Delta-9 fatty acid desaturase gene containing a carboxyl-terminal cytochrome b5 domain from the red alga Cyanidioschyzon merolae

Membrane fatty acid desaturase: biosynthesis, mechanism, and architecture

Fatty acid desaturase catalyzes the desaturation reactions by inserting double bonds into the fatty acyl chain, producing unsaturated fatty acids, which play a vital part in the synthesis of polyunsaturated fatty acids. Though soluble fatty acid desaturases have been described extensively in advanced organisms, there are very limited studies of membrane fatty acid desaturases due to their difficulties in producing a sufficient amount of recombinant desaturases. However, the advancement of technology has shown substantial progress towards the development of elucidating crystal structures of membrane fatty acid desaturase, thus, allowing modification of structure to be manipulated. Understanding the structure, mechanism, and biosynthesis of fatty acid desaturase lay a foundation for the potential production of various strategies associated with alteration and modifications of polyunsaturated fatty acids. This manuscript presents the current state of knowledge and understanding about the structure, mechanisms, and biosynthesis of fatty acid desaturase. In addition, the role of unsaturated fatty acid desaturases in health and diseases is also encompassed. This will be useful in understanding the molecular basis and structural protein of fatty acid desaturase that are significant for the advancement of therapeutic strategies associated with the improvement of health status. KEY POINTS: • Current state of knowledge and understanding about the biosynthesis, mechanisms, and structure of fatty acid desaturase. • The role of unsaturated fatty acid desaturase. • The molecular basis and structural protein elucidated the crystal structure of fatty acid desaturase.

Microalgae as a Source for VLC-PUFA Production

Microalgae present a huge and still insufficiently tapped resource of very long-chain omega-3 and omega-6 polyunsaturated fatty acids (VLC-PUFA) for human nutrition and medicinal applications. This chapter describes the diversity of unicellular eukaryotic microalgae in respect to VLC-PUFA biosynthesis. Then, we outline the major biosynthetic pathways mediating the formation of VLC-PUFA by sequential desaturation and elongation of C18-PUFA acyl groups. We address the aspects of spatial localization of those pathways and elaborate on the role for VLC-PUFA in microalgal cells. Recent progress in microalgal genetic transformation and molecular engineering has opened the way to increased production efficiencies for VLC-PUFA. The perspectives of photobiotechnology and metabolic engineering of microalgae for altered or enhanced VLC-PUFA production are also discussed.

Construction of Global Acyl Lipid Metabolic Map by Comparative Genomics and Subcellular Localization Analysis in the Red Alga Cyanidioschyzon merolae

Pathways of lipid metabolism have been established in land plants, such as Arabidopsis thaliana, but the information on exact pathways is still under study in microalgae. In contrast with Chlamydomonas reinhardtii, which is currently studied extensively, the pathway information in red algae is still in the state in which enzymes and pathways are estimated by analogy with the knowledge in plants. Here we attempt to construct the entire acyl lipid metabolic pathways in a model red alga, Cyanidioschyzon merolae, as an initial basis for future genetic and biochemical studies, by exploiting comparative genomics and localization analysis. First, the data of whole genome clustering by Gclust were used to identify 121 acyl lipid-related enzymes. Then, the localization of 113 of these enzymes was analyzed by GFP-based techniques. We found that most of the predictions on the subcellular localization by existing tools gave erroneous results, probably because these tools had been tuned for plants or green algae. The experimental data in the present study as well as the data reported before in our laboratory will constitute a good training set for tuning these tools. The lipid metabolic map thus constructed show that the lipid metabolic pathways in the red alga are essentially similar to those in A. thaliana, except that the number of enzymes catalyzing individual reactions is quite limited. The absence of fatty acid desaturation to produce oleic and linoleic acids within the plastid, however, highlights the central importance of desaturation and acyl editing in the endoplasmic reticulum, for the synthesis of plastid lipids as well as other cellular lipids. Additionally, some notable characteristics of lipid metabolism in C. merolae were found. For example, phosphatidylcholine is synthesized by the methylation of phosphatidylethanolamine as in yeasts. It is possible that a single 3-ketoacyl-acyl carrier protein synthase is involved in the condensation reactions of fatty acid synthesis in the plastid. We will also discuss on the redundant β-oxidation enzymes, which are characteristic to red algae.

Identification and functional analysis of delta-9 desaturase, a key enzyme in PUFA Synthesis, isolated from the oleaginous diatom Fistulifera

Oleaginous microalgae are one of the promising resource of nonedible biodiesel fuel (BDF) feed stock alternatives. Now a challenge task is the decrease of the long-chain polyunsaturated fatty acids (PUFAs) content affecting on the BDF oxidative stability by using gene manipulation techniques. However, only the limited knowledge has been available concerning the fatty acid and PUFA synthesis pathways in microalgae. Especially, the function of Δ9 desaturase, which is a key enzyme in PUFA synthesis pathway, has not been determined in diatom. In this study, 4 Δ(9) desaturase genes (fD9desA, fD9desB, fD9desC and fD9desD) from the oleaginous diatom Fistulifera were newly isolated and functionally characterized. The putative Δ(9) acyl-CoA desaturases in the endoplasmic reticulum (ER) showed 3 histidine clusters that are well-conserved motifs in the typical Δ(9) desaturase. Furthermore, the function of these Δ(9) desaturases was confirmed in the Saccharomyces cerevisiae ole1 gene deletion mutant (Δole1). All the putative Δ(9) acyl-CoA desaturases showed Δ(9) desaturation activity for C16∶0 fatty acids; fD9desA and fD9desB also showed desaturation activity for C18∶0 fatty acids. This study represents the first functional analysis of Δ(9) desaturases from oleaginous microalgae and from diatoms as the first enzyme to introduce a double bond in saturated fatty acids during PUFA synthesis. The findings will provide beneficial insights into applying metabolic engineering processes to suppressing PUFA synthesis in this oleaginous microalgal strain.

Porphyra (Bangiophyceae) Transcriptomes Provide Insights Into Red Algal Development And Metabolism

The red seaweed Porphyra (Bangiophyceae) and related Bangiales have global economic importance. Here, we report the analysis of a comprehensive transcriptome comprising ca. 4.7 million expressed sequence tag (EST) reads from P. umbilicalis (L.) J. Agardh and P. purpurea (Roth) C. Agardh (ca. 980 Mbp of data generated using 454 FLX pyrosequencing). These ESTs were isolated from the haploid gametophyte (blades from both species) and diploid conchocelis stage (from P. purpurea). In a bioinformatic analysis, only 20% of the contigs were found to encode proteins of known biological function. Comparative analysis of predicted protein functions in mesophilic (including Porphyra) and extremophilic red algae suggest that the former has more putative functions related to signaling, membrane transport processes, and establishment of protein complexes. These enhanced functions may reflect general mesophilic adaptations. A near-complete repertoire of genes encoding histones and ribosomal proteins was identified, with some differentially regulated between the blade and conchocelis stage in P. purpurea. This finding may reflect specific regulatory processes associated with these distinct phases of the life history. Fatty acid desaturation patterns, in combination with gene expression profiles, demonstrate differences from seed plants with respect to the transport of fatty acid/lipid among subcellular compartments and the molecular machinery of lipid assembly. We also recovered a near-complete gene repertoire for enzymes involved in the formation of sterols and carotenoids, including candidate genes for the biosynthesis of lutein. Our findings provide key insights into the evolution, development, and biology of Porphyra, an important lineage of red algae.

Genomic and biochemical analysis of lipid biosynthesis in the unicellular rhodophyte Cyanidioschyzon merolae: lack of a plastidic desaturation pathway results in the coupled pathway of galactolipid synthesis

The acyl lipids making up the plastid membranes in plants and algae are highly enriched in polyunsaturated fatty acids and are synthesized by two distinct pathways, known as the prokaryotic and eukaryotic pathways, which are located within the plastids and the endoplasmic reticulum, respectively. Here we report the results of biochemical as well as genomic analyses of lipids and fatty acids in the unicellular rhodophyte Cyanidioschyzon merolae. All of the glycerolipids usually found in photosynthetic algae were found, such as mono- and digalactosyl diacylglycerol, sulfolipid, phosphatidylglycerol, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol. However, the fatty acid composition was extremely simple. Only palmitic, stearic, oleic, and linoleic acids were found as major acids. In addition, 3-trans-hexadecanoic acid was found as a very minor component in phosphatidylglycerol. Unlike the case for most other photosynthetic eukaryotes, polyenoic fatty acids having three or more double bonds were not detected. These results suggest that polyunsaturated fatty acids are not necessary for photosynthesis in eukaryotes. Genomic analysis suggested that C. merolae lacks acyl lipid desaturases of cyanobacterial origin as well as stearoyl acyl carrier protein desaturase, both of which are major desaturases in plants and green algae. The results of labeling experiments with radioactive acetate showed that the desaturation leading to linoleic acid synthesis occurs on phosphatidylcholine located outside the plastids. Monogalactosyl diacylglycerol is therefore synthesized by the coupled pathway, using plastid-derived palmitic acid and endoplasmic reticulum-derived linoleic acid. These results highlight essential differences in lipid biosynthetic pathways between the red algae and the green lineage, which includes plants and green algae.

Recent advances in the study of fatty acid desaturases from animals and lower eukaryotes

The biosynthesis of polyunsaturated fatty acids (PUFAs) in different organisms can involve a variety of pathways, catalyzed by a complex series of desaturation and elongation steps. A range of different desaturases have been identified to date, capable of introducing double bonds at various locations on the fatty acyl chain. Some recently identified novel desaturases include a delta4 desaturase from marine fungi, and a bi-functional delta5/delta6 desaturase from zebrafish. Using molecular genetics approaches, these desaturase genes have been isolated, identified, and expressed in variety of heterologous hosts. Results from these studies will help increase our understanding of the biochemistry of desaturases and the regulation of PUFA biosynthesis. This is of significance because PUFAs play critical roles in multiple aspects of membrane physiology and signaling mechanisms which impact human health and development.

A desaturase-like protein from white spruce is a Delta(9) desaturase

Gymnospermae seed lipids are characterized by a high degree of desaturation, most having a Delta(9) double bond. By degenerate polymerase chain reaction (PCR) we have isolated a white spruce (Picea glauca) cDNA clone that encodes an amino acid sequence sharing a high degree of homology with other putative plant acyl-coenzyme A (CoA) Des9 desaturases. Both in vivo and in vitro expression studies in a Delta(9) desaturase-deficient yeast strain demonstrated the desaturation functionality of the white spruce clone, and gas chromatography-mass spectrometry (GC-MS) analyses confirmed the regioselectivity of the encoded enzyme. This is the first report of the functional characterization of a plant membrane-bound acyl-CoA-like protein Delta(9) desaturase by heterologous expression in yeast.

Expression of borage Δ6 desaturase in Saccharomyces cerevisiae and oilseed crops

The borage Δ6 desaturase is a modular protein that contains a cytochrome b5-like domain attached to the N-terminus of the main catalytic domain. When the full-length open reading frame was expressed in two types of Saccharomyces cerevisiae strains, the endogenous cytochrome b5-disrupted strain and the cytochrome b5-competent wild type strain, the expressed enzyme introduced a double bond at the Δ6 position into the endogenous substrates 16:1(9) and 18:1(9), and also into the exogenously fed fatty acid substrates 18:2(9,12) and 18:3(9,12,15). However, the preferred substrates of the enzyme when expressed in yeast were 18:2(9,12) and 18:3(9,12,15), whereas 16:1(9) and 18:1(9) were relatively poor substrates. When the partial desaturase sequence with a deletion of the cytochrome b5-like domain at the N-terminus was introduced into the yeast strains, the truncated enzyme could not desaturate any of the supplied substrates. Introduction of the full-length cDNA into flax (Linum usitatissimum L.) under the control of a constitutive (35S) promoter resulted in accumulation of the two Δ6 desaturated fatty acids (18:3(6,9,12) and 18:4(6,9,12,15)), in one elite line, forming up to 22% of the total fatty acids in the stem, 19% in the root, and 11% in the leaf. Introduction of the desaturase into Brassica juncea (L.) Czern. under the control of the Brassica napus L. napin promoter resulted in accumulation of the Δ6 desaturated fatty acids at levels of up to 13% of the total fatty acids in mature seeds.Key words: borage, Δ6 desaturase, γ-linolenic acid, stearidonic acid, Brassica juncea, flax.

Molecular cloning and sequence analysis of stearoyl-CoA desaturase in milkfish, Chanos chanos

Stearoyl-CoA desaturase (EC 1.14.99.5) is a key enzyme in the biosynthesis of polyunsaturated fatty acids and the maintenance of the homeoviscous fluidity of biological membranes. The stearoyl-CoA desaturase cDNA in milkfish (Chanos chanos) was cloned by RT-PCR and RACE, and it was compared with the stearoyl-CoA desaturase in cold-tolerant teleosts, common carp and grass carp. Nucleotide sequence analysis revealed that the cDNA clone has a 972-bp open reading frame encoding 323 amino acid residues. Alignments of the deduced amino acid sequence showed that the milkfish stearoyl-CoA desaturase shares 79% and 75% identity with common carp and grass carp, and 63%-64% with other vertebrates such as sheep, hamsters, rats, mice, and humans. Like common carp and grass carp, the deduced amino acid sequence in milkfish well conserves three histidine cluster motifs (one HXXXXH and two HXXHH) that are essential for catalysis of stearoyl-CoA desaturase activity. However, RT-PCR analysis showed that stearoyl-CoA desaturase expression in milkfish is detected in the tissues of liver, muscle, kidney, brain, and gill, and more expression sites were found in milkfish than in common carp and grass carp. Phylogenic relationships among the deduced stearoyl-CoA desaturase amino acid sequence in milkfish and those in other vertebrates showed that the milkfish stearoyl-CoA desaturase amino acid sequence is phylogenetically closer to those of common carp and grass carp than to other higher vertebrates.

Mechanism of fatty acid desaturation in the green alga Chlorella vulgaris

The hypothesis that the Delta9 desaturase of Chlorella vulgaris might operate by a synchronous mechanism has been tested using a kinetic isotope effect (KIE) approach. Thus the intermolecular primary deuterium KIE on the individual C-H bond cleavage steps involved in Delta9 desaturation have been determined by incubating growing cultures of C. vulgaris (strain 211/8K) with mixtures of the appropriate regiospecifically deuterated fatty acid analogues. Our analysis shows that the introduction of a double bond between C-9 and C-10 occurs in two discrete steps as the cleavage of the C9-H bond is very sensitive to isotopic substitution (kH/kD = 6.6 +/- 0.3) whereas a negligible isotope effect (kH/kD = 1.05 +/- 0.05) was observed for the C10-H bond-breaking step. Similar results were obtained for linoleic acid biosynthesis (Delta12 desaturation). These data clearly rule out a synchronous mechanism for these reactions.

Molecular cloning of full-length cDNA encoding delta-9 desaturase through PCR strategies and its genomic organization and expression in grass carp (Ctenopharyngodon idella)

Desaturases are enzymes that catalyze double bond formation in fatty acids, which is a critical step in the synthesis of unsaturated fatty acids in organisms. Desaturase cDNA has been cloned from various species. Here we report the cloning of a full-length cDNA of Delta(9)-desaturase from grass carp (Ctenopharyngodon idella), using a combination of PCR techniques: reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). The resolved cDNA encompasses 2420 bp, containing an open reading frame corresponding to 324 amino acids. The deduced amino acid sequence shares high homology with those of mammalian desaturases. Northern blot and RT-PCR analyses demonstrated a high abundance of the transcript in liver tissue but low abundance in brain tissue. Furthermore, the structure of the gene has been resolved by screening its cognate genomic DNA library. The analysis shows that this gene is composed of six exons and five introns, encompassing a region of 8.5 kb. In particular, the last exon contains a length of the 3' untranslated region as long as 1382 bp. Although the primary sequence and the genomic organization are phylogenetically conserved between fish and mammals, the regulation of the gene expression appears to be divergent among species.

Histidine-41 of the cytochrome b5 domain of the borage delta6 fatty acid desaturase is essential for enzyme activity

Unlike most other plant microsomal desaturases, the Delta6-fatty acid desaturase from borage (Borago officinalis) contains an N-terminal extension that shows homology to the small hemoprotein cytochrome (Cyt) b5. To determine if this domain serves as a functional electron donor for the Delta6-fatty acid desaturase, mutagenesis and functional analysis by expression in transgenic Arabidopsis was carried out. Although expression of the wild-type borage Delta6-fatty acid desaturase resulted in the synthesis and accumulation of Delta6-unsaturated fatty acids, this was not observed in plants transformed with N-terminally deleted forms of the desaturase. Site-directed mutagenesis was used to disrupt one of the axial heme-binding residues (histidine-41) of the Cyt b5 domain; expression of this mutant form of the Delta6-desaturase in transgenic plants failed to produce Delta6-unsaturated fatty acids. These data indicate that the Cyt b5 domain of the borage Delta6-fatty acid desaturase is essential for enzymatic activity.

Mucor rouxii delta9-desaturase gene is transcriptionally regulated during cell growth and by low temperature

Unsaturated fatty acids are essential lipid components of Mucor rouxii. Gamma-linolenic acid (GLA) is synthesized via the desaturase enzymes: delta9-desaturase catalyzes mono-unsaturated fatty acids that are utilized as substrate for GLA biosynthesis. We cloned and characterized a M. rouxii gene highly homologous to delta9-desaturase genes. This sequence encodes for a protein of 452 amino acids and contains two introns of 60 and 61 nucleotides. Delta9-desaturase of M. rouxii is expressed during cell growth when cells are subjected to temperature shifts. At 30 degrees C, the mRNA level of late log phase is about 6.4-fold higher than that of early log phase. A shift from 30 to 15 degrees C induced transcription of delta9-desaturase gene in both early and late log phases. However, the pattern of increased transcription by cold induction varied depending on growth conditions: transcription of late log phase is higher than that of early log phase. These results indicate that cell growth and low temperature influence the expression of delta9-desaturase gene and fatty acid composition of M. rouxii.

A sphingolipid desaturase from higher plants. Identification of a new cytochrome b5 fusion protein

A recently cloned cDNA from sunflower codes for a fusion protein composed of an N-terminal cytochrome b5 and a domain similar to membrane-bound acyl lipid desaturases. For a functional identification, homologous cDNAs from Brassica napus and Arabidopsis thaliana were expressed in Saccharomyces cerevisiae, and sphingolipid long chain bases were analyzed. The expression of the heterologous enzyme results in significant proportions of new Delta8, 9-cis/trans-phytosphingenines that accompany the residual C18-phytosphinganine predominating in wild-type yeast cells. These results represent the first identification of a gene coding for a sphingolipid desaturase and for a stereounselective desaturase showing trans-activity from any organism. Furthermore, this fusion protein is a new member of the cytochrome b5 superfamily. The formation of the two regioisomeric phytosphingenines in the transformed yeast sheds new light on the factors controlling regioselectivity.