Photoactive protochlorophyllide-enzyme complexes reconstituted with PORA, PORB and PORC proteins of A. thaliana: fluorescence and catalytic properties

Photoactive Pchlide-POR-NADPH complexes were reconstituted using protochlorophyllide (Pchlide) and recombinant light-dependent protochlorophyllide oxidoreductase (POR) proteins, His₆-PORA, His₆-PORB and His₆-PORC, from Arabidopsis thaliana. We did not observe any differences in the kinetics of the protochlorophyllide photoreduction at room temperature among the PORA, PORB and PORC proteins. In contrast, the PORC protein showed lower yield of Chlide formation than PORA and PORB when preincubated in the dark for 30 min and then illuminated for a short time. The most significant observation was that reconstituted Pchlide-POR-NADPH complexes showed fluorescence maxima at 77 K similar to those observed for highly aggregated Pchlide-POR-NADPH complexes in prolamellar bodies (PLBs) in vivo. Homology models of PORA, PORB and PORC of Arabidopsis thaliana were developed to compare predicted structures of POR isoforms. There were only slight structural differences, mainly in the organisation of helices and loops, but not in the shape of whole molecules. This is the first comparative analysis of all POR isoforms functioning at different stages of A. thaliana development.

EPR spin labeling measurements of thylakoid membrane fluidity during barley leaf senescence

Physical properties of thylakoid membranes isolated from barley were investigated by the electron paramagnetic resonance (EPR) spin labeling technique. EPR spectra of stearic acid spin labels 5-SASL and 16-SASL were measured as a function of temperature in secondary barley leaves during natural and dark-induced senescence. Oxygen transport parameter was determined from the power saturation curves of the spin labels obtained in the presence and absence of molecular oxygen at 25°C. Parameters of EPR spectra of both spin labels showed an increase in the thylakoid membrane fluidity during senescence, in the headgroup area of the membrane, as well as in its interior. The oxygen transport parameter also increased with age of barley, indicating easier diffusion of oxygen within the membrane and its higher fluidity. The data are consistent with age-related changes of the spin label parameters obtained directly by EPR spectroscopy. Similar outcome was also observed when senescence was induced in mature secondary barley leaves by dark incubation. Such leaves showed higher membrane fluidity in comparison with leaves of the same age, grown under light conditions. Changes in the membrane fluidity of barley secondary leaves were compared with changes in the levels of carotenoids (car) and proteins, which are known to modify membrane fluidity. Determination of total car and proteins showed linear decrease in their level with senescence. The results indicate that thylakoid membrane fluidity of barley leaves increases with senescence; the changes are accompanied with a decrease in the content of car and proteins, which could be a contributing factor.

Antagonistic Effects of α-Tocopherol and α-Tocoquinone in the Regulation of Cyclic Electron Transport around Photosystem II

α-Tocoquinone (α-TQ ) and α-tocopherol (α-TOC) which cannot substitute for plastoquinone-9 (PQ-A) as an electron acceptor from photosystem II (PS II), influence the oxygen evolution activity of thylakoid membranes under continuous illumination. In the presence of the herbicide DCMU and the protonophore FCCP which stimulate cyclic electron transport around PS II, α-TQ decreased oxygen evolution whereas α-TOC enhanced it. The effects are attributed to a stimulation or an inhibition of cyclic electron transport around PS II by α-TQ and α-TOC, respectively. Results of flash light experiments on PS II preparations show that both α-TQ and α-TOC increased the d-parameter which describes the transition probability from the S3- to the S0-state of the oxygen-evolving complex, although to a smaller extent when PQ-A is added alone to the preparations. The initial S-state distribution in darkadapted samples was changed only upon PQ-A addition and influenced neither by α-TQ nor by α-TO C supplementation. These effects indicate different kinds of interaction of PQ-A, α-TQ and α-TOC with the PS II components. α-TQ increased and α-TOC decreased the “total miss” parameter both in the presence or absence of PQ-A. A possible site of interaction of α-TQ and α-TO C with the cyclic electron transport around PS II is suggested.

Effect of copper on pro- and antioxidative reactions in radish (Raphanus sativus L.) in vitro and in vivo

The generation of superoxide radicals, lipid peroxidation (as measured by malone dialdehyde formation) and the activity of selected antioxidant enzymes (superoxide dismutase, catalase, ascorbate peroxidase) were assessed in radish (Raphanus sativus L.), in response to elevated concentrations of copper ions in the culture medium in vitro and in vivo. Experiments were performed on 7-day-old seedlings and 5-week-old calluses grown on media supplemented with CuSO4 in concentrations of 10, 100 and 1000μМ. The exposure to elevated Cu concentrations in the medium significantly reduced both callogenesis and the proliferation of radish calluses in vitro. Cu treatment resulted in the increased generation of the superoxide radical (O2(-)) in radish seedlings and calluses indicating the occurrence of oxidative stress in radish cells, whereas the level of lipid peroxidation (LPO) remained unchanged. Both in calluses and in radish seedlings in vivo, the relative level of oxidative stress was maximal at micromolar Cu concentrations and became attenuated with increasing Cu concentrations. Stronger oxidative stress occurred in the radish seedlings in vivo, compared with radish calluses in vitro. The observed lower sensitivity of calluses to Cu-induced oxidative stress and their ability to proliferate upon exposure to Cu concentrations of up to 1000μМ demonstrate the potential of in vitro cell-selection to obtain metal-tolerant radish plant lines.

Visualization and characterization of prolamellar bodies with atomic force microscopy

Prolamellar bodies (PLBs) isolated from etiolated wheat seedlings were studied with the use of atomic force microscopy (AFM), transmission electron microscopy (TEM) and fluorescence spectroscopy. With AFM, PLBs were seen as spherical structures about 1-2μm in diameter, more elastic than mica and poly-l-lysine substrate. TEM analyses confirmed that PLBs of wheat leaf etioplasts also had an average diameter of appr. 1μm. Illumination induced the photoreduction of photoactive protochlorophyllide (Pchlide), i.e. Pchlide bound to protochlorophyllide oxidoreductase, which was shown in fluorescence spectra. The photoreduction was followed by the disruption of PLB structures, which started with the enlargement of PLB spheres and then their fragmentation into small balls as seen with AFM. Light-induced vesicle formation and the outgrowth of lamellar (pro)thylakoid membranes on the PLB surface were also confirmed by TEM analyses, and resulted in the apparent enlargement of the PLB diameter. The blue-shift of the fluorescence emission maximum of chlorophyllide observed for PLBs at room temperature after Pchlide photoreduction was completed within 25min. However, structural changes in PLBs were still observed after the completion of the blue-shift. The incubation of PLBs in darkness with HgCl2 also resulted in PLB enlargement and a loosening of their structure. AFM provides a unique opportunity to observe PLBs at a physiological temperature without the necessity of fixation.

Structural effects of the Solanum steroids solasodine, diosgenin and solanine on human erythrocytes and molecular models of eukaryotic membranes

This report presents evidence that the following Solanum steroids: solasodine, diosgenin and solanine interact with human erythrocytes and molecular models of their membranes as follows: a) X-ray diffraction studies showed that the compounds at low molar ratios (0.1-10.0mol%) induced increasing structural perturbation to dimyristoylphosphatidylcholine bilayers and to a considerable lower extent to those of dimyristoylphosphatidylethanolamine; b) differential scanning calorimetry data showed that the compounds were able to alter the cooperativity of dimyristoylphosphatidylcholine, dimyristoylphosphatidylethanolamine and dimyristoylphosphatidylserine phase transitions in a concentration-dependent manner; c) in the presence of steroids, the fluorescence of Merocyanine 540 incorporated to the membranes decreased suggesting a fluidization of the lipid system; d) scanning electron microscopy observations showed that all steroids altered the normal shape of human erythrocytes inducing mainly echinocytosis, characterized by the formation of blebs in their surfaces, an indication that their molecules are located into the outer monolayer of the erythrocyte membrane.

Protochlorophyllide in model systems--an approach to in vivo conditions

Absorption and fluorescence properties of protochlorophyllide (Pchlide) monomers and aggregates in various model systems are presented in this study. The absorption and fluorescence maxima, and fluorescence lifetimes of Pchlide monomers were not dependent on liposome composition. Fluorescence quenching experiments using KI and SASLs as fluorescence quenchers, revealed that Pchlide molecules entered a lipid bilayer and were localized close to the polar lipid headgroup area. The process of Pchlide aggregation was evident for high (i.e. at least 9 mol%) Pchlide content in liposomes prepared from galactolipids. To our knowledge, this is the first study of Pchlide aggregation in membrane-mimicking model systems. The aggregates showed absorption maxima at 480 and 650 nm. Fluorescence of the aggregates measured for excitation at 480 nm had a maximum at 656 nm and was characterized with two fluorescence lifetime components, i.e. 0.1 and 1-2 ns. Pchlide aggregates observed in the buffer had similar position of absorption and fluorescence bands to those observed in liposomes, although the overall fluorescence intensity was considerably lower. Some differences in the relative intensity of Soret absorption bands were observed. These results showed that the presence of liposomes decreased the efficiency of the process of Pchlide aggregation. Water bound at the interface region of AOT/isooctane/water reversed micelles induced disaggregation of the Pchlide aggregates indicating that Pchlide aggregates are buried into hydrophilic core of micelles. The results are discussed with respect to the role of lipids in Pchlide aggregation found in plant etioplasts and their significance for light-induced Pchlide photoreduction.

Temperature effect on growth, and selected parameters of Phaeodactylum tricornutum in batch cultures

The effect of optimal and stress temperatures on the growth kinetics of the Phaeodactylum tricornutum CCAP/1055/1 strain (a model diatom with a known genome sequence) in batch cultures was examined. The analysis of the obtained results showed two phases of culture growth. There were significant positive correlations between OD increase of chlorophyll a chlorophyll c and protein concentration at different temperatures. The Fv/Fm parameter achieved a maximum level on the 6(th) or 7(th) day and then decreased to the values registered on the first day of observation. Genetic material undergoes gradual degradation 10 days after inoculation. The size of the cells was invariable.

Expression of three diadinoxanthin de-epoxidase genes of Phaeodacylum tricornutum in Escherichia coli Origami b and BL21 strain

In the diadinoxanthin cycle the epoxy group is removed from diadinoxanthin and diatoxanthin is created. This conversion takes place e.g. in diatoms with the involvement of the enzyme diadinoxanthin de-epoxidase. In one of the diatom species, Phaeodactylum tricornutum (CCAP 1055/1 strain with genome sequenced) three de-epoxidase genes (PtVDE, PtVDL1, PtVDL2) have been identified, but only one of them (PtVDE) corresponds to violaxanthin de-epoxidase, an enzyme which is commonly found in higher plants. In these studies, the expression of two de-epoxidase genes of another Phaeodactylum tricornutum strain (UTEX 646), which is commonly used in diatom studies, were obtained in Origami b and BL21 E. coli strains. The molecular masses of the mature proteins are about 49 kDa and 60 kDa, respectively, for VDE and VDL2. Both enzymes are active with violaxanthin as a substrate.

Protochlorophyllide and protochlorophyll in model membranes - an influence of hydrophobic side chain moiety

In the present work, a comparative study of protochlorophyllide- and protochlorophyll-lipid interaction was performed on liposomes prepared from phospholipids and galactolipids, which had a pigment content varying from 0.1 to 4mol%. The incorporation of pigment molecules into the lipid bilayer and pigment-pigment interactions were investigated. Protochlorophyllide entered the lipid bilayer spontaneously and showed fluorescence spectra characteristic of its monomers. Similar spectra were observed for protochlorophyll where its concentration was low. However, the fluorescence maxima of protochlorophyll monomers were blue-shifted compared to those of protochlorophyllide by about 5nm. Protochlorophyll at high concentrations formed transient aggregates that showed an additional fluorescence band with a maximum at around 685nm, especially in liposomes prepared from phospholipids. For both compounds, the Stern-Volmer constant for KI quenching was much lower in liposomes than in solution, which confirmed the incorporation of these compounds into the lipid bilayer. Two populations of protochlorophyll that differed in their accessibility to quenching by KI were determined, and the proportions between them for different lipids are discussed. Protochlorophyllide showed such heterogeneity only in DPPC membranes. Quenching with 5- and 16-SASL revealed a localization of the porphyrin ring of both Pchl and Pchlide in the polar headgroup area of the lipid bilayer. The side chain of protochlorophyll forced these molecules to localize deeper in the bilayer in the case of DPPC in gel phase.

The dynamics of the non-heme iron in bacterial reaction centers from Rhodobacter sphaeroides

We investigate the dynamical properties of the non-heme iron (NHFe) in His-tagged photosynthetic bacterial reaction centers (RCs) isolated from Rhodobacter (Rb.) sphaeroides. Mössbauer spectroscopy and nuclear inelastic scattering of synchrotron radiation (NIS) were applied to monitor the arrangement and flexibility of the NHFe binding site. In His-tagged RCs, NHFe was stabilized only in a high spin ferrous state. Its hyperfine parameters (IS=1.06±0.01mm/s and QS=2.12±0.01mm/s), and Debye temperature (θ(D0)~167K) are comparable to those detected for the high spin state of NHFe in non-His-tagged RCs. For the first time, pure vibrational modes characteristic of NHFe in a high spin ferrous state are revealed. The vibrational density of states (DOS) shows some maxima between 22 and 33meV, 33 and 42meV, and 53 and 60meV and a very sharp one at 44.5meV. In addition, we observe a large contribution of vibrational modes at low energies. This iron atom is directly connected to the protein matrix via all its ligands, and it is therefore extremely sensitive to the collective motions of the RC protein core. A comparison of the DOS spectra of His-tagged and non-His-tagged RCs from Rb. sphaeroides shows that in the latter case the spectrum was overlapped by the vibrations of the heme iron of residual cytochrome c(2), and a low spin state of NHFe in addition to its high spin one. This enabled us to pin-point vibrations characteristic for the low spin state of NHFe.

Investigating the interaction between the violaxanthin cycle enzyme zeaxanthin epoxidase and the thylakoid membrane

In the present study the interaction between the violaxanthin cycle enzyme zeaxanthin epoxidase (ZEP) and the thylakoid membrane was investigated. Isolated, active thylakoid membranes of spinach (Spinacia oleracea L.) were subjected to different salt and detergent treatments that are generally used to isolate peripheral and integral membrane proteins. These salt and detergent treatments included the use of the salts NaBr, Na(2)CO(3) and Tris and the detergents octylglucoside (OG) and dodecylmaltoside (DM). After the treatments the activity of the ZEP was determined in washed thylakoid membranes. To obtain additional information about the mode of ZEP binding to the membrane a hydrophobicity plot based on the amino acid sequence of the protein was constructed. The plot was then compared to a diagram obtained for the photosystem II antenna Lhcb1 protein whose integration into the thylakoid membrane is known. The results of the salt and detergent treatments of the thylakoid membrane suggest that the ZEP is a peripheral, rather weakly bound membrane protein. Results from the hydrophobicity plots indicate the existence of specialized protein domains which may realize the partial integration and binding of the ZEP to the thylakoid membrane.

Variations in xanthophyll composition in etiolated seedlings of Arabidopsis thaliana correlate with protochlorophyllide accumulation

Protochlorophyllide (Pchlide) accumulation and xantophyll composition were studied in 5-day old etiolated seedlings of three ecotypes of Arabidopsis thaliana: Columbia (Col-0), Landsberg erecta (Ler) and Wassiliewska (Ws). The total Pchlide level as measured by fluorescence spectroscopy varied significantly between ecotypes. A rapid HPLC method revealed quantitative differences in carotenoid composition. It was found that in the Ler ecotype any enhanced accumulation of Pchlide correlates with an increased level of lutein, suggesting the role of enzymes involved in lutein synthesis in cross-regulation between chlorophyll and carotenoid biosynthetic pathways. The function of the dark-accumulated carotenoid pool in seedling de-etiolation is discussed.

Atomic force microscopy studies of the adhesive properties of DPPC vesicles containing β-carotene

A role of carotenoids as modulators of physical properties of model and biological membranes has been already postulated. However, there is a lack of information on the influence of these pigments on interactions between the lipids which form such membranes. This paper applies atomic force microscopy (AFM) in to study the effects of β-carotene on the adhesion properties of DPPC multilamellar liposomes. This allowed us to gain, for the first time, a direct insight into the interactions between the components in model systems on a molecular level. We observe that the adhesive forces in DPPC multilamellar liposomes containing 1mol% of β-carotene decrease exponentially with increasing temperature, and that at about 37°C they diminish. In the case of pure liposomes the decline in adhesion is of a different nature and the adhesive forces disappear at 34°C. The adhesive forces are about 5 times higher at 31°C in the presence of β-carotene than in its absence. However, measurements using differential scanning calorimetry (DSC) showed a shift of the lamellar-to-undulled-lamellar phase transition toward lower temperatures by about 0.8 ± 0.2°C in a system containing β-carotene. The enthalpy changes (ΔH) of this transition are similar for both systems. For the main transition, gel-to-liquid crystalline, the peak is shifted by about 0.5 ± 0.1°C, and ΔH decreases by about 30% in liposomes treated with β-carotene in comparison to pure liposomes. Our results suggest increased cooperation between liposome components in a system with enriched β-carotene, which cause a change in phase transition temperatures. Moreover, these interactions are very sensitive to temperature.

Zeaxanthin epoxidation - an in vitro approach

Zeaxanthin epoxidase (ZE) is an enzyme operating in the violaxanthin cycle, which is involved in photoprotective mechanisms. In this work model systems to study zeaxanthin (Zx) epoxidation were developed. Two assay systems are presented in which epoxidation of Zx was observed. In these assays two mutants of Arabidopsis thaliana which have active only one of the two xanthophyll cycle enzymes were used. The npq1 mutant possesses an active ZE and is thus able to convert Zx to violaxanthin (Vx) but the violaxanthin de-epoxidase (VDE) is inactive, so that Vx cannot be converted to Zx. The other mutant, npq2, possesses an active VDE and can convert exogenous Vx to Zx under strong light conditions but reverse reaction is not possible. The first assay containing thylakoids from npq1 and npq2 mutants of A. thaliana gave positive results and high efficiency of epoxidation reaction was observed. The amount of Zx was reduced by 25%. To optimize high efficiency of epoxidation reaction additional factors facilitating both fusion of the two types of thylakoids and incorporation of Zx to their membranes were also studied. The second kind of assay contained npq1 mutant thylakoids of A. thaliana supplemented with exogenous Zx and monogalactosyldiacylglycerol (MGDG). Experiments with different proportions of Zx and MGDG showed that their optimal ratio is 1:60. In such system, due to epoxidation, the amount of Zx was reduced by 38% of its initial level. The in vitro systems of Zx epoxidation described in this paper enable analysis some properties of the ZE without necessity of its isolation.

Zeaxanthin epoxidation - an in vitro approach

Zeaxanthin epoxidase (ZE) is an enzyme operating in the violaxanthin cycle, which is involved in photoprotective mechanisms. In this work model systems to study zeaxanthin (Zx) epoxidation were developed. Two assay systems are presented in which epoxidation of Zx was observed. In these assays two mutants of Arabidopsis thaliana which have active only one of the two xanthophyll cycle enzymes were used. The npq1 mutant possesses an active ZE and is thus able to convert Zx to violaxanthin (Vx) but the violaxanthin de-epoxidase (VDE) is inactive, so that Vx cannot be converted to Zx. The other mutant, npq2, possesses an active VDE and can convert exogenous Vx to Zx under strong light conditions but reverse reaction is not possible. The first assay containing thylakoids from npq1 and npq2 mutants of A. thaliana gave positive results and high efficiency of epoxidation reaction was observed. The amount of Zx was reduced by 25%. To optimize high efficiency of epoxidation reaction additional factors facilitating both fusion of the two types of thylakoids and incorporation of Zx to their membranes were also studied. The second kind of assay contained npq1 mutant thylakoids of A. thaliana supplemented with exogenous Zx and monogalactosyldiacylglycerol (MGDG). Experiments with different proportions of Zx and MGDG showed that their optimal ratio is 1:60. In such system, due to epoxidation, the amount of Zx was reduced by 38% of its initial level. The in vitro systems of Zx epoxidation described in this paper enable analysis some properties of the ZE without necessity of its isolation.

Violaxanthin and diadinoxanthin de-epoxidation in various model lipid systems

The xanthophyll cycle is an important photoprotective process functioning in plants. One of its forms, the violaxanthin (Vx) cycle, involves interconversion between: Vx, antheraxanthin (Ax) and zeaxanthin (Zx). Another kind of the xanthophyll cycle is the diadinoxanthin (Ddx) cycle in which interconversion between Ddx and diatoxanthin (Dtx) occurs. In this study an information on molecular mechanism and regulation of these two types of the xanthophyll cycle is presented. The influence of lipids on the de-epoxidation of the xanthophyll cycle pigments was investigated, with special focus put on the significance of physical properties of the aggregates formed by inverted lipid micelles, which are necessary for activity of the xanthophyll cycle enzymes. In particular, thickness of the hydrophobic fraction of the aggregates, size of the inverted micelles, suggested by mathematical description of the structures and solubility of Vx and Ddx in various kind of lipids were studied. Obtained results show that the rate of de-epoxidation is strongly dependent on the physicochemical properties of the lipids used. The key role for enzyme activation play non-bilayer lipids and the parameters of inverted micelles such as thickness, fluidity of hydrophobic core and their diameter. The presented results show that MGDG and other non-lamellar lipids like different forms of phosphatidylethanolamine are necessary for the Vx and Ddx de-epoxidation because they provide the three-dimensional structures, which are needed for the binding of de-epoxidases and for the accessibility of Vx and Ddx to these enzymes.

EPR study of thylakoid membrane dynamics in mutants of the carotenoid biosynthesis pathway of Synechocystis sp. PCC6803

EPR spectroscopy using 5-doxylstearic acid (5-SASL) and 16-doxylstearic acid (16-SASL) spin probes was used to study the fluidity of thylakoid membranes. These were isolated from wild type Synechocystis and from several mutants in genes encoding selected enzymes of the carotenoid biosynthesis pathway and/or acyl-lipid desaturases. Cyanobacteria were cultivated at 25°C and 35°C under different light regimes: photoautotrophically (PAG) and/or in light-activated heterotrophic conditions (LAHG). The relative fluidity of membranes was estimated from EPR spectra based on the empirical outermost splitting parameter in a temperature range from 15°C to 40°C. Our findings demonstrate that in native thylakoid membranes the elimination of xanthophylls decreased fluidity in the inner membrane region under optimal growth conditions (25°C) and increased it under sublethal heat stress (35°C). This indicated that the overall fluidity of native photosynthetic membranes in cyanobacteria may be influenced by the ratio of polar to non-polar carotenoid pools under different environmental conditions.

Atomic force microscopy studies of the adhesive properties of DPPC vesicles containing β-carotene

A role of carotenoids as modulators of physical properties of model and biological membranes has been already postulated. However, there is a lack of information on the influence of these pigments on interactions between the lipids which form such membranes. This paper applies atomic force microscopy (AFM) in to study the effects of β-carotene on the adhesion properties of DPPC multilamellar liposomes. This allowed us to gain, for the first time, a direct insight into the interactions between the components in model systems on a molecular level. We observe that the adhesive forces in DPPC multilamellar liposomes containing 1mol% of β-carotene decrease exponentially with increasing temperature, and that at about 37°C they diminish. In the case of pure liposomes the decline in adhesion is of a different nature and the adhesive forces disappear at 34°C. The adhesive forces are about 5 times higher at 31°C in the presence of β-carotene than in its absence. However, measurements using differential scanning calorimetry (DSC) showed a shift of the lamellar-to-undulled-lamellar phase transition toward lower temperatures by about 0.8 ± 0.2°C in a system containing β-carotene. The enthalpy changes (ΔH) of this transition are similar for both systems. For the main transition, gel-to-liquid crystalline, the peak is shifted by about 0.5 ± 0.1°C, and ΔH decreases by about 30% in liposomes treated with β-carotene in comparison to pure liposomes. Our results suggest increased cooperation between liposome components in a system with enriched β-carotene, which cause a change in phase transition temperatures. Moreover, these interactions are very sensitive to temperature.

Violaxanthin and diadinoxanthin de-epoxidation in various model lipid systems

The xanthophyll cycle is an important photoprotective process functioning in plants. One of its forms, the violaxanthin (Vx) cycle, involves interconversion between: Vx, antheraxanthin (Ax) and zeaxanthin (Zx). Another kind of the xanthophyll cycle is the diadinoxanthin (Ddx) cycle in which interconversion between Ddx and diatoxanthin (Dtx) occurs. In this study an information on molecular mechanism and regulation of these two types of the xanthophyll cycle is presented. The influence of lipids on the de-epoxidation of the xanthophyll cycle pigments was investigated, with special focus put on the significance of physical properties of the aggregates formed by inverted lipid micelles, which are necessary for activity of the xanthophyll cycle enzymes. In particular, thickness of the hydrophobic fraction of the aggregates, size of the inverted micelles, suggested by mathematical description of the structures and solubility of Vx and Ddx in various kind of lipids were studied. Obtained results show that the rate of de-epoxidation is strongly dependent on the physicochemical properties of the lipids used. The key role for enzyme activation play non-bilayer lipids and the parameters of inverted micelles such as thickness, fluidity of hydrophobic core and their diameter. The presented results show that MGDG and other non-lamellar lipids like different forms of phosphatidylethanolamine are necessary for the Vx and Ddx de-epoxidation because they provide the three-dimensional structures, which are needed for the binding of de-epoxidases and for the accessibility of Vx and Ddx to these enzymes.

EPR study of thylakoid membrane dynamics in mutants of the carotenoid biosynthesis pathway of Synechocystis sp. PCC6803

EPR spectroscopy using 5-doxylstearic acid (5-SASL) and 16-doxylstearic acid (16-SASL) spin probes was used to study the fluidity of thylakoid membranes. These were isolated from wild type Synechocystis and from several mutants in genes encoding selected enzymes of the carotenoid biosynthesis pathway and/or acyl-lipid desaturases. Cyanobacteria were cultivated at 25°C and 35°C under different light regimes: photoautotrophically (PAG) and/or in light-activated heterotrophic conditions (LAHG). The relative fluidity of membranes was estimated from EPR spectra based on the empirical outermost splitting parameter in a temperature range from 15°C to 40°C. Our findings demonstrate that in native thylakoid membranes the elimination of xanthophylls decreased fluidity in the inner membrane region under optimal growth conditions (25°C) and increased it under sublethal heat stress (35°C). This indicated that the overall fluidity of native photosynthetic membranes in cyanobacteria may be influenced by the ratio of polar to non-polar carotenoid pools under different environmental conditions.

Variations in xanthophyll composition in etiolated seedlings of Arabidopsis thaliana correlate with protochlorophyllide accumulation

Protochlorophyllide (Pchlide) accumulation and xantophyll composition were studied in 5-day old etiolated seedlings of three ecotypes of Arabidopsis thaliana: Columbia (Col-0), Landsberg erecta (Ler) and Wassiliewska (Ws). The total Pchlide level as measured by fluorescence spectroscopy varied significantly between ecotypes. A rapid HPLC method revealed quantitative differences in carotenoid composition. It was found that in the Ler ecotype any enhanced accumulation of Pchlide correlates with an increased level of lutein, suggesting the role of enzymes involved in lutein synthesis in cross-regulation between chlorophyll and carotenoid biosynthetic pathways. The function of the dark-accumulated carotenoid pool in seedling de-etiolation is discussed.

Cadmium inhibitory action leads to changes in structure of ferredoxin:NADP(+) oxidoreductase

This study deals with the influence of cadmium on the structure and function of ferredoxin:NADP(+) oxidoreductase (FNR), one of the key photosynthetic enzymes. We describe changes in the secondary and tertiary structure of the enzyme upon the action of metal ions using circular dichroism measurements, Fourier transform infrared spectroscopy and fluorometry, both steady-state and time resolved. The decrease in FNR activity corresponds to a gentle unfolding of the protein, caused mostly by a nonspecific binding of metal ions to multiple sites all over the enzyme molecule. The final inhibition event is most probably related to a bond created between cadmium and cysteine in close proximity to the FNR active center. As a result, the flavin cofactor is released. The cadmium effect is compared to changes related to ionic strength and other ions known to interact with cysteine. The complete molecular mechanism of FNR inhibition by heavy metals is discussed.Electronic supplementary material The online version of this article (doi:10.1007/s10867-012-9262-z) contains supplementary material, which is available to authorized users.