The evolution of fatty acid desaturases and cytochrome b5 in eukaryotes

Characterization of Unique Eukaryotic Sphingolipids with Temperature-Dependent Δ8-Unsaturation from the Picoalga Ostreococcus tauri

Sphingolipids (SLs) are ubiquitous components of eukaryotic cell membranes and are found in some prokaryotic organisms and viruses. They are composed of a sphingoid backbone that may be acylated and glycosylated. Assembly of various sphingoid base, fatty acyl and glycosyl moieties results in highly diverse structures. The functional significance of variations in SL chemical diversity and abundance is still in the early stages of investigation. Among SL modifications, Δ8-desaturation of the sphingoid base occurs only in plants and fungi. In plants, SL Δ8-unsaturation is involved in cold hardiness. Our knowledge of the structure and functions of SLs in microalgae lags far behind that of animals, plants and fungi. Original SL structures have been reported from microalgae. However, functional studies are still missing. Ostreococcus tauri is a minimal microalga at the base of the green lineage and is therefore a key organism for understanding lipid evolution. In the present work, we achieved the detailed characterization of O. tauri SLs and unveiled unique glycosylceramides as sole complex SLs. The head groups are reminiscent of bacterial SLs, as they contain hexuronic acid residues and can be polyglycosylated. Ceramide backbones show a limited variety, and SL modification is restricted to Δ8-unsaturation. The Δ8-SL desaturase from O. tauri only produced E isomers. Expression of both Δ8-SL desaturase and Δ8-unsaturation of sphingolipids varied with temperature, with lower levels at 24°C than at 14°C. Overexpression of the Δ8-SL desaturase dramatically increases the level of Δ8 unsaturation at 24°C and is paralleled by a failure to increase cell size. Our work provides the first characterization of O. tauri SLs and functional evidence for the involvement of SL Δ8-unsaturation for temperature acclimation in microalgae, suggesting that this function is an ancestral feature in the green lineage.

De Novo Biosynthesis of Free Vaccenic Acid with a Low Content of Oleic Acid in Saccharomyces cerevisiae

Omega-7 (ω-7) fatty acids have potential application in the fields of nutraceutical, agricultural, and food industry. The natural ω-7 fatty acids are currently from plants or vegetable oils, which are unsustainable and limited by the availability of plant sources. Here, we developed an innovative biosynthetic route to produce vaccenic acid (C18:1 ω-7) while minimizing oleic acid (C18:1 ω-9) content in Saccharomyces cerevisiae. We have engineered S. cerevisiaeto produce C18:1 ω-7 by expressing a fatty acid elongase from Rattus norvegicus. To reduce the content of C18:1 ω-9, the endogenous desaturase Ole1 was replaced by the desaturase, which has specific activity on palmitoyl-coenzyme A (C16:0-CoA). Finally, the production of free C18:1 ω-7 was improved by optimizing the source of cytochrome b5 and overexpressing endoplasmic reticulum chaperones. After combining these strategies, the yield of C18:1 ω-7 was increased from 0 to 9.3 mg/g DCW and C18:1 ω-9 was decreased from 25.2 mg/g DCW to 1.6 mg/g DCW. This work shows a de novo synthetic pathway to produce the highest amount of free C18:1 ω-7 with a low content of C18:1 ω-9 in S. cerevisiae.

Review of Eukaryote Cellular Membrane Lipid Composition, with Special Attention to the Fatty Acids

Biological membranes, primarily composed of lipids, envelop each living cell. The intricate composition and organization of membrane lipids, including the variety of fatty acids they encompass, serve a dynamic role in sustaining cellular structural integrity and functionality. Typically, modifications in lipid composition coincide with consequential alterations in universally significant signaling pathways. Exploring the various fatty acids, which serve as the foundational building blocks of membrane lipids, provides crucial insights into the underlying mechanisms governing a myriad of cellular processes, such as membrane fluidity, protein trafficking, signal transduction, intercellular communication, and the etiology of certain metabolic disorders. Furthermore, comprehending how alterations in the lipid composition, especially concerning the fatty acid profile, either contribute to or prevent the onset of pathological conditions stands as a compelling area of research. Hence, this review aims to meticulously introduce the intricacies of membrane lipids and their constituent fatty acids in a healthy organism, thereby illuminating their remarkable diversity and profound influence on cellular function. Furthermore, this review aspires to highlight some potential therapeutic targets for various pathological conditions that may be ameliorated through dietary fatty acid supplements. The initial section of this review expounds on the eukaryotic biomembranes and their complex lipids. Subsequent sections provide insights into the synthesis, membrane incorporation, and distribution of fatty acids across various fractions of membrane lipids. The last section highlights the functional significance of membrane-associated fatty acids and their innate capacity to shape the various cellular physiological responses.

Genome-wide comparative identification and analysis of membrane-FADS-like superfamily genes in freshwater economic fishes

Membrane fatty acid desaturase (FADS)-like superfamily proteins (FADSs) are essential for the synthesis of unsaturated fatty acids (UFAs). Recently, studies on FADS in fishes have mostly focused on marine species, and a comprehensive analysis of the FADS superfamily, including the FADS, stearoyl-CoA desaturase (SCD), and sphingolipid delta 4-desaturase (DEGS) families, in freshwater economic fishes is urgently required. To this end, we conducted a thorough analysis of the number, gene/protein structure, chromosomal location, gene linkage map, phylogeny, and expression of the FADS superfamily. We identified 156 FADSs genes in the genome of 27 representative species. Notably, FADS1 and SCD5 were lost in most freshwater fish and other teleosts. All FADSs proteins contain 4 transmembrane helices and 2-3 amphipathic α-helices. FADSs in the same family are often linked on the same chromosome; moreover, FADS and SCD or DEGS are frequently collocated on the same chromosome. In addition, FADS, SCD, and DEGS family proteins share similar evolutionary patterns. Interestingly, FADS6, as a member of the FADS family, exhibits a similar gene structure and chromosome location to that of SCD family members, which may be the transitional form of FADS and SCD. This study shed light on the type, structure, and phylogenetic relationship of FADSs in freshwater fishes, offering a new perspective into the functional mechanism analysis of FADSs.

Desaturases: Structural and mechanistic insights into the biosynthesis of unsaturated fatty acids

This review highlights the key role of fatty acid desaturases in the synthesis of naturally occurring, more common and not unsaturated fatty acids. The three major classes of fatty acid desaturases, such as acyl-lipid, acyl-acyl carrier protein and acyl-coenzyme A, are described in detail, with particular attention to the cellular localisation, the structure, the substrate and product specificity and the expression and regulation of desaturase genes. The review also gives an insight into the biocatalytic reaction of fatty acid desaturation by covering the general and more class-specific mechanistic studies around the synthesis of unsaturated fatty acids Finally, we conclude the review by looking at the numerous novel applications for desaturases in order to meet the very high demand for polyunsaturated fatty acids, taking into account the opportunity for the development of new, more efficient, easily reproducible, sustainable bioengineering advances in the field.

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.

Melatonin: Regulation of Biomolecular Condensates in Neurodegenerative Disorders

Biomolecular condensates are membraneless organelles (MLOs) that form dynamic, chemically distinct subcellular compartments organizing macromolecules such as proteins, RNA, and DNA in unicellular prokaryotic bacteria and complex eukaryotic cells. Separated from surrounding environments, MLOs in the nucleoplasm, cytoplasm, and mitochondria assemble by liquid-liquid phase separation (LLPS) into transient, non-static, liquid-like droplets that regulate essential molecular functions. LLPS is primarily controlled by post-translational modifications (PTMs) that fine-tune the balance between attractive and repulsive charge states and/or binding motifs of proteins. Aberrant phase separation due to dysregulated membrane lipid rafts and/or PTMs, as well as the absence of adequate hydrotropic small molecules such as ATP, or the presence of specific RNA proteins can cause pathological protein aggregation in neurodegenerative disorders. Melatonin may exert a dominant influence over phase separation in biomolecular condensates by optimizing membrane and MLO interdependent reactions through stabilizing lipid raft domains, reducing line tension, and maintaining negative membrane curvature and fluidity. As a potent antioxidant, melatonin protects cardiolipin and other membrane lipids from peroxidation cascades, supporting protein trafficking, signaling, ion channel activities, and ATPase functionality during condensate coacervation or dissolution. Melatonin may even control condensate LLPS through PTM and balance mRNA- and RNA-binding protein composition by regulating N6-methyladenosine (m6A) modifications. There is currently a lack of pharmaceuticals targeting neurodegenerative disorders via the regulation of phase separation. The potential of melatonin in the modulation of biomolecular condensate in the attenuation of aberrant condensate aggregation in neurodegenerative disorders is discussed in this review.

Optimization of Multiparameters for Increased Yields of Cytochrome B5 in Bioreactors

The production of recombinant proteins is gaining increasing importance as the market requests high quality proteins for several applications. However, several process parameters affect both the growth of cells and product yields. This study uses high throughput systems and statistical methods to assess the influence of fermentation conditions in lab-scale bioreactors. Using this methodology, it was possible to find the best conditions to produce cytochrome b5 with recombinant cells of Escherichia coli. Using partial least squares, the height-to-diameter ratio of the bioreactor, aeration rate, and PID controller parameters were found to contribute significantly to the final biomass and cytochrome concentrations. Hence, we could use this information to fine-tune the process parameters, which increased cytochrome production and yield several-fold. Using aeration of 1 vvm, a bioreactor with a height-to-ratio of 2.4 and tuned PID parameters, a production of 72.72 mg/L of cytochrome b5 in the culture media, and a maximum of product to biomass yield of 24.97 mg/g could be achieved.

Δ6 fatty acid desaturases in polyunsaturated fatty acid biosynthesis: insights into the evolution, function with substrate specificities and biotechnological use

Δ6 fatty acid desaturases (FADS6) have different substrate specificities that impact the ratio of omega-6/omega-3 polyunsaturated fatty acids, which are involved in regulating multiple signalling pathways associated with various diseases. For decades, FADS6 with different substrate specificities have been characterized and the functions of these crucial enzymes have been investigated, while it remains enigmatic that the substrate specificities of FADS6 from various species have a huge difference. This review summarizes the substrate specificities of FADS6 in different species and reveals the underlying relationship. Further evaluation of biochemical properties has revealed that the FADS6 prefer linoleic acid that is more hydrophilic and stable. Domain-swapping and site-directed mutagenesis have been employed to delineate the regions and sites that affect the substrate specificities of FADS6. These analyses improve our understanding of the functions of FADS6 and offer information for the discovery of novel biological resources. KEY POINTS: • Outline of the excavation and identification of Δ6 fatty acid desaturases. • Overview of methods used to determine the pivotal resides of desaturases. • Application of substrate properties to generate specific fatty acids.

Fatty Acid Biosynthesis in Chromerids

Fatty acids are essential components of biological membranes, important for the maintenance of cellular structures, especially in organisms with complex life cycles like protozoan parasites. Apicomplexans are obligate parasites responsible for various deadly diseases of humans and livestock. We analyzed the fatty acids produced by the closest phototrophic relatives of parasitic apicomplexans, the chromerids Chromera velia and Vitrella brassicaformis, and investigated the genes coding for enzymes involved in fatty acids biosynthesis in chromerids, in comparison to their parasitic relatives. Based on evidence from genomic and metabolomic data, we propose a model of fatty acid synthesis in chromerids: the plastid-localized FAS-II pathway is responsible for the de novo synthesis of fatty acids reaching the maximum length of 18 carbon units. Short saturated fatty acids (C14:0-C18:0) originate from the plastid are then elongated and desaturated in the cytosol and the endoplasmic reticulum. We identified giant FAS I-like multi-modular enzymes in both chromerids, which seem to be involved in polyketide synthesis and fatty acid elongation. This full-scale description of the biosynthesis of fatty acids and their derivatives provides important insights into the reductive evolutionary transition of a phototropic algal ancestor to obligate parasites.

Assessment of Fatty Acid Desaturase (Fads2) Structure-Function Properties in Fish in the Context of Environmental Adaptations and as a Target for Genetic Engineering

Fatty acid desaturase 2 (Fads2) is the key enzyme of long-chain polyunsaturated fatty acid (LC-PUFA) biosynthesis. Endogenous production of these biomolecules in vertebrates, if present, is insufficient to meet demand. Hence, LC-PUFA are considered as conditionally essential. At present, however, LC-PUFA are globally limited nutrients due to anthropogenic factors. Research attention has therefore been paid to finding ways to maximize endogenous LC-PUFA production, especially in production species, whereby deeper knowledge on molecular mechanisms of enzymatic steps involved is being generated. This review first briefly informs about the milestones in the history of LC-PUFA essentiality exploration before it focuses on the main aim-to highlight the fascinating Fads2 potential to play roles fundamental to adaptation to novel environmental conditions. Investigations are summarized to elucidate on the evolutionary history of fish Fads2, providing an explanation for the remarkable plasticity of this enzyme in fish. Furthermore, structural implications of Fads2 substrate specificity are discussed and some relevant studies performed on organisms other than fish are mentioned in cases when such studies have to date not been conducted on fish models. The importance of Fads2 in the context of growing aquaculture demand and dwindling LC-PUFA supply is depicted and a few remedies in the form of genetic engineering to improve endogenous production of these biomolecules are outlined.

Cellular and Molecular Mechanisms of Recessive Hereditary Methaemoglobinaemia Type II

Cytochrome b5 reductase 3 (CYB5R3) is a membrane-bound NADH-dependent redox enzyme anchored to the mitochondrial outer membrane, endoplasmic reticulum, and plasma membrane. Recessive hereditary methaemoglobinaemia (RHM) type II is caused by CYB5R3 deficiency and is an incurable disease characterized by severe encephalopathy with mental retardation, microcephaly, generalized dystonia, and movement disorders. Currently, the etiology of type II RHM is poorly understood and there is no treatment for encephalopathy associated with this disease. Defective CYB5R3 leads to defects in the elongation and desaturation of fatty acids and cholesterol biosynthesis, which are conventionally linked with neurological disorders of type II RHM. Nevertheless, this abnormal lipid metabolism cannot explain all manifestations observed in patients. Current molecular and cellular studies indicate that CYB5R3 deficiency has pleiotropic tissue effects. Its localization in lipid rafts of neurons indicates its role in interneuronal contacts and its presence in caveolae of the vascular endothelial membrane suggests a role in the modulation of nitric oxide diffusion. Its role in aerobic metabolism and oxidative stress in fibroblasts, neurons, and cardiomyocytes has been reported to be due to its ability to modulate the intracellular ratio of NAD⁺/NADH. Based on the new molecular and cellular functions discovered for CYB5R3 linked to the plasma membrane and mitochondria, the conventional conception that the cause of type II RHM is a lipid metabolism disorder should be revised. We hypothesized that neurological symptoms of the disease could be caused by disorders in the synapse, aerobic metabolism, and/or vascular homeostasis rather than in disturbances of lipid metabolism.

Insights into the phylogenetic and molecular evolutionary histories of Fad and Elovl gene families in Actiniaria

The biosynthesis of long-chain polyunsaturated fatty acids (LC-PUFAs, ≥ C20) is reliant on the action of desaturase and elongase enzymes, which are encoded by the fatty acyl desaturase (Fad) and elongation of very long-chain fatty acid (Elovl) gene families, respectively. In Metazoa, research investigating the distribution and evolution of these gene families has been restricted largely to Bilateria. Here, we provide insights into the phylogenetic and molecular evolutionary histories of the Fad and Elovl gene families in Cnidaria, the sister phylum to Bilateria. Four model cnidarian genomes and six actiniarian transcriptomes were interrogated. Analysis of the fatty acid composition of a candidate cnidarian species, Actinia tenebrosa, was performed to determine the baseline profile of this species. Phylogenetic analysis revealed lineage-specific gene duplication in actiniarians for both the Fad and Elovl gene families. Two distinct cnidarian Fad clades clustered with functionally characterized Δ5 and Δ6 proteins from fungal and plant species, respectively. Alternatively, only a single cnidarian Elovl clade clustered with functionally characterized Elovl proteins (Elovl4), while two additional clades were identified, one actiniarian-specific (Novel ElovlA) and the another cnidarian-specific (Novel ElovlB). In actiniarians, selection analyses revealed pervasive purifying selection acting on both gene families. However, codons in the Elovl gene family show patterns of nucleotide variation consistent with the action of episodic diversifying selection following gene duplication events. Significantly, these codons may encode amino acid residues that are functionally important for Elovl proteins to target and elongate different precursor fatty acids. In A. tenebrosa, the fatty acid analysis revealed an absence of LC-PUFAs > C20 molecules and implies that the Elovl enzymes are not actively contributing to the elongation of these LC-PUFAs. Overall, this study has revealed that actiniarians possess Fad and Elovl genes required for the biosynthesis of some LC-PUFAs, and that these genes appear to be distinct from bilaterians.

Identification of cytochrome b5 CYTB-5.1 and CYTB-5.2 in C. elegans; evidence for differential regulation of SCD

Unsaturated fatty acids (UFAs) play crucial roles in living organisms regarding development, energy metabolism, stress resistance, etc. The biosynthesis of UFAs starts from the introduction of the first double bond by stearoyl-CoA desaturase (SCD), converting saturated fatty acids (SFAs) to monounsaturated fatty acids (MUFAs). This desaturation is considered to be an aerobic process that requires cytochrome b5 reductase, cytochrome b5 and SCD. However, this enzyme system remains elusive in Caenorhabditis elegans. Here, we show that inactivation by RNAi knockdown or mutation (gk442189) of putative cytochrome b5 genes cytb-5.1 led to reduced conversion of C18:0 to C18:1(n-9) by SCD desaturases FAT-6/7 in C. elegans. On the contrary, cytb-5.2RNAi and cytb-5.2(gk113588) mutant worms showed decreased conversion of C16:0 to C16:1(n-7) by FAT-5 desaturase. Dietary supplementation with C18:1(n-9) and C18:2(n-6) also showed that CYTB-5.1 is likely required for the activity of FAT-6/7 desaturases, but not for FAT-1 to FAT-4 desaturases. Interestingly, co-immunoprecipitation (Co-IP) demonstrated that either FAT-7 or FAT-5 has ability to interact with both CYTB-5.1 and CYTB-5.2. Moreover, RNAi knockdown of cytb-5.1 upregulates the transcriptional and translational expression of fat-5 to fat-7, which may be due to the feedback induction by reduced C18:1(n-9) and downstream fatty acids. Furthermore, both CYTB-5.1 and CYTB-5.2 are involved in fat accumulation, fertility and lifespan in worms, which may be independent of changes in fatty acid compositions. Collectively, these findings for the first time reveal the differential regulation of various SCDs by distinct cytochrome b5 CYTB-5.1 and CYTB-5.2 in the biosynthesis of UFAs in C. elegans.

Identification, Characterization and Expression Profiling of Stress-Related Genes in Easter Lily (Lilium formolongi)

Biotic and abiotic stresses are the major causes of crop loss in lily worldwide. In this study, we retrieved 12 defense-related expressed sequence tags (ESTs) from the NCBI database and cloned, characterized, and established seven of these genes as stress-induced genes in Lilium formolongi. Using rapid amplification of cDNA ends PCR (RACE-PCR), we successfully cloned seven full-length mRNA sequences from L. formolongi line Sinnapal lily. Based on the presence of highly conserved characteristic domains and phylogenetic analysis using reference protein sequences, we provided new nomenclature for the seven nucleotide and protein sequences and submitted them to GenBank. The real-time quantitative PCR (qPCR) relative expression analysis of these seven genes, including LfHsp70-1, LfHsp70-2, LfHsp70-3, LfHsp90, LfUb, LfCyt-b5, and LfRab, demonstrated that they were differentially expressed in all organs examined, possibly indicating functional redundancy. We also investigated the qPCR relative expression levels under two biotic and four abiotic stress conditions. All seven genes were induced by Botrytis cinerea treatment, and all genes except LfHsp70-3 and LfHsp90 were induced by Botrytis elliptica treatment; these genes might be associated with disease tolerance mechanisms in L. formolongi. In addition, LfHsp70-1, LfHsp70-2, LfHsp70-3, LfHsp90, LfUb, and LfCyt-b5 were induced by heat treatment, LfHsp70-1, LfHsp70-2, LfHsp70-3, LfHsp90, and LfCyt-b5 were induced by cold treatment, and LfHsp70-1, LfHsp70-2, LfHsp70-3, LfHsp90, LfCy-b5, and LfRab were induced by drought and salt stress, indicating their likely association with tolerance to these stress conditions. The stress-induced candidate genes identified in this study provide a basis for further functional analysis and the development of stress-resistant L. formolongi cultivars.

Cloning and functional analysis of Δ6-desaturase gene and its upstream region from Mortierella sp. AGED

BACKGROUND

Δ6-desaturase belonging to membrane-bound enzyme is a key enzyme involved in the synthesis of polyunsaturated fatty acids (PUFAs). This study aimed to clone and characterise Δ6-desaturase gene and its upstream regulatory region of Mortierella sp. AGED.

RESULTS

Glucose and soybean meal are best for lipid and arachidonic acid accumulation of Mortierella sp. AGED. A 1375-bp Δ6-desaturase gene AGfad6 which contains a 1275-bp open reading frame encoding 424 amino acids without signal peptide was cloned. The putative protein contained three conserved histidine-rich motifs and a conserved cytochrome b5 HPGG (H: Histine, P: Proline, G: Glycine, G: Glycine) motif, with a mass of 48.3 kDa and an isoelectric point of 5.96. AGfad6 was successfully expressed in Pichia pastoris GS115, which exerted the effect on converting linoleic acid to γ-linolenic acid. The 1712-bp upstream region contained basic transcriptional elements including TATA, GC and GATA box, putative target-binding sites for transcription factors such as TATA binding protein, transcription activator, CCAAT-enhancer-binding protein, activator protein 1, alcohol dehydrogenase gene regulator 1 and metabolic regulators p40x in fungi, stress-related elements including GT-1 (light-responsive, salicylic acid-inducible), stress response element, heat stress-responsive element, which might participate in regulation of PUFAs synthesis.

CONCLUSION

The present finding could enable us to understand the evolution and regulatory mechanism of Δ6-desaturase gene.

Identification of amino acid residues that determine the substrate specificity of mammalian membrane-bound front-end fatty acid desaturases

Membrane-bound desaturases are physiologically and industrially important enzymes that are involved in the production of diverse fatty acids such as polyunsaturated fatty acids and their derivatives. Here, we identified amino acid residues that determine the substrate specificity of rat Δ6 desaturase (D6d) acting on linoleoyl-CoA by comparing its amino acid sequence with that of Δ5 desaturase (D5d), which converts dihomo-γ-linolenoyl-CoA. The N-terminal cytochrome b5-like domain was excluded as a determinant by domain swapping analysis. Substitution of eight amino acid residues (Ser209, Asn211, Arg216, Ser235, Leu236, Trp244, Gln245, and Val344) of D6d with the corresponding residues of D5d by site-directed mutagenesis switched the substrate specificity from linoleoyl-CoA to dihomo-γ-linolenoyl-CoA. In addition, replacement of Leu323 of D6d with Phe323 on the basis of the amino acid sequence of zebra fish Δ5/6 bifunctional desaturase was found to render D6d bifunctional. Homology modeling of D6d using recent crystal structure data of human stearoyl-CoA (Δ9) desaturase revealed that Arg216, Trp244, Gln245, and Leu323 are located near the substrate-binding pocket. To our knowledge, this is the first report on the structural basis of the substrate specificity of a mammalian front-end fatty acid desaturase, which will aid in efficient production of value-added fatty acids.

A common tRNA modification at an unusual location: the discovery of wyosine biosynthesis in mitochondria

Establishment of the early genetic code likely required strategies to ensure translational accuracy and inevitably involved tRNA post-transcriptional modifications. One such modification, wybutosine/wyosine is crucial for translational fidelity in Archaea and Eukarya; yet it does not occur in Bacteria and has never been described in mitochondria. Here, we present genetic, molecular and mass spectromery data demonstrating the first example of wyosine in mitochondria, a situation thus far unique to kinetoplastids. We also show that these modifications are important for mitochondrial function, underscoring their biological significance. This work focuses on TyW1, the enzyme required for the most critical step of wyosine biosynthesis. Based on molecular phylogeny, we suggest that the kinetoplastids pathways evolved via gene duplication and acquisition of an FMN-binding domain now prevalent in TyW1 of most eukaryotes. These findings are discussed in the context of the extensive U-insertion RNA editing in trypanosome mitochondria, which may have provided selective pressure for maintenance of mitochondrial wyosine in this lineage.

Comparative transcriptional analysis reveals differential gene expression between Sand Daffodil tissues

Sand Daffodil (Pancratium maritimum) is a world-wide endangered Amayllidaceae species and represents an important anti-cancer medicinal resource due to alkaloids production. Despite its increasing pharmaceutical importance, there are not molecular resources that can be utilized toward improving genetic traits. In our research, the suppression subtractive hybridization (SSH) method conducted to generate large-scale expressed sequence tags (EST), was designed to identify gene candidates related to the morphological and physiological differences between the two tissues, leaves and bulbs, since lycorine, the main anti-cancer compound, is there synthesized. We focused on identification of transcripts in different tissues from Sand Daffodil using PCR-based suppression SSH to identify genes involved in global pathway control. Sequencing of 2,000 differentially screened clones from the SSH libraries resulted in 136 unigenes. Functional annotation and gene ontology analysis of up-regulated EST libraries showed several known biosynthetic genes and novel transcripts that may be involved in signaling, cellular transport, or metabolism. Real time RT-PCR analysis of a set of 8 candidate genes further confirmed the differential gene expression.

Functional characterization of eight human CYP1A2 variants: the role of cytochrome b5

BACKGROUND

Interindividual variability in cytochrome P450 (CYP)-mediated xenobiotic metabolism is extensive. CYP metabolism requires two electrons, which can be donated by NADPH cytochrome P450 oxidoreductase (CYPOR) and/or cytochrome b5 (b5). Although substantial number of studies have reported on the function and effect of b5 in CYP-mediated catalysis, its mode of action is still not fully understood.

OBJECTIVE

The aim of this work was to examine the effect of b5 on the activities of eight natural-occurring variants of human CYP1A2, namely, T83M, S212C, S298R, G299S, I314V, I386F, C406Y, and R456H.

MATERIALS AND METHODS

An approach, as used in our former study was applied, coexpressing these polymorphic CYP1A2 variants separately with CYPOR and b5 in the bacterial cell model BTC-CYP. For each variant, 16 different activity parameters were measured, using eight different substrates. This heterogeneous data set was merged with the one of our former study (i.e. without b5) and a multivariate analysis was carried out.

RESULTS

This analysis indicated that b5 seems to have the ability to affect CYP1A2 variants to behave more like the wild-type variant. This was especially the case for variant I386F, for which the presence of b5 was crucial to show activity. Variants T83M and C406Y showed considerably different activity-profiles when in the presence of b5. Furthermore, our data seem to implicate CYP1A2 residue G299 in its interaction with CYPOR and/or b5.

CONCLUSION

Results indicate the ability of b5 to affect CYP1A2 variants to behave more like the wild-type variant, attenuating detrimental effects of structural mutations of these variants, seemingly through extensive allosteric effects.

Cytochrome b₅ coexpression increases Tetrahymena thermophila Δ6 fatty acid desaturase activity in Saccharomyces cerevisiae

Very-long-chain polyunsaturated fatty acids such as arachidonic, eicosapentaenoic, and docosahexaenoic acids, are important to the physiology of many microorganisms and metazoans and are vital to human development and health. The production of these and related fatty acids depends on Δ6 desaturases, the final components of an electron transfer chain that introduces double bonds into 18-carbon fatty acid chains. When a Δ6 desaturase identified from the ciliated protist Tetrahymena thermophila was expressed in Saccharomyces cerevisiae cultures supplemented with the 18:2(Δ9,12) substrate, only 4% of the incorporated substrate was desaturated. Cytochrome b₅ protein sequences identified from the genome of T. thermophila included one sequence with two conserved cytochrome b₅ domains. Desaturation by the Δ6 enzyme increased as much as 10-fold when T. thermophila cytochrome b₅s were coexpressed with the desaturase. Coexpression of a cytochrome b₅ from Arabidopsis thaliana with the Δ6 enzyme also increased desaturation. A split ubiquitin growth assay indicated that the strength of interaction between cytochrome b₅ proteins and the desaturase plays a vital role in fatty acid desaturase activity, illustrating the importance of protein-protein interactions in this enzyme activity.

The cytochrome b5 dependent C-5(6) sterol desaturase DES5A from the endoplasmic reticulum of Tetrahymena thermophila complements ergosterol biosynthesis mutants in Saccharomyces cerevisiae

Tetrahymena thermophila is a free-living ciliate with no exogenous sterol requirement. However, it can perform several modifications on externally added sterols including desaturation at C5(6), C7(8), and C22(23). Sterol desaturases in Tetrahymena are microsomal enzymes that require Cyt b(5), Cyt b(5) reductase, oxygen, and reduced NAD(P)H for their activity, and some of the genes encoding these functions have recently been identified. The DES5A gene encodes a C-5(6) sterol desaturase, as shown by gene knockout in Tetrahymena. To confirm and extend that result, and to develop new approaches to gene characterization in Tetrahymena, we have now, expressed DES5A in Saccharomyces cerevisiae. The DES5A gene was codon optimized and expressed in a yeast mutant, erg3Δ, which is disrupted for the gene encoding the S. cerevisiae C-5(6) sterol desaturase ERG3. The complemented strain was able to accumulate 74% of the wild type level of ergosterol, and also lost the hypersensitivity to cycloheximide associated with the lack of ERG3 function. C-5(6) sterol desaturases are expected to function at the endoplasmic reticulum. Consistent with this, a GFP-tagged copy of Des5Ap was localized to the endoplasmic reticulum in both Tetrahymena and yeast. This work shows for the first time that both function and localization are conserved for a microsomal enzyme between ciliates and fungi, notwithstanding the enormous evolutionary distance between these lineages. The results suggest that heterologous expression of ciliate genes in S. cerevisiae provides a useful tool for the characterization of genes in Tetrahymena, including genes encoding membrane protein complexes.

Isolation of a Δ5 desaturase gene from Euglena gracilis and functional dissection of its HPGG and HDASH motifs

Delta (Δ) 5 desaturase is a key enzyme for the biosynthesis of health-beneficial long chain polyunsaturated fatty acids such as arachidonic acid (ARA, C20:4n-6), eicosapentaenoic acid (C20:5n-3) and docosahexaenoic acid (C22:6n-3) via the “desaturation and elongation” pathways. A full length Δ5 desaturase gene from Euglena gracilis (EgΔ5D) was isolated by cloning the products of polymerase chain reaction with degenerate oligonucleotides as primers, followed by 5′ and 3′ rapid amplification of cDNA ends. The whole coding region of EgΔ5D was 1,350 nucleotides in length and encoded a polypeptide of 449 amino acids. BlastP search showed that EgΔ5D has about 39 % identity with a Δ5 desaturase of Phaeodactylum tricornutum. In a genetically modified dihomo-gamma-linoleic acid (DGLA, C20:3n-6) producing Yarrowia lipolytica strain, EgΔ5D had strong Δ5 desaturase activity with DGLA to ARA conversion of more than 24 %. Functional dissection of its HPGG and HDASH motifs demonstrated that both motifs were important, but not necessary in the exact form as encoded for the enzyme activity of EgΔ5D. A double mutant EgΔ5D-34G158G with altered sequences within both HPGG and HDASH motifs was generated and exhibited Δ5 desaturase activity similar to the wild type EgΔ5D. Codon optimization of the N-terminal region of EgΔ5D-34G158G and substitution of the arginine with serine at residue 347 improved substrate conversion to 27.6 %.

Mechanistic and structural insights into the regioselectivity of an acyl-CoA fatty acid desaturase via directed molecular evolution

Membrane-bound fatty acid desaturases and related enzymes play a pivotal role in the biosynthesis of unsaturated and various unusual fatty acids. Structural insights into the remarkable catalytic diversity and wide range of substrate specificities of this class of enzymes remain limited due to the lack of a crystal structure. To investigate the structural basis of the double bond positioning (regioselectivity) of the desaturation reaction in more detail, we relied on a combination of directed evolution in vitro and a powerful yeast complementation assay to screen for Δx regioselectivity. After two selection rounds, variants of the bifunctional Δ12/Δ9-desaturase from the house cricket (Acheta domesticus) exhibited increased Δ9-desaturation activity on shorter chain fatty acids. This change in specificity was the result of as few as three mutations, some of them near the putative active site. Subsequent analysis of individual substitutions revealed an important role of residue Phe-52 in facilitating Δ9-desaturation of shorter chain acyl substrates and allowed for the redesign of the cricket Δ12/Δ9-desaturase into a 16:0-specific Δ9-desaturase. Our results demonstrate that a minimal number of mutations can have a profound impact on the regioselectivity of acyl-CoA fatty acid desaturases and include the first biochemical data supporting the acyl-CoA acyl carrier specificity of a desaturase able to carry out Δ12-desaturation.