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1.
The generation of superoxide anion radical (O2 ·−) in the cytochrome b 6 f complex (Cyt b 6 f) of spinach under high-light illumination was studied using electron paramagnetic resonance spectroscopy. The generation of O2 ·− was lost in the absence of molecular oxygen. It was also suppressed in the presence of NaN3 and could be scavenged by extraneous antioxidants such as ascorbate, β-carotene, and glutathione. The results also indicate that O2 ·−, which is produced under high-light illumination of the Cyt b 6 f from spinach, might be generated from a reaction involing 1O2, and the Rieske Fe-S protein could serve as the electron donor in the O2 ·− production. The mechanism of photoprotection of the Cyt b 6 f complex by antioxidants is discussed.  相似文献   

2.
Li B  Mao D  Liu Y  Li L  Kuang T 《Photosynthesis research》2005,83(3):297-305
A pure, active cytochrome b 6 f was isolated from the chloroplasts of the marine green alga, Bryopsis corticulans. To investigate and characterize this cytochrome b 6 f complex, sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS–PAGE), absorption spectra measurement and HPLC were employed. It was shown that this purified complex contained four large subunits with apparent molecular masses of 34.8, 24, 18.7 and 16.7 kD. The ratio of Cyt b 6 to Cytf was 2.01 : 1. The cytochromeb 6 f was shown to catalyze the transfer of 73 electrons from decylplastoquinol to plastocyanin–ferricyanide per Cyt f per second. α-Carotene, one kind of carotenoid that has not been found to present in cytochrome b 6 f complex, was discovered in this preparation by reversed phase HPLC. It was different from β-carotene usually found in cytochrome b 6 f complex. The configuration of the major α-carotene component was assigned to be 9-cis by resonance Raman spectroscopy. Different from the previous reports, the configuration of this α-carotene in dissociated state was determined to be all-trans. Besides this carotene, chlorophyll a was also found in this complex. It was shown that the molecular ratios of chlorophylla, cis and all-trans-α-carotene to Cyt f in this complex were 1.2, 0.7 and 0.2, respectively.  相似文献   

3.
The cytochrome b 6 f (Cyt b 6 f) complex, which functions as a plastoquinol-plastocyanin oxidoreductase and mediates the linear electron flow between photosystem II (PSII) and photosystem I (PSI) and the cyclic electron flow around PSI, was isolated from spinach (Spinacia oleracea L.) chloroplasts using n-octyl-β-D-glucopyranoside (β-OG). The preparation was also able to catalyze the peroxidase-like reaction in the presence of hydrogen peroxide (H2O2) and guaiacol. The optimal conditions for peroxidase activity of the preparation included: pH 3.6, ionic strength 0.1, and temperature 35°C. The apparent Michaelis constant (K m) values for H2O2 and guaiacol were 50 mM and 2 mM, respectively. The bimolecular rate constant (k obs) was about 26 M−1 s−1 and the turnover number (K cat) was about 60 min−1 (20 mM guaiacol, 100 mM sodium phosphate, pH 3.6, 25°C, [H2O2]<100mM). These parameters were similar to those of several other heme-containing proteins, such as myoglobin and Cyt c.  相似文献   

4.
This study examined the effects of season-long exposure of Chinese pine (Pinus tabulaeformis) to elevated carbon dioxide (CO2) and/or ozone (O3) on indole-3-acetic acid (IAA) content, activities of IAA oxidase (IAAO) and peroxidase (POD) in needles. Trees grown in open-top chambers (OTC) were exposed to control (ambient O3, 55 nmol mol−1 + ambient CO2, 350 μmol mol−1, CK), elevated CO2 (ambient O3 + high CO2, 700 μmol mol−1, EC) and elevated O3 (high O3, 80 ± 8 nmol mol−1 + ambient CO2, EO) OTCs from 1 June to 30 September. Plants grown in elevated CO2 OTC had a growth increase of axial shoot and needle length, compared to control, by 20% and 10% respectively, while the growth in elevated O3 OTC was 43% and 7% less respectively, than control. An increase in IAA content and POD activity and decrease in IAAO activity were observed in trees exposed to elevated CO2 concentration compared with control. Elevated O3 decreased IAA content and had no significant effect on IAAO activity, but significantly increased POD activity. When trees pre-exposed to elevated CO2 were transferred to elevated O3 (EC–EO) or trees pre-exposed to elevated O3 were transferred to elevated CO2 (EO–EC), IAA content was lower while IAAO activity was higher than that transferred to CK (EC–CK or EO–CK), the change in IAA content was also related to IAAO activity. The results indicated that IAAO and POD activities in Chinese pine needles may be affected by the changes in the atmospheric environment, resulting in the change of IAA metabolism which in turn may cause changes in Chinese pine’s growth. An erratum to this article can be found at  相似文献   

5.
6.
The cytochrome b 6 f complex of oxygenic photosynthesis produces substantial levels of reactive oxygen species (ROS). It has been observed that the ROS production rate by b 6 f is 10–20 fold higher than that observed for the analogous respiratory cytochrome bc1 complex. The types of ROS produced (O2??, 1O2, and, possibly, H2O2) and the site(s) of ROS production within the b 6 f complex have been the subject of some debate. Proposed sources of ROS have included the heme b p , PQ p ?? (possible sources for O2??), the Rieske iron–sulfur cluster (possible source of O2?? and/or 1O2), Chl a (possible source of 1O2), and heme c n (possible source of O2?? and/or H2O2). Our working hypothesis is that amino acid residues proximal to the ROS production sites will be more susceptible to oxidative modification than distant residues. In the current study, we have identified natively oxidized amino acid residues in the subunits of the spinach cytochrome b 6 f complex. The oxidized residues were identified by tandem mass spectrometry using the MassMatrix Program. Our results indicate that numerous residues, principally localized near p-side cofactors and Chl a, were oxidatively modified. We hypothesize that these sites are sources for ROS generation in the spinach cytochrome b 6 f complex.  相似文献   

7.
This study examined the effects of carbon dioxide (CO2)-, ozone (O3)-, and genotype-mediated changes in quaking aspen (Populus tremuloides) chemistry on performance of the forest tent caterpillar (Malacosoma disstria) and its dipteran parasitoid (Compsilura concinnata) at the Aspen Free-Air CO2 Enrichment (FACE) site. Parasitized and non-parasitized forest tent caterpillars were reared on two aspen genotypes under elevated levels of CO2 and O3, alone and in combination. Foliage was collected for determination of the chemical composition of leaves fed upon by forest tent caterpillars during the period of endoparasitoid larval development. Elevated CO2 decreased nitrogen levels but had no effect on concentrations of carbon-based compounds. In contrast, elevated O3 decreased nitrogen and phenolic glycoside levels, but increased concentrations of starch and condensed tannins. Foliar chemistry also differed between aspen genotypes. CO2, O3, genotype, and their interactions altered forest tent caterpillar performance, and differentially so between sexes. In general, enriched CO2 had little effect on forest tent caterpillar performance under ambient O3, but reduced performance (for insects on one aspen genotype) under elevated O3. Conversely, elevated O3 improved forest tent caterpillar performance under ambient, but not elevated, CO2. Parasitoid larval survivorship decreased under elevated O3, depending upon levels of CO2 and aspen genotype. Additionally, larval performance and masses of mature female parasitoids differed between aspen genotypes. These results suggest that host-parasitoid interactions in forest systems may be altered by atmospheric conditions anticipated for the future, and that the degree of change may be influenced by plant genotype.  相似文献   

8.
The ba 3-type cytochrome c oxidase from Thermus thermophilus is phylogenetically very distant from the aa 3–type cytochrome c oxidases. Nevertheless, both types of oxidases have the same number of redox-active metal sites and the reduction of O2 to water is catalysed at a haem a 3-CuB catalytic site. The three-dimensional structure of the ba 3 oxidase reveals three possible proton-conducting pathways showing very low homology compared to those of the mitochondrial, Rhodobacter sphaeroides and Paracoccus denitrificans aa 3 oxidases. In this study we investigated the oxidative part of the catalytic cycle of the ba 3 -cytochrome c oxidase using the flow-flash method. After flash-induced dissociation of CO from the fully reduced enzyme in the presence of oxygen we observed rapid oxidation of cytochrome b (k ≅ 6.8 × 104 s−1) and formation of the peroxy (PR) intermediate. In the next step a proton was taken up from solution with a rate constant of ~1.7 × 104 s−1, associated with formation of the ferryl (F) intermediate, simultaneous with transient reduction of haem b. Finally, the enzyme was oxidized with a rate constant of ~1,100 s−1, accompanied by additional proton uptake. The total proton uptake stoichiometry in the oxidative part of the catalytic cycle was ~1.5 protons per enzyme molecule. The results support the earlier proposal that the PR and F intermediate spectra are similar (Siletsky et al. Biochim Biophys Acta 1767:138, 2007) and show that even though the architecture of the proton-conducting pathways is different in the ba 3 oxidases, the proton-uptake reactions occur over the same time scales as in the aa 3-type oxidases. Smirnova and Zaslavsky contributed equally to the work described in this paper.  相似文献   

9.
《BBA》1985,808(1):39-45
Numbers of the Photosystem I reaction center complexes and the cytochrome b6-f complexes with which a cytochrome c-553 molecule can interact within the limiting time of photosynthetic electron transport were examined by measuring flash-induced absorption changes of P-700, cytochrome c-553 and cytochrome f in the thermophilic cyanobacterium Synechococcus sp. The addition of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) did not affect the common 2 ms half-time of P-700, cytochrome c-553 and cytochrome f reduction, which is ascribed to electron transfer from the plastoquinone pool. The inhibitor decreased, however, amounts of the three electron carriers which underwent the 2 ms reduction in the order of cytochrome f, cytochrome c-553 and P-700. On excitation with weak flashes which oxidized only a small fraction of cytochrome c-553 molecules present in cells, P-700 remained in the oxidized state after the flashes was reduced with electrons from the Rieske center or plastoquinone but not from cytochrome c-553. The ratios of cytochrome c-553 to cytochrome f oxidized at various flash intensities were constant and similar to the ratio of the two cytochromes present in cells. It is concluded that cytochrome c-553 cannot exchange electrons with large numbers of the Photosystem I reaction center complexes and the cytochrome b6-f complexes in the limiting time, but has a mobility sufficient to mediate electron transfer between the two complexes, which are present at an unbalanced ratio in Synechococcus cells.  相似文献   

10.
In Escherichia coli, the F1FO ATP synthase b subunits house a conserved arginine in the tether domain at position 36 where the subunit emerges from the membrane. Previous experiments showed that substitution of isoleucine or glutamate result in a loss of enzyme activity. Double mutants have been constructed in an attempt to achieve an intragenic suppressor of the b arg36→ile and the b arg36→glu mutations. The b arg36→ile mutation could not be suppressed. In contrast, the phenotypic defect resulting from the b arg36→glu mutation was largely suppressed in the b arg36→glu,glu39→arg double mutant. E. coli expressing the b arg36→glu,glu39→arg subunit grew well on succinate-based medium. F1FO ATP synthase complexes were more efficiently assembled and ATP driven proton pumping activity was improved. The evidence suggests that efficient coupling in F1FO ATP synthase is dependent upon a basic amino acid located at the base of the peripheral stalk.  相似文献   

11.
Yeast flavocytochrome b 2 tranfers reducing equivalents from lactate to oxygen via cytochrome c and cytochrome c oxidase. The enzyme catalytic cycle includes FMN reduction by lactate and reoxidation by intramolecular electron transfer to heme b 2. Each subunit of the soluble tetrameric enzyme consists of an N terminal b 5-like heme-binding domain and a C terminal flavodehydrogenase. In the crystal structure, FMN and heme are face to face, and appear to be in a suitable orientation and at a suitable distance for exchanging electrons. But in one subunit out of two, the heme domain is disordered and invisible. This raises a central question: is this mobility required for interaction with the physiological acceptor cytochrome c, which only receives electrons from the heme and not from the FMN? The present review summarizes the results of the variety of methods used over the years that shed light on the interactions between the flavin and heme domains and between the enzyme and cytochrome c. The conclusion is that one should consider the interaction between the flavin and heme domains as a transient one, and that the cytochrome c and the flavin domain docking areas on the heme b 2 domain must overlap at least in part. The heme domain mobility is an essential component of the flavocytochrome b 2 functioning. In this respect, the enzyme bears similarity to a variety of redox enzyme systems, in particular those in which a cytochrome b 5-like domain is fused to proteins carrying other redox functions.  相似文献   

12.
In sunflower (Helianthus annuus L.) grown under controlled conditions and subjected to drought by withholding watering, net photosynthetic rate (P N) and stomatal conductance (g s) of attached leaves decreased as leaf water potential (Ψw) declined from −0.3 to −2.9 MPa. Although g s decreased over the whole range of Ψw, nearly constant values in the intercellular CO2 concentrations (C i) were observed as Ψw decreased to −1.8 MPa, but C i increased as Ψw decreased further. Relative quantum yield, photochemical quenching, and the apparent quantum yield of photosynthesis decreased with water deficit, whereas non-photochemical quenching (qNP) increased progressively. A highly significant negative relationship between qNP and ATP content was observed. Water deficit did not alter the pyridine nucleotide concentration but decreased ATP content suggesting metabolic impairment. At a photon flux density of 550 μmol m−2 s−1, the allocation of electrons from photosystem (PS) 2 to O2 reduction was increased by 51 %, while the allocation to CO2 assimilation was diminished by 32 %, as Ψw declined from −0.3 to −2.9 MPa. A significant linear relationship between mean P N and the rate of total linear electron transport was observed in well watered plants, the correlation becoming curvilinear when water deficit increased. The maximum quantum yield of PS2 was not affected by water deficit, whereas qP declined only at very severe stress and the excess photon energy was dissipated by increasing qNP indicating that a greater proportion of the energy was thermally dissipated. This accounted for the apparent down-regulation of PS2 and supported the protective role of qNP against photoinhibition in sunflower.  相似文献   

13.
Cyanobacteria dominate the world's oceans where iron is often barely detectable. One manifestation of low iron adaptation in the oligotrophic marine environment is a decrease in levels of iron-rich photosynthetic components, including the reaction center of photosystem I and the cytochrome b6f complex [R.F. Strzepek and P.J. Harrison, Photosynthetic architecture differs in coastal and oceanic diatoms, Nature 431 (2004) 689-692.]. These thylakoid membrane components have well characterised roles in linear and cyclic photosynthetic electron transport and their low abundance creates potential impediments to photosynthetic function. Here we show that the marine cyanobacterium Synechococcus WH8102 exhibits significant alternative electron flow to O2, a potential adaptation to the low iron environment in oligotrophic oceans. This alternative electron flow appears to extract electrons from the intersystem electron transport chain, prior to photosystem I. Inhibitor studies demonstrate that a propyl gallate-sensitive oxidase mediates this flow of electrons to oxygen, which in turn alleviates excessive photosystem II excitation pressure that can often occur even at relatively low irradiance. These findings are also discussed in the context of satisfying the energetic requirements of the cell when photosystem I abundance is low.  相似文献   

14.
Cytochrome bd from Escherichia coli is able to oxidize such substrates as guaiacol, ferrocene, benzohydroquinone, and potassium ferrocyanide through the peroxidase mechanism, while none of these donors is oxidized in the oxidase reaction (i.e. in the reaction that involves molecular oxygen as the electron acceptor). Peroxidation of guaiacol has been studied in detail. The dependence of the rate of the reaction on the concentration of the enzyme and substrates as well as the effect of various inhibitors of the oxidase reaction on the peroxidase activity have been tested. The dependence of the guaiacol-peroxidase activity on the H2O2 concentration is linear up to the concentration of 8 mM. At higher concentrations of H2O2, inactivation of the enzyme is observed. Guaiacol markedly protects the enzyme from inactivation induced by peroxide. The peroxidase activity of cytochrome bd increases with increasing guaiacol concentration, reaching saturation in the range from 0.5 to 2.5 mM, but then starts falling. Such inhibitors of the ubiquinol-oxidase activity of cytochrome bd as cyanide, pentachlorophenol, and 2-n-heptyl 4-hydroxyquinoline-N-oxide also suppress its guaiacol-peroxidase activity; in contrast, zinc ions have no influence on the enzyme-catalyzed peroxidation of guaiacol. These data suggest that guaiacol interacts with the enzyme in the center of ubiquinol binding and donates electrons into the di-heme center of oxygen reduction via heme b 558, and H2O2 is reduced by heme d. Although the peroxidase activity of cytochrome bd from E. coli is low compared to peroxidases, it might be of physiological significance for the bacterium itself and plays a pathophysiological role for humans and animals.  相似文献   

15.
Sulfur deprivation of algal cultures selectively and partially inactivates photosystem II (PSII)-catalyzed O2 evolution, induces anaerobiosis and hydrogenase expression, and results in sustained H2 photoproduction for several days. We show that re-addition of limiting amounts of sulfate (1–10 μM final concentration) to the cultures during the H2-production phase temporarily reactivates PSII photochemical and O2-evolution activity and re-establishes higher rates of electron transport through the photosynthetic electron transport chain. The reactivation of PSII occurs by de novo D1 protein synthesis, but does not result in the re-establishment of aerobic conditions in the reactor, detectable by dissolved-O2 sensors. However, concomitant H2 photoproduction is inhibited, possibly due to excessive intra-cellular levels of photosynthetically-evolved O2. The partial recovery of electron transport rates correlates with the re-oxidation of the plastoquinone (PQ) pool, as observed by pulse-amplitude modulated (PAM) and fluorescence-induction measurements. These results show that the presence of a more oxidized PQ pool releases some of the down-regulation of electron transport caused by the anaerobic conditions.  相似文献   

16.
17.
The enzymatic extracts from seven species of microalgae (Pediastrum duplex, Dactylococcopsis fascicularis, Halochlorococcum porphyrae, Oltmannsiellopsis unicellularis, Achnanthes longipes, Navicula sp. and Amphora coffeaeformis) collected from three habitats (freshwater, tidal pool, and coastal benthic) at Jeju Island in Korea were investigated for their antioxidant activity. Of the extracts tested, the AMG 300 L (an exo 1, 4-α-d-glucosidase) extract of P. duplex, the Viscozyme extract of Navicula sp., and the Celluclast extract of A. longipes provided the most potential as antioxidants. Meanwhile, the Termamyl extract of P. duplex in an H2O2 scavenging assay exhibited an approximate 60% scavenging effect. In this study, we report that the DNA damage inhibitory effects of P. duplex (Termamyl extract) and D. fascicularis (Kojizyme extract) were nearly 80% and 69% respectively at a concentration of 100 μg/ml. Thus, it is suggested that the microalgae tested in this study yield promising DNA damage inhibitory properties on mouse lymphoma L 5178 cells that are treated with H2O2. Therefore, microalgae such as P. duplex may be an excellent source of naturally occurring antioxidant compounds with potent DNA damage inhibition potential.  相似文献   

18.
Bernacchi CJ  Morgan PB  Ort DR  Long SP 《Planta》2005,220(3):434-446
Down-regulation of light-saturated photosynthesis (Asat) at elevated atmospheric CO2 concentration, [CO2], has been demonstrated for many C3 species and is often associated with inability to utilize additional photosynthate and/or nitrogen limitation. In soybean, a nitrogen-fixing species, both limitations are less likely than in crops lacking an N-fixing symbiont. Prior studies have used controlled environment or field enclosures where the artificial environment can modify responses to [CO2]. A soybean free air [CO2] enrichment (FACE) facility has provided the first opportunity to analyze the effects of elevated [CO2] on photosynthesis under fully open-air conditions. Potential ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation (Vc,max) and electron transport through photosystem II (Jmax) were determined from the responses of Asat to intercellular [CO2] (Ci) throughout two growing seasons. Mesophyll conductance to CO2 (gm) was determined from the responses of Asat and whole chain electron transport (J) to light. Elevated [CO2] increased Asat by 15–20% even though there was a small, statistically significant, decrease in Vc,max. This differs from previous studies in that Vc,max/Jmax decreased, inferring a shift in resource investment away from Rubisco. This raised the Ci at which the transition from Rubisco-limited to ribulose-1,5-bisphosphate regeneration-limited photosynthesis occurred. The decrease in Vc,max was not the result of a change in gm, which was unchanged by elevated [CO2]. This first analysis of limitations to soybean photosynthesis under fully open-air conditions reveals important differences to prior studies that have used enclosures to elevate [CO2], most significantly a smaller response of Asat and an apparent shift in resources away from Rubisco relative to capacity for electron transport.Abbreviations FACE Free air [CO2] enrichment - Rubisco Ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP Ribulose-1,5-bisphosphate - SoyFACE Soybean free air [CO2] enrichment - VPD Vapor pressure deficit  相似文献   

19.
Cyanobacterial thylakoids catalyze both photosynthetic and respiratory activities. In a photosystem I-less Synechocystis sp. PCC 6803 strain, electrons generated by photosystem II appear to be utilized by cytochrome oxidase. To identify the lumenal electron carriers (plastocyanin and/or cytochromes c 553, c 550, and possibly c M) that are involved in transfer of photosystem II-generated electrons to the terminal oxidase, deletion constructs for genes coding for these components were introduced into a photosystem I-less Synechocystis sp. PCC 6803 strain, and electron flow out of photosystem II was monitored in resulting strains through chlorophyll fluorescence yields. Loss of cytochrome c 553 or plastocyanin, but not of cytochrome c 550, decreased the rate of electron flow out of photosystem II. Surprisingly, cytochrome c M could not be deleted in a photosystem I-less background strain, and also a double-deletion mutant lacking both plastocyanin and cytochromec 553 could not be obtained. Cytochrome c M has some homology with the cytochrome c-binding regions of the cytochromecaa3 -type cytochrome oxidase from Bacillus spp. and Thermus thermophilus. We suggest that cytochrome c M is a component of cytochrome oxidase in cyanobacteria that serves as redox intermediate between soluble electron carriers and the cytochromeaa3 complex, and that either plastocyanin or cytochrome c 553 can shuttle electrons from the cytochrome b6f complex to cytochrome c M.  相似文献   

20.
During daffodil flower development, chloroplasts differentiate into photosynthetically inactive chromoplasts having lost functional photosynthetic reaction centers. Chromoplasts exhibit a respiratory activity reducing oxygen to water and generating ATP. Immunoblots revealed the presence of the plastid terminal oxidase (PTOX), the NAD(P)H dehydrogenase (NDH) complex, the cytochrome b6f complex, ATP synthase and several isoforms of ferredoxin‐NADP+ oxidoreductase (FNR), and ferredoxin (Fd). Fluorescence spectroscopy allowed the detection of chlorophyll a in the cytochrome b6f complex. Here we characterize the electron transport pathway of chromorespiration by using specific inhibitors for the NDH complex, the cytochrome b6f complex, FNR and redox‐inactive Fd in which the iron was replaced by gallium. Our data suggest an electron flow via two separate pathways, both reducing plastoquinone (PQ) and using PTOX as oxidase. The first oxidizes NADPH via FNR, Fd and cytochrome bh of the cytochrome b6f complex, and does not result in the pumping of protons across the membrane. In the second, electron transport takes place via the NDH complex using both NADH and NADPH as electron donor. FNR and Fd are not involved in this pathway. The NDH complex is responsible for the generation of the proton gradient. We propose a model for chromorespiration that may also be relevant for the understanding of chlororespiration and for the characterization of the electron input from Fd to the cytochrome b6f complex during cyclic electron transport in chloroplasts.  相似文献   

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