Involvement of ferredoxin in desaturation of lipid-bound oleate in chloroplasts

Rieske Oxygenases and Other Ferredoxin-Dependent Enzymes: Electron Transfer Principles and Catalytic Capabilities

Enzymes that depend on sophisticated electron transfer via ferredoxins (Fds) exhibit outstanding catalytic capabilities, but despite decades of research, many of them are still not well understood or exploited for synthetic applications. This review aims to provide a general overview of the most important Fd-dependent enzymes and the electron transfer processes involved. While several examples are discussed, we focus in particular on the family of Rieske non-heme iron-dependent oxygenases (ROs). In addition to illustrating their electron transfer principles and catalytic potential, the current state of knowledge on structure-function relationships and the mode of interaction between the redox partner proteins is reviewed. Moreover, we highlight several key catalyzed transformations, but also take a deeper dive into their engineerability for biocatalytic applications. The overall findings from these case studies highlight the catalytic capabilities of these biocatalysts and could stimulate future interest in developing additional Fd-dependent enzyme classes for synthetic applications.

Understanding desaturation/hydroxylation activity of castor stearoyl Δ9-Desaturase through rational mutagenesis

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Rationally designed mutations in the Δ⁹ desaturase promoted hydroxylation activity.

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Proton and electron transfer to the active site is crucial for the Δ⁹D to desaturate

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Detailed analysis of all enzymatic products of the Δ⁹D was carried out

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Insight into the chemo-, and stereoselectivity of non-heme diiron enzymes was obtained

A recently proposed reaction mechanism of soluble Δ⁹ desaturase (Δ⁹D) allowed us to identify auxiliary residues His203, Asp101, Thr206 and Cys222 localized near the di-iron active site that are supposedly involved in the proton transfer (PT) to and from the active site. The PT, along with the electron transfer (ET), seems to be crucial for efficient desaturation. Thus, perturbing the major PT chains is expected to impair the native reaction and (potentially) amplify minor reaction channels, such as the substrate hydroxylation. To verify this hypothesis, we mutated the four residues mentioned above into their counterparts present in a soluble methane monooxygenase (sMMO), and determined the reaction products of mutants. We found that the mutations significantly promote residual monohydroxylation activities on stearoyl-CoA, often at the expense of native desaturation activity. The favored hydroxylation positions are C₉, followed by C₁₀ and C₁₁. Reactions with unsaturated substrate, oleoyl-CoA, yield erythro-9,10-diol, cis-9,10-epoxide and a mixture of allylic alcohols. Additionally, using 9- and 11-hydroxystearoyl-CoA, we showed that the desaturation reaction can proceed only with the hydroxyl group at position C₁₁, whereas the hydroxylation reaction is possible in both cases, i.e. with hydroxyl at position C₉ or C₁₁. Despite the fact that the overall outcome of hydroxylation is rather modest and that it is mostly the desaturation/hydroxylation ratio that is affected, our results broaden understanding of the origin of chemo- and stereoselectivity of the Δ⁹D and provide further insight into the catalytic action of the NHFe₂ enzymes.

Plant monounsaturated fatty acids: Diversity, biosynthesis, functions and uses

Monounsaturated fatty acids are straight-chain aliphatic monocarboxylic acids comprising a unique carbon‑carbon double bond, also termed unsaturation. More than 50 distinct molecular structures have been described in the plant kingdom, and more remain to be discovered. The evolution of land plants has apparently resulted in the convergent evolution of non-homologous enzymes catalyzing the dehydrogenation of saturated acyl chain substrates in a chemo-, regio- and stereoselective manner. Contrasted enzymatic characteristics and different subcellular localizations of these desaturases account for the diversity of existing fatty acid structures. Interestingly, the location and geometrical configuration of the unsaturation confer specific characteristics to these molecules found in a variety of membrane, storage, and surface lipids. An ongoing research effort aimed at exploring the links existing between fatty acid structures and their biological functions has already unraveled the importance of several monounsaturated fatty acids in various physiological and developmental contexts. What is more, the monounsaturated acyl chains found in the oils of seeds and fruits are widely and increasingly used in the food and chemical industries due to the physicochemical properties inherent in their structures. Breeders and plant biotechnologists therefore develop new crops with high monounsaturated contents for various agro-industrial purposes.

Aerobic Barley Mg-protoporphyrin IX Monomethyl Ester Cyclase is Powered by Electrons from Ferredoxin

Chlorophyll is the light-harvesting molecule central to the process of photosynthesis. Chlorophyll is synthesized through 15 enzymatic steps. Most of the reactions have been characterized using recombinant proteins. One exception is the formation of the isocyclic E-ring characteristic of chlorophylls. This reaction is catalyzed by the Mg-protoporphyrin IX monomethyl ester cyclase encoded by Xantha-l in barley (Hordeum vulgare L.). The Xantha-l gene product (XanL) is a membrane-bound diiron monooxygenase, which requires additional soluble and membrane-bound components for its activity. XanL has so far been impossible to produce as an active recombinant protein for in vitro assays, which is required for deeper biochemical and structural analyses. In the present work, we performed cyclase assays with soluble and membrane-bound fractions of barley etioplasts. Addition of antibodies raised against ferredoxin or ferredoxin-NADPH oxidoreductase (FNR) inhibited assays, strongly suggesting that reducing electrons for the cyclase reaction involves ferredoxin and FNR. We further developed a completely recombinant cyclase assay. Expression of active XanL required co-expression with an additional protein, Ycf54. In vitro cyclase activity was obtained with recombinant XanL in combination with ferredoxin and FNR. Our experiment demonstrates that the cyclase is a ferredoxin-dependent enzyme. Ferredoxin is part of the photosynthetic electron-transport chain, which suggests that the cyclase reaction might be connected to photosynthesis under light conditions.

Primary leaf-type ferredoxin 1 participates in photosynthetic electron transport and carbon assimilation in rice

Ferredoxins (Fds) play a crucial role in photosynthesis by regulating the distribution of electrons to downstream enzymes. Multiple Fd genes have been annotated in the Oryza sativa L. (rice) genome; however, their specific functions are not well understood. Here, we report the functional characterization of rice Fd1. Sequence alignment, phylogenetic analysis of seven rice Fd proteins and quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis showed that rice Fd1 is a primary leaf-type Fd. Electron transfer assays involving NADP⁺ and cytochrome c indicated that Fd1 can donate electrons from photosystem I (PSI) to ferredoxin-NADP⁺ reductase. Loss-of-function fd1 mutants showed chlorosis and seedling lethality at the three-leaf stage. The deficiency of Fd1 impaired photosynthetic electron transport, which affected carbon assimilation. Exogenous glucose treatment partially restored the mutant phenotype, suggesting that Fd1 plays an important role in photosynthetic electron transport in rice. In addition, the transcript levels of Fd-dependent genes were affected in fd1 mutants, and the trend was similar to that observed in fdc2 plants. Together, these results suggest that OsFd1 is the primary Fd in photosynthetic electron transport and carbon assimilation in rice.

Proton-Electron Transfer to the Active Site Is Essential for the Reaction Mechanism of Soluble Δ9-Desaturase

A full understanding of the catalytic action of non-heme iron (NHFe) and non-heme diiron (NHFe₂) enzymes is still beyond the grasp of contemporary computational and experimental techniques. Many of these enzymes exhibit fascinating chemo-, regio-, and stereoselectivity, in spite of employing highly reactive intermediates which are necessary for activations of most stable chemical bonds. Herein, we study in detail one intriguing representative of the NHFe₂ family of enzymes: soluble Δ⁹ desaturase (Δ⁹D), which desaturates rather than performing the thermodynamically favorable hydroxylation of substrate. Its catalytic mechanism has been explored in great detail by using QM(DFT)/MM and multireference wave function methods. Starting from the spectroscopically observed 1,2-μ-peroxo diferric P intermediate, the proton-electron uptake by P is the favored mechanism for catalytic activation, since it allows a significant reduction of the barrier of the initial (and rate-determining) H-atom abstraction from the stearoyl substrate as compared to the "proton-only activated" pathway. Also, we ruled out that a Q-like intermediate (high-valent diamond-core bis-μ-oxo-[Fe^IV]₂ unit) is involved in the reaction mechanism. Our mechanistic picture is consistent with the experimental data available for Δ⁹D and satisfies fairly stringent conditions required by Nature: the chemo-, stereo-, and regioselectivity of the desaturation of stearic acid. Finally, the mechanisms evaluated are placed into a broader context of NHFe₂ chemistry, provided by an amino acid sequence analysis through the families of the NHFe₂ enzymes. Our study thus represents an important contribution toward understanding the catalytic action of the NHFe₂ enzymes and may inspire further work in NHFe₍₂₎ biomimetic chemistry.

Endoplasmic reticulum retention signaling and transmembrane channel proteins predicted for oilseed ω3 fatty acid desaturase 3 (FAD3) genes

Oilseed crop oils contain a variety of unsaturated fatty acids that are synthesized and regulated by fatty acid desaturases (FADs). In this study, 14 FAD3 (ω3 desaturase) protein sequences from oilseeds are analyzed and presented through the application of several computational tools. The results indicated a close relationship between Brassica napus and Camelina sativa, as well as between Salvia hispanica and Perilla frutescens FAD3s, due to a high similarity in codon preferences in codon usage clusters and the phylogenetic tree. The cis-acting element results reveal that the seed-specific promoter region of BnFAD3 contains the critical conserved boxes such as HSE and ABRE, which are involved in responsiveness to heat stress and abscisic acid. The presence of the aforementioned conserved boxes may increase cold acclimation as well as tolerance to drought and high salinity. Omega(ω)3 desaturases contain a Skn-1 motif which is a cis-acting regulatory element required involved in endosperm development. In oilseed FAD3s, leucine is the most repeated amino acid in FAD3 proteins. The study conveyed that B. napus, Camelina sativa, Linum usitatissimum, Vernicia fordii, Gossypium hirsutum, S. hispanica, Cannabis sativa, and P. frutescens have retention signal KXKXX/XKXX at their c-terminus sites, which is one of the most important characteristics of FADs. Additionally, it was found that BnFAD3 is a transmembrane protein that can convert ω6 to ω3 fatty acids and may simultaneously act as a potassium ion channel in the ER.

Overexpressing Ferredoxins in Chlamydomonas reinhardtii Increase Starch and Oil Yields and Enhance Electric Power Production in a Photo Microbial Fuel Cell

Ferredoxins (FDX) are final electron carrier proteins in the plant photosynthetic pathway, and function as major electron donors in diverse redox-driven metabolic pathways. We previously showed that overexpression of a major constitutively expressed ferredoxin gene PETF in Chlamydomonas decreased the reactive oxygen species (ROS) level and enhanced tolerance to heat stress. In addition to PETF, an endogenous anaerobic induced FDX5 was overexpressed in transgenic Chlamydomonas lines here to address the possible functions of FDX5. All the independent FDX transgenic lines showed decreased cellular ROS levels and enhanced tolerance to heat and salt stresses. The transgenic Chlamydomonas lines accumulated more starch than the wild-type line and this effect increased almost three-fold in conditions of nitrogen depletion. Furthermore, the lipid content was higher in the transgenic lines than in the wild-type line, both with and without nitrogen depletion. Two FDX-overexpressing Chlamydomonas lines were assessed in a photo microbial fuel cell (PMFC); power density production by the transgenic lines was higher than that of the wild-type cells. These findings suggest that overexpression of either PETF or FDX5 can confer tolerance against heat and salt stresses, increase starch and oil production, and raise electric power density in a PMFC.

Plant type ferredoxins and ferredoxin-dependent metabolism

Ferredoxin (Fd) is a small [2Fe-2S] cluster-containing protein found in all organisms performing oxygenic photosynthesis. Fd is the first soluble acceptor of electrons on the stromal side of the chloroplast electron transport chain, and as such is pivotal to determining the distribution of these electrons to different metabolic reactions. In chloroplasts, the principle sink for electrons is in the production of NADPH, which is mostly consumed during the assimilation of CO2 . In addition to this primary function in photosynthesis, Fds are also involved in a number of other essential metabolic reactions, including biosynthesis of chlorophyll, phytochrome and fatty acids, several steps in the assimilation of sulphur and nitrogen, as well as redox signalling and maintenance of redox balance via the thioredoxin system and Halliwell-Asada cycle. This makes Fds crucial determinants of the electron transfer between the thylakoid membrane and a variety of soluble enzymes dependent on these electrons. In this article, we will first describe the current knowledge on the structure and function of the various Fd isoforms present in chloroplasts of higher plants and then discuss the processes involved in oxidation of Fd, introducing the corresponding enzymes and discussing what is known about their relative interaction with Fd.

Plant lipids: metabolism, mutants, and membranes

The mechanisms that regulate plant lipid metabolism determine the dietary and industrial value of storage oils found in economically important species and may control the ability of many plants to survive exposure to temperature extremes. Many of the problems researchers have in defining the pathways, enzymes, and genes involved in plant lipid metabolism appear to be amenable to analysis by genetic approaches. Mutants with alterations in membrane lipid composition have also been used to study the structural and adaptive roles of lipids. The application of genetic engineering methods affords opportunities for researchers to apply knowledge gained about plant lipid metabolism toward enhanced use of plant oils as abundant and renewable sources of reduced carbon.

cDNA cloning, expression levels and gene mapping of photosynthetic and non-photosynthetic ferredoxin genes in sunflower (Helianthus annuus L.)

Fatty acid desaturation in plastids and chloroplasts depends on the electron-donor activity of ferredoxins. Using degenerate oligonucleotides designed from known photosynthetic and heterotrophic plant ferredoxin sequences, two full-length ferredoxin cDNAs were cloned from sunflower (Helianthus annuus L.) leaves and developing seeds, HaFd1 and HaFd2, homologous to photosynthetic and non-photosynthetic ferredoxins, respectively. Based on these cDNAs, the respective genomic sequences were obtained and the presence of DNA polymorphisms was investigated. Complete sequencing of the HaFd1 and HaFd2 genes in different lines indicated the presence of two haplotypes for HaFd2 and their alignment showed that sequence polymorphisms are restricted to the 5'-NTR intron. In addition, specific DNA markers for the HaFd1 and HaFd2 genes were developed that enabled the genes to be mapped. Accordingly, the HaFd1 locus maps to linkage group 10 of the public sunflower map, while the HaFd2 locus maps to linkage group 11. Both ferredoxins display different spatial-temporal patterns of expression. While HaFd2 is expressed at similar levels in all tissues tested (leaves, stem, roots, cotyledons and developing seeds), HaFd1 is more strongly expressed in green tissues than in all the other tissues tested. Both photosynthetic- and heterotrophic-ferredoxins are present in sunflower seeds and may contribute to fatty acid desaturation during oil accumulation. Nevertheless, the levels of HaFd2 expression during seed formation are distinct in lines that only varied in the HaFd2 haplotypes they expressed.

In situ molecular identification of the plastid omega3 fatty acid desaturase FAD7 from soybean: evidence of thylakoid membrane localization

omega3 fatty acid desaturases are the enzymes responsible for the synthesis of trienoic fatty acids in plants. These enzymes have been mainly investigated using molecular, biochemical, and genetic approaches but very little is known about their subcellular distribution in plant cells. In this work, the precise subcellular localization of the omega3 desaturase FAD7 was elucidated by immunofluorescence and immunogold labeling using a monospecific GmFAD7 polyclonal antibody in soybean (Glycine max) photoautotrophic cell suspension cultures. Confocal analysis revealed the localization of the GmFAD7 protein within the chloroplast; i.e. signals from FAD7 and chlorophyll autofluorescence showed specific colocalization. Immunogold labeling was pursued on cryofixed and freeze-substituted samples for convenient preservation of antigenicity and ultrastructure of membrane subcompartments. Our data revealed that the FAD7 protein was preferentially localized in the thylakoid membranes. Biochemical fractionation of purified chloroplasts and western analysis of the subfractions further confirmed these results. These findings suggest that not only the envelope, but also the thylakoid membranes could be sites of lipid desaturation in higher plants.

Spectroscopic and electronic structure studies of intermediate X in ribonucleotide reductase R2 and two variants: a description of the FeIV-oxo bond in the FeIII-O-FeIV dimer

Spectroscopic and electronic structure studies of the class I Escherichia coli ribonucleotide reductase (RNR) intermediate X and three computationally derived model complexes are presented, compared, and evaluated to determine the electronic and geometric structure of the FeIII-FeIV active site of intermediate X. Rapid freeze-quench (RFQ) EPR, absorption, and MCD were used to trap intermediate X in R2 wild-type (WT) and two variants, W48A and Y122F/Y356F. RFQ-EPR spin quantitation was used to determine the relative contributions of intermediate X and radicals present, while RFQ-MCD was used to specifically probe the FeIII/FeIV active site, which displayed three FeIV d-d transitions between 16,700 and 22,600 cm(-1), two FeIV d-d spin-flip transitions between 23,500 and 24,300 cm(-1), and five oxo to FeIV and FeIII charge transfer (CT) transitions between 25,000 and 32,000 cm(-1). The FeIV d-d transitions were perturbed in the two variants, confirming that all three d-d transitions derive from the d-pi manifold. Furthermore, the FeIV d-pi splittings in the WT are too large to correlate with a bis-mu-oxo structure. The assignment of the FeIV d-d transitions in WT intermediate X best correlates with a bridged mu-oxo/mu-hydroxo [FeIII(mu-O)(mu-OH)FeIV] structure. The mu-oxo/mu-hydroxo core structure provides an important sigma/pi superexchange pathway, which is not present in the bis-mu-oxo structure, to promote facile electron transfer from Y122 to the remote FeIV through the bent oxo bridge, thereby generating the tyrosyl radical for catalysis.

The induction of CP43′ by iron-stress in Synechococcus sp. PCC 7942 is associated with carotenoid accumulation and enhanced fatty acid unsaturation

Comparative lipid analysis demonstrated reduced amount of PG (50%) and lower ratio of MGDG/DGDG in iron-stressed Synechococcus sp. PCC 7942 cells compared to cells grown under iron sufficient conditions. In parallel, the monoenoic (C:1) fatty acids in MGDG, DGDG and PG increased from 46.8%, 43.7% and 45.6%, respectively in control cells to 51.6%, 48.8% and 48.7%, respectively in iron-stressed cells. This suggests increased membrane dynamics, which may facilitate the diffusion of PQ and keep the PQ pool in relatively more oxidized state in iron-stressed compared to control cells. This was confirmed by chlorophyll fluorescence and thermoluminescence measurements. Analysis of carotenoid composition demonstrated that the induction of isiA (CP43') protein in response to iron stress is accompanied by significant increase of the relative abundance of all carotenoids. The quantity of carotenoids calculated on a Chl basis increased differentially with nostoxanthin, cryptoxanthin, zeaxanthin and beta-carotene showing 2.6-, 3.1-, 1.9- and 1.9-fold increases, respectively, while the relative amount of caloxanthin was increased only by 30%. HPLC analyses of the pigment composition of Chl-protein complexes separated by non-denaturating SDS-PAGE demonstrated even higher relative carotenoids content, especially of cryptoxanthin, in trimer and monomer PSI Chl-protein complexes co-migrating with CP43' from iron-stressed cells than in PSI complexes from control cells where CP43' is not present. This implies a carotenoid-binding role for the CP43' protein which supports our previous suggestion for effective energy quenching and photoprotective role of CP43' protein in cyanobacteria under iron stress.

Identification of Rv3230c as the NADPH oxidoreductase of a two-protein DesA3 acyl-CoA desaturase in Mycobacterium tuberculosis H37Rv

DesA3 is a membrane-bound stearoyl-CoA Delta(9)-desaturase that produces oleic acid, a precursor of mycobacterial membrane phospholipids and triglycerides. The sequence of DesA3 is homologous with those of other membrane desaturases, including the presence of the eight-His motif proposed to bind the diiron center active site. This family of desaturases function as multicomponent complexes and thus require electron transfer proteins for efficient catalytic turnover. Here we present evidence that Rv3230c from Mycobacterium tuberculosis H37Rv is a biologically relevant electron transfer partner for DesA3 from the same pathogen. For these studies, Rv3230c was expressed as a partially soluble protein in Escherichia coli; recombinant DesA3 was expressed in Mycobacterium smegmatis as a catalytically active membrane protein. The addition of E. coli lysates containing Rv3230c to lysates of M. smegmatis expressing DesA3 gave strong conversion of [1-(14)C]-18:0-CoA to [1-(14)C]-cis-Delta(9)-18:1-CoA and of [1-(14)C]-16:0-CoA to [1-(14)C]-cis-Delta(9)-16:1-CoA. Both M. tuberculosis proteins were required for reconstitution of activity, as various combinations of control lysates lacking either Rv3230c or DesA3 gave minimal or no activity. Furthermore, the specificity of interaction between Rv3230c and DesA3 was implied by the inability of other related redox systems to substitute for Rv3230c. The reconstituted activity was dependent upon the presence of NADPH, could be saturated by increasing the amount of Rv3230c added, and was also sensitive to the salt concentration in the buffer. The results are consistent with the formation of a protein-protein complex, possibly with electrostatic character. This work defines a multiprotein, acyl-CoA desaturase complex from M. tuberculosis H37Rv to minimally consist of a soluble Rv3230c reductase and integral membrane DesA3 desaturase. Further implications of this finding relative to the properties of other multiprotein iron-enzyme complexes are discussed.

Nature of the Peroxo Intermediate of the W48F/D84E Ribonucleotide Reductase Variant: Implications for O2 Activation by Binuclear Non-Heme Iron Enzymes

Analysis of the spectroscopic signatures of the R2-W48F/D84E biferric peroxo intermediate identifies a cis mu-1,2 peroxo coordination geometry. DFT geometry optimizations on both R2-W48F/D84E and R2-wild-type peroxo intermediate models including constraints imposed by the protein also identify the cis mu-1,2 peroxo geometry as the most stable peroxo intermediate structure. This study provides significant insight into the electronic structure and reactivity of the R2-W48F/D84E peroxo intermediate, structurally related cis mu-1,2 peroxo model complexes, and other enzymatic biferric peroxo intermediates.

Density functional studies of oxidized and reduced methane monooxygenase. Optimized geometries and exchange coupling of active site clusters

The conflicting protein crystallography data for the oxidized form (MMOH(ox)) of methane monooxygenase present a dilemma regarding the identity of the solvent-derived bridging ligands within the active site: do they comprise a diiron unit bridged by 1H2O and 1OH(-) as postulated for Methylococcus capsulatus or 2OH(-) ligands as suggested for Methylosinus trichosporium? Using models derived explicitly from the M. capsulatus and M. trichosporium protein data, spin-unrestricted density functional methods have been used to study two structurally characterized forms of the hydroxylase component of methane monooxygenase. The active site geometries of the oxidized (MMOH(ox)) and two-electron-reduced (MMOH(red)) states have been geometry optimized using several quantum cluster models which take into account the antiferromagnetic (AF) and ferromagnetic (F) coupling of electron spins. Trends in cluster geometries, energetics, and Heisenberg J values have been evaluated. For the majority of models, calculated geometries are in good agreement with the X-ray analyses and appear relatively insensitive to the F or AF alignment of electron spins on adjacent Fe sites. Discrepancies between calculation and experiment appear in the orientation of the coordinated His and Glu amino acid side chains for both MMOH(ox) and MMOH(red) and also in unexpected intramolecular proton transfer in the MMOH(ox) cluster models. There is additional dispersion between (and among) calculated and experimental Fe(3+)-OH(-) distances with relevance to the correct protonation state of the solvent-derived ligands. In an accompanying paper (Lovell, T.; Li, J.; Noodleman, L. Inorg. Chem. 2001, 40, 5267), a comparison of the related energetics of the active site models examined herein is further evaluated in the full protein and solvent environment.

Changes in fatty acid composition in plant tissues expressing a mammalian delta9 desaturase

Plant tissues expressing a mammalian stearoyl-CoA delta9 desaturase were reported to accumulate delta9 hexadecenoic acid (16:1), normally very minor in most plant tissues. The transgenic plants were thoroughly analyzed for alterations of individual lipids in different subcellular sites. Western blot analysis indicated that the animal desaturase was targeted to the microsomes. The delta9 16:1 was incorporated into both the sn-1 and sn-2 positions of all the major membrane lipids tested, indicating that the endoplasmic reticulum acyltransferases do not exclude unsaturated C16 fatty acids from the sn-2 position. In addition to increases in monounsaturated and decreases in saturated fatty acids, accumulation of 16:1 was accompanied by a reduction in 18:3 in all the lipids tested except phosphatidylglycerol, and increases in 18:2 in phospholipids. Total C16 fatty acid content in the galactolipids of the transgenics was significantly higher than that in the control, but those in the phospholipids were unchanged. In transgenics, delta11 18:1 was detected in the sn-1 position of the lipids tested except phosphatidylinositol and phosphatidylserine. Introduction of the animal desaturase, controlled by a seed-specific phaseolin promoter, into soybean somatic embryo resulted in a significant reduction in saturated fatty acids. Such effects were greater in cotyledons than hypocotyl-radicles. This study demonstrated that the animal desaturase can be used to decrease the levels of saturated fatty acids in a crop plant.

The CrIIL reduction of [2Fe-2S] ferredoxins and site of attachment of CrIII using 1H NMR and site-directed mutagenesis

The recently reported NMR solution structure of FeIIIFeIII parsley FdI has made possible 2D NOESY NMR studies to determine the point of attachment of CrIIIL in FeIIIFeIII...CrIIIL. The latter Cr-modified product was obtained by reduction of FeIIIFeIII parsley and spinach FdI forms with [Cr(15-aneN4) (H2O)2]2+ (15-aneN4 = 1,4,8,12-tetraazacyclopentadecane), referred to here as CrIIL, followed by air oxidation and chromatographic purification. From a comparison of NMR cross-peak intensities of native and Cr-modified proteins, two surface sites designated A and B, giving large paramagnetic CrIIIL broadening of a number of amino acid peaks, have been identified. The effects at site A (residues 19-22, 27, and 30) are greater than those at site B (residues 92-94 and 96), which is on the opposite side of the protein. From metal (ICP-AES) and electrospray ionization mass spectrometry (EIMS) analyses on the Cr-modified protein, attachment of a single CrIIIL only is confirmed for both parsley and spinach FdI and FdII proteins. Electrostatic interaction of the 3+ CrIIIL center covalently attached to one protein molecule (charge approximately -18) with a second (like) molecule provides an explanation for the involvement of two regions. Thus for 3-4 mM FeIIIFeIII...CrIIIL solutions used in NMR studies (CrIIIL attached at A), broadening effects due to electrostatic interactions at B on a second molecule are observed. Experiments with the Cys18Ala spinach FdI variant have confirmed that the previously suggested Cys-18 at site A is not the site of CrIIIL attachment. Line broadening at Val-22 of A gives the largest effect, and CrIIIL attachment at one or more adjacent (conserved) acidic residues in this region is indicated. The ability of CrIIL to bind in some (parsley and spinach) but not all cases (Anabaena variabilis) suggests that intramolecular H-bonding of acidic residues at A is relevant. The parsley and spinach FeIIFeIII...CrIIIL products undergo a second stage of reduction with the formation of FeIIFeII...CrIIIL. However, the spinach Glu92Ala (site B) variant undergoes only the first stage of reduction, and it appears that Glu-92 is required for the second stage of reduction to occur. A sample of CrIIIL-modified parsley FeIIIFeIII Fd is fully active as an electron carrier in the NADPH-cytochrome c reductase reaction catalyzed by ferredoxin-NADP+ reductase.

The structure of iron-sulfur proteins

Ferredoxins are a group of iron-sulfur proteins for which a wealth of structural and mutational data have recently become available. Previously unknown structures of ferredoxins which are adapted to halophilic, acidophilic or hyperthermophilic environments and new cysteine patterns for cluster ligation and non-cysteine cluster ligation have been described. Site-directed mutagenesis experiments have given insight into factors that influence the geometry, stability, redox potential, electronic properties and electron-transfer reactivity of iron-sulfur clusters.

Identification of a novel delta 6-acyl-group desaturase by targeted gene disruption in Physcomitrella patens

The moss Physcomitrella patens contains high levels of arachidonic acid. For its synthesis from linoleic acid by desaturation and elongation, novel delta 5- and delta 6-desaturases are required. To isolate one of these, PCR-based cloning was used, and resulted in the isolation of a full-length cDNA coding for a putatively new desaturase. The deduced amino acid sequence has three domains: a N-terminal segment of about 100 amino acids, with no similarity to any sequence in the data banks, followed by a cytochrome b5-related region and a C-terminal sequence with low similarity (27% identify) to acyl-lipid desaturases. To elucidate the function of this protein, we disrupted its gene by transforming P. patens with the corresponding linear genomic sequence, into which a positive selection marker had been inserted. The molecular analysis of five transformed lines showed that the selection cartridge had been inserted into the corresponding genomic locus of all five lines. The gene disruption resulted in a dramatic alteration of the fatty acid pattern in the knockout plants. The large increase in linoleic acid and the concomitant disappearance of gamma-linolenic and arachidonic acid in all knockout lines suggested that the new cDNA coded for a delta 6-desaturase. This was confirmed by expression of the cDNA in yeast and analysis of the resultant fatty acids by GC-MS. Only the transformed yeast cells were able to introduce a further double bond into the delta 6-position of unsaturated fatty acids. To our knowledge, this is the first report of a successful gene disruption in a multicellular plant resulting in a specific biochemical phenotype.

DESATURATION AND RELATED MODIFICATIONS OF FATTY ACIDS1

Desaturation of a fatty acid first involves the enzymatic removal of a hydrogen from a methylene group in an acyl chain, a highly energy-demanding step that requires an activated oxygen intermediate. Two types of desaturases have been identified, one soluble and the other membrane-bound, that have different consensus motifs. Database searching for these motifs reveals that these enzymes belong to two distinct multifunctional classes, each of which includes desaturases, hydroxylases, and epoxidases that act on fatty acids or other substrates. The soluble class has a consensus motif consisting of carboxylates and histidines that coordinate an active site diiron cluster. The integral membrane class contains a different consensus motif composed of histidines. Biochemical and structural similarities between the integral membrane enzymes suggest that this class also uses a diiron cluster for catalysis. Soluble and membrane enzymes have been successfully re-engineered for substrate specificity and reaction outcome. It is anticipated that rational design of these enzymes will result in new and desired activities that may form the basis for improved oil crops.

Biochemical characterization of a delta12 acyl-lipid desaturase after overexpression of the enzyme in Escherichia coli

The Delta12 acyl-lipid desaturase of Synechocystis sp. PCC 6803 was overexpressed in Escherichia coli as an active enzyme. The overexpressed protein was associated with cell membranes; it represented about 10% of the total cellular protein and 25% of the total membrane protein. The enzyme in the membrane fraction exhibited strong fatty-acid desaturase activity. The desaturase in salt-washed membranes was stabilized by the presence of sorbitol. Storage of salt-washed membranes in 2 M sorbitol at 4 degrees C and at pH 7-8 for six days resulted in the loss of less than 10% of the desaturase activity. The desaturase activity had a positive temperature coefficient, a result that suggests that the increase in the desaturation of fatty acids at low temperature might not be caused by the activation of desaturases at low temperature but, rather, by the increased synthesis of desaturases de novo.

Formation, Properties, and Characterization of a Fully Reduced Fe(II)Fe(II) Form of Spinach (and Parsley) [2Fe-2S] Ferredoxin with the Macrocyclic Complex [Cr(15-aneN(4))(H(2)O)(2)](2+) as Reductant

Reduction of spinach and parsley ferredoxin FdI in the Fe(III)Fe(III) state with the 1,4,8,12-tetraazacyclopentadecane complex [Cr(15-aneN(4))(H(2)O)(2)](2+), here written as Cr(II)L, provides the first evidence for two 1-equiv steps yielding an Fe(II)Fe(II) product. Rate constants (25 degrees C) for spinach FdI are 2760 and 660 M(-)(1) s(-)(1), respectively, at pH 7.5, I = 0.100 M (NaCl). An important observation is that the Cr(III)L generated in the first step remains attached to the Fe(II)Fe(III) product and perturbs the protein active site sufficiently to make the second stage possible. The second Cr(II)L reduction is of the "outer-sphere" type, and the Cr(III)L generated is not attached to the protein. Anaerobic reoxidation of the fully reduced protein with [Co(NH(3))(6)](3+) is rapid and can be achieved with approximately 80% recovery of the Fe(III)Fe(III) oxidation state over 40 min. Air oxidation yields the Cr(III)L product Fe(III)Fe(III).Cr(III)L (Fe:Cr = 2:1). With Anabaena variabilis only a one-step reduction is observed and there is no Cr(III)L attachment. From a comparison of amino acid sequences with spinach (and parsley) FdI, a likely point of Cr(III)L attachment is indicated. Comparisons are made with dithionite as reductant. In addition, square-wave voltammetry on spinach Fe(III)Fe(III).Cr(III)L gives two reduction potentials -273 and -410 mV vs NHE. The different redox products have been characterized by EPR. Using (1)H NMR line-broadening techniques, evidence for Cr(III)L binding at a surface site close to Tyr-25/Tyr-82 is obtained. Also from investigations with redox-inactive [Cr(en)(3)](3+) as a competitive inhibitor for Cr(II)L reduction of spinach Fe(III)Fe(III), Tyr-25/Tyr-82 is proposed as the site for Cr(II)L reduction. From an extension of studies to include reduction of Fe(III)Fe(III).Cr(III)L with Cr(II)L, evidence is obtained for a second reaction site when that at Tyr-25/Tyr-82 is no longer available.

CHILLING SENSITIVITY IN PLANTS AND CYANOBACTERIA: The Crucial Contribution of Membrane Lipids

The contribution of membrane lipids, particularly the level of unsaturation of fatty acids, to chilling sensitivity of plants has been intensively discussed for many years. We have demonstrated that the chilling sensitivity can be manipulated by modulating levels of unsaturation of fatty acids of membrane lipids by the action of acyl-lipid desaturases and glycerol-3-phosphate acyltransferase. This review covers recent studies on genetic manipulation of these enzymes in transgenic tobacco and cyanobacteria with special emphasis on the crucial importance of the unsaturation of membrane lipids in protecting the photosynthetic machinery from photoinhibition under cold conditions. Furthermore, we review the molecular mechanism of temperature-induced desaturation of fatty acids and introduce our hypothesis that changes in the membrane fluidity is the initial event of the expression of desaturase genes.

Isolation and characterization of mutants affected in lipid metabolism of Chlamydomonas reinhardtii

Two mutants affected in lipid metabolism of Chlamydomonas reinhardtii were isolated by treating cells with ultraviolet light. Both mutants showed high chlorophyll fluorescent yields, as compared with parents, and were designated as hf-2 and hf-9 (for high fluorescence). hf-2 was shown to be defective in the synthesis of a chloroplast-specific lipid, sulfoquinovosyl diacylglycerol. hf-9 was shown to be defective in desaturation at the omega 6 position of fatty acids of monogalactosyl diacylglycerol, digalactosyl diacylglycerol, sulfoquinovosyl diacylglycerol and phosphatidylglycerol. The mutants exhibited alterations in photosynthetic activity and chloroplast ultrastructure.

Acyl-lipid desaturases and their importance in the tolerance and acclimatization to cold of cyanobacteria

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Cloning of omega 3 desaturase from cyanobacteria and its use in altering the degree of membrane-lipid unsaturation

Cyanobacteria respond to a decrease in temperature by desaturating fatty acids of membrane lipids to compensate for the decrease in membrane fluidity. Among various desaturation reactions in cyanobacteria, the desaturation of the omega 3 position of fatty acids is the most sensitive to the change in temperature. In the present study, we isolated a gene, designated desB, for the omega 3 desaturase from the cyanobacterium, Synechocystis sp. PCC 6803. The desB gene encodes a protein a 359 amino-acid residues with molecular mass of 41.9 kDa. The desB gene is transcribed as a monocistronic operon that produced a single transcript of 1.4 kb. The level of the desB transcript in cells grown at 22 degrees C was 10 times higher than that in cells grown at 34 degrees C. In order to manipulate the fatty-acid unsaturation of membrane lipids, the desB gene in Synechocystis sp. PCC 6803 was mutated by insertion of a kanamycin-resistance gene cartridge. The resultant mutant was unable to desaturate fatty acids at the omega 3 position. The desA gene, which encodes the delta 12 desaturase of Synechocystis sp. PCC 6803, and the desB gene were introduced into Synechococcus sp. PCC 7942. Whilst the parent cyanobacterium can only desaturate membrane lipids at the delta 9 position of fatty acids, the resultant transformant was able to desaturate fatty acids of membrane lipids at the delta 9, delta 12 and omega 3 positions. These results confirm the function of the desB gene and demonstrate that it is possible to genetically manipulate the fatty-acid unsaturation of membrane lipids in cyanobacteria.

Purification and PCR-based cDNA cloning of a plastidial n-6 desaturase

A plastidial membrane-bound n-6 desaturase from spinach (Spinacia oleracea) was purified from chloroplast envelope membranes by anion exchange, cation exchange and ferredoxin-affinity chromatography. The molecular mass of the protein was estimated by SDS-PAGE to be 40 kDa. The highest specific activity of the desaturase in the final preparation was 196 nmol/min per mg protein with free oleic acid as the substrate. The N-terminal amino acid sequence of the blotted protein was determined and used for the construction of a degenerated and inosine-containing oligonucleotide primer for PCR experiments with cDNA transcribed from leaf mRNA. A 3'-RACE experiment with this primer amplified a single band of 1500 bp that after sequencing showed an open reading frame of 382 amino acids corresponding to a protein of 43 kDa. The 5' end of the cDNA was amplified by a 5'-RACE experiment and isolated as a 500 bp fragment. Sequencing of this DNA revealed an additional 65 amino acids at the N-terminus of the native protein that are attributed to a plastidial leader peptide. With appropriate primers derived from these sequences a full-length clone was amplified by PCR and sequenced. Comparison of the plastidial oleate desaturase with the homologous enzyme from cyanobacteria showed about 50% amino acid homology. Comparison with other desaturases revealed three histidine boxes with the general sequence HXXXH that are highly conserved in all membrane-bound desaturases. These boxes might be involved in metal ion complexation required for reduction of oxygen.

Identification of conserved domains in the delta 12 desaturases of cyanobacteria

Cyanobacterial genes for enzymes that desaturate fatty acids at the delta 12 position, designated desA, were isolated from Synechocystis PCC6714, Synechococcus PCC7002 and Anabaena variabilis by cross-hybridization with a DNA probe derived from the desA gene of Synechocystis PCC6803. The genes of Synechocystis PCC6714, Synechococcus PCC7002 and A. variabilis encode proteins of 349, 347 and 350 amino acid residues, respectively. The transformation of Synechococcus PCC7942 with the desA genes from Synechocystis PCC6714, Synechococcus PCC7002 and A. variabilis was associated with the ability to introduce a second double bond at the delta 12 position of fatty acids. The amino acid sequence of the products of the desA genes revealed the presence of four conserved domains. Since one of the conserved domains was also found in the amino acid sequences of omega 3 desaturases of Brassica napus and mung bean, this domain may play an essential role in the introduction of a double bond into fatty acids bound to membrane lipids.

Octadecatetraenoate synthesis in the unicellular alga Isochrysis galbana: studies with intact and broken chloroplasts

(1) Monogalactosyldiacylglycerol (MGD) is the major lipid component of Isochrysis galbana. In cells incubated for 3 h with [1-14C]]acetate or [1-14C]oleate. MGD contained 35.9% and 52.8%, respectively, of the label incorporated into cellular lipids. (2) 18:4 amounted to 50-60% of the total FA of MGD. Separation of MGD species of cells grown with [1-14C]oleate on AgNO3 impregnated plates revealed 20 distinct spots. The slowest spot was identified as dioctadecatetraenoyl MGD. Fast moving species were enriched with 18:1. (3) In cells incubated for 3 h with [1-14C]oleate, approx. 60% of the radioactivity was associated with 18:1. Subsequent chase resulted in a gradual shift of label and after 48 h [14C]18:1 declined to 10% and [14C]18:4 reached 52%. This shift was also reflected in the labeling pattern of the MGD-species. Dioctadecatetraenoyl-MGD became labelled only after 24 h. (4) Addition of the substituted pyridazinone herbicide (SAN 9785) during the chase period inhibited [14C]18:4 formation; [14C]18:2 and [14C]18:3 accumulated instead. (5) Isolated chloroplast readily incorporated [14C]oleate into MGD and PA. Considerable amounts of [14C]18:1 were desaturated to 18:2 and 18:3. Only very small amounts of 18:4 were formed. O2 was required for desaturation. Cofactor requirement could not be shown. (6) Membranes isolated from broken chloroplasts retained the ability to incorporate [14C]oleate into MGD and PA and desaturate 18:1 to 18:2 and 18:3.

Direct desaturation of intact galactolipids by a desaturase solubilized from spinach (Spinacia oleracea) chloroplast envelopes

In plants, polyenoic fatty acids are synthesized by desaturase enzymes which use acyl groups of membrane lipids as substrates. To provide direct 'in vitro' evidence for this reaction, we solubilized envelope membranes from spinach (Spinacia oleracea) chloroplasts with Triton X-100 to release a membrane-bound n-6 desaturase. In the presence of oxygen and reduced ferredoxin, the solubilized enzyme desaturated a variety of substrates, such as free oleic acid, free erucic acid, 1-oleoyl-sn-glycerol 3-phosphate and the three galactolipids 1-oleoyl-2-(7'-cis-hexadecenoyl)-3-beta-D-galactopyranosyl-sn-glycerol, 1,2-dioleoyl-3-beta-D-galactopyranosyl-sn-glycerol and the ether analogue 1,2-di-(9'-cis-octadecenyl)-3-beta-D-galactopyranosyl-sn- glycerol. The in vitro desaturation of these exogenously added complex lipids with ester- and ether-linked substrate chains is unambiguous evidence for lipid-linked desaturation. The enzyme measures the insertion of the new double bond from the methyl end and the existing (n-9)-cis-double bond of an appropriate acyl or alkyl chain. The distal part of the substrate group, normally the carboxy end of a fatty acyl residue, is of less importance and, in particular, its activation in thioester form is not required.

In vitro ferredoxin-dependent desaturation of fatty acids in cyanobacterial thylakoid membranes

Thylakoid membranes isolated from the cyanobacterium Synechocystis sp. strain PCC6803 were capable of desaturating the acyl groups in monogalactosyl diacylglycerol. This desaturation reaction required the reduced form of ferredoxin.

Biosynthesis of linolenate in developing embryos and cell-free preparations of high-linolenate linseed (Linum usitatissimum) and low-linolenate mutants

Biosynthesis of alpha-linolenate was investigated in developing embryos of the high-linolenic (45%) linseed cv. Glenelg, two mutant lines (M1589 and M1722) having reduced linolenic acid content (30%), and a very low linolenic (2%) genotype (Zero) obtained by recombination of the M1589 and M1722 mutations. Glenelg embryos showed an exponential rate of linolenate synthesis that paralleled their exponential pattern of triacylglycerol accumulation. The Zero line, although showing a pattern of triacylglycerol accumulation similar to that of Glenelg, accumulated linolenate at only a very low and constant rate throughout embryo development. An NADH- and O2-dependent decrease in oleate and increase in linolenate content of phosphatidylcholine was observed in dilute homogenates prepared from Glenelg embryos at 21 days after flowering, indicating active oleoyl- and linoleoyl-phosphatidylcholine desaturases in these preparations. While oleate decreased similarly in both sn positions of phosphatidylcholine, the increase in linolenate was confined mostly to the sn-2 position. Homogenates prepared from the mutant lines showed decreases in oleoyl-phosphatidylcholine similar to those of the wild-type Glenelg, whereas the increase in linolenoyl-phosphatidylcholine was substantially lower in M1589 and M1722 and barely detected in Zero. In vivo labeling experiments with detached embryos at 17 days after flowering, as well as analysis of endogenous linolenate content in various lipids, indicated that only delta 15-phospholipid desaturases, and not delta 15-galactolipid desaturases, were affected by the mutations. Embryos from M1722 had amounts of both radioactive and endogenous linolenate at position sn-1 of phosphatidylcholine that were close to those of the wild-type embryos, whereas M1589 had only 30 and 50% of these levels, respectively. The regulation of linolenic acid content in oilseeds is discussed on the basis of the results obtained.

Glycerolipid-fatty-acid desaturase deficiencies in chloroplasts from fruits of Capsicum annuum L

Chloroplasts from fruits and leaves of Capsicum annuum cv. 'Bell Tower' were purified on sucrose gradients, and the lipids were separated by column and thin-layer chromatography. The glycerolipids mono- and digalactosyldiacylglycerol (MGDG, DGDG), sulfoquinovosyldiacylglycerol (SQDG), and phosphatidylglycerol (PG) were quantified, and the fatty-acid composition at the 1 and 2 positions of the glycerol moiety (sn-1 and sn-2) was determined after hydrolysis with position-specific lipases. In fruit chloroplasts, Δ (3)-trans hexadecenoate (trans-3-16∶1) was absent and replaced by palmitate (16∶0) at sn-2 of PG, and Δ (7,10,13)-hexadecatrienoate (16∶3) at sn-2 of MGDG was greatly reduced and largely replaced by linoleate (18∶2). The ratio of 18∶2 to linolenate (18∶3) was consistently greater in glycerolipids from fruit compared with leaf chloroplasts. The lower percentage of C-16 fatty acids at sn-2 indicated that "prokaryotic" molecular species were reduced by ≤15% in DGDG, ∼40% in SQDG, and ∼90% in MGDG, in fruit compared with leaf chloroplasts. The MGDG∶DGDG ratios in fruit and leaf chloroplasts were 1.2∶1 and 2.2∶1, respectively. Taken together, the data indicate that chloroplasts in Capsicum fruit are deficient in three desaturases: those that convert 1) 16∶0 to Δ (3)-trans-16∶1 at sn-2 of PG, 2) 16∶0 to Δ (7)-cis-16∶1 at sn-2 of MGDG, and 3) 18∶2 to 18∶3 at both sn-1 and sn-2 of various chloroplast glycerolipids.