Only small differences between the dynamic properties of the membranes from cells grown at different temperatures could be detected. This suggests that the observed change in fatty acid composition of the membranes following the change in growth temperature does not serve to maintain a constant viscosity at the growth temperature.

The molecular mechanism of the bicarbonate effect at the plastoquinone reductase site of photosynthesis

It has been known for some time that bicarbonate reverses the inhibition, by formate under HCO3 (-)-depletion conditions, of electron transport in thylakoid membranes. It has been shown that the major effect is on the electron acceptor side of photosystem II, at the site of plastoquinone reduction. After presenting a historical introduction, and a minireview of the bicarbonate effect, we present a hypothesis on how HCO3 (-) functions in vivo as (a) a proton donor to the plastoquinone reductase site in the D1-D2 protein; and (b) a ligand to Fe(2+) in the QA-Fe-QB complex that keeps the D1-D2 proteins in their proper functional conformation. They key points of the hypothesis are: (1) HCO3 (-) forms a salt bridge between Fe(2+) and the D2 protein. The carboxyl group of HCO3 (-) is a bidentate ligand to Fe(2+), while the hydroxyl group H-bonds to a protein residue. (2) A second HCO3 (-) is involved in protonating a histidine near the QB site to stabilize the negative charge on QB. HCO3 (-) provides a rapidly available source of H(+) for this purpose. (3) After donation of a H(+), CO3 (2-) is replaced by another HCO3 (-). The high pKa of CO3 (2-) ensures rapid reprotonation from the bulk phase. (4) An intramembrane pool of HCO3 (-) is in equilibrium with a large number of low affinity sites. This pool is a H(+) buffering domain functionally connecting the external bulk phase with the quinones. The low affinity sites buffer the intrathylakoid [HCO3 (-)] against fluctuations in the intracellular CO2. (5) Low pH and high ionic strength are suggested to disrupt the HCO3 (-) salt bridge between Fe(2+) and D2. The resulting conformational change exposes the intramembrane HCO3 (-) pool and low affinity sites to the bulk phase.Two contrasting hypotheses for the action of formate are: (a) it functions to remove bicarbonate, and the low electron transport left in such samples is due to the left-over (or endogenous) bicarbonate in the system; or (b) bicarbonate is less of an inhibitor and so appears to relieve the inhibition by formate. Hypothesis (a) implies that HCO3 (-) is an essential requirement for electron transport through the plastoquinones (bound plastoquinones QA and QB and the plastoquinone pool) of photosystem II. Hypothesis (b) implies that HCO3 (-) does not play any significant role in vivo. Our conclusion is that hypothesis (a) is correct and HCO3 (-) is an essential requirement for electron transport on the electron acceptor side of PS II. This is based on several observations: (i) since HCO3 (-), not CO2, is the active species involved (Blubaugh and Govindjee 1986), the calculated concentration of this species (220 μM at pH 8, pH of the stroma) is much higher than the calculated dissociation constant (Kd) of 35-60 μM; thus, the likelihood of bound HCO3 (-) in ambient air is high; (ii) studies on HCO3 (-) effect in thylakoid samples with different chlorophyll concentrations suggest that the "left-over" (or "endogenous") electron flow in bicarbonate-depleted chloroplasts is due to "left-over" (or endogenous) HCO3 (-) remaining bound to the system (Blubaugh 1987).

Inhibition of photosynthetic electron transport by metabolites produced by Phialophora gregata

In soybeans, adzuki beans, and mung beans the disease brown stem rot, which is caused by Phialophora gregata , is characterized by vascular browning of the stem and, in some cases, by chlorosis and necrosis of the leaves. We investigated the effect of compounds produced by P. gregata on light-driven electron transport in thylakoid membranes. The compounds investigated were contained in three different extracts isolated from the fungal culture: crude extract, gregatin A and the mother liquor from the crystallization of gregatin A. Electron transport rates were measured potentiometrically using a Clark-type oxygen electrode and spectrophotometrically by monitoring light-induced absorbancc changes due to cytochrome f . The data show that at least one metabolite produced by P. gregata can completely inhibit photosynthetic electron transport. The most potent inhibitor was the mother liquor which inhibited the reaction from H 2 O to methylviologen by 50% at a concentration of 20 μg ml −1 . Comparison of the efficacy of the different extracts indicates that gregatin A is a less effective inhibitor. The effect of the extracts on electron transport reactions that include limited segments of the photosynthetic apparatus indicates that there are at least two sites of inhibition, the Photosystem II complex and the cytochrome b/f complex. The results raise the possibility that the leaf symptoms observed in brown rot may be due to inhibition of photosynthetic electron transport by a metabolite produced by P. gregata .