Characterization of a structurally and functionally diverged acyl-acyl carrier protein desaturase from milkweed seed

A toolkit for plant lipid engineering: Surveying the efficacies of lipogenic factors for accumulating specialty lipids

Plants produce energy-dense lipids from carbohydrates using energy acquired via photosynthesis, making plant oils an economically and sustainably attractive feedstock for conversion to biofuels and value-added bioproducts. A growing number of strategies have been developed and optimized in model plants, oilseed crops and high-biomass crops to enhance the accumulation of storage lipids (mostly triacylglycerols, TAGs) for bioenergy applications and to produce specialty lipids with increased uses and value for chemical feedstock and nutritional applications. Most successful metabolic engineering strategies involve heterologous expression of lipogenic factors that outperform those from other sources or exhibit specialized functionality. In this review, we summarize recent progress in engineering the accumulation of triacylglycerols containing - specialized fatty acids in various plant species and tissues. We also provide an inventory of specific lipogenic factors (including accession numbers) derived from a wide variety of organisms, along with their reported efficacy in supporting the accumulation of desired lipids. A review of previously obtained results serves as a foundation to guide future efforts to optimize combinations of factors to achieve further enhancements to the production and accumulation of desired lipids in a variety of plant tissues and species.

Modification of Fatty Acid Composition of Escherichia coli by Co-Expression of Fatty Acid Desaturase and Thioesterase from Arabidopsis thaliana

Fatty acid composition has an important influence on the fluidity of biological membranes, which is a key factor for the survival of Escherichia coli. With the aim to modify fatty acid composition in this experimentally friendly microorganism, the AtFab2 gene, encoding the Arabidopsis thaliana fatty acid desaturase, was expressed separately and jointly with AtFatA, a fatty acid thioesterase of the same plant origin. The expression of ATFab2 desaturase resulted in an enhancement of cis-vaccenic acid (18:1Δ11) contents, while amounts of palmitioleic acid (16:1Δ9) accumulated by E. coli were increased by 130% for the expression of the AtFatA thioesterase. In the final engineered strain co-expressing AtFab2 and AtFatA, the percentage of palmitic acid (16:0), the most abundant saturated fatty acid found in E. coli, was reduced to 29.9% and the ratio of unsaturated fatty acid to saturated fatty acid reached 2:1. Free fatty acids accounted for about 40% of total fatty acid profiles in the recombinant strain expressing both two genes, and the unsaturated fatty acid contents reached nearly 75% in the free fatty acid profiles. The increase of unsaturated fatty acid level might provide some implication for the construction of cold tolerant strains.

Acyl-Acyl Carrier Protein Desaturases and Plant Biotic Interactions

Interactions between land plants and other organisms such as pathogens, pollinators, or symbionts usually involve a variety of specialized effectors participating in complex cross-talks between organisms. Fatty acids and their lipid derivatives play important roles in these biological interactions. While the transcriptional regulation of genes encoding acyl–acyl carrier protein (ACP) desaturases appears to be largely responsive to biotic stress, the different monounsaturated fatty acids produced by these enzymes were shown to take active part in plant biotic interactions and were assigned with specific functions intrinsically linked to the position of the carbon–carbon double bond within their acyl chain. For example, oleic acid, an omega-9 monounsaturated fatty acid produced by Δ⁹-stearoyl–ACP desaturases, participates in signal transduction pathways affecting plant immunity against pathogen infection. Myristoleic acid, an omega-5 monounsaturated fatty acid produced by Δ⁹-myristoyl–ACP desaturases, serves as a precursor for the biosynthesis of omega-5 anacardic acids that are active biocides against pests. Finally, different types of monounsaturated fatty acids synthesized in the labellum of orchids are used for the production of a variety of alkenes participating in the chemistry of sexual deception, hence favoring plant pollination by hymenopterans.

Genetic and environmental variation impact the cuticular hydrocarbon metabolome on the stigmatic surfaces of maize

BACKGROUND

Simple non-isoprenoid hydrocarbons accumulate in discrete regions of the biosphere, including within bacteria and algae as a carbon and/or energy store, and the cuticles of plants and insects, where they may protect against environmental stresses. The extracellular cuticular surfaces of the stigmatic silks of maize are rich in linear hydrocarbons and therefore provide a convenient system to study the biological origins and functions of these unique metabolites.

RESULTS

To test the hypotheses that genetics and environment influence the accumulation of surface hydrocarbons on silks and to examine the breadth of metabolome compositions across diverse germplasm, cuticular hydrocarbons were analyzed on husk-encased silks and silks that emerged from the husk leaves from 32 genetically diverse maize inbred lines, most of which are commonly utilized in genetics experiments. Total hydrocarbon accumulation varied ~ 10-fold among inbred lines, and up to 5-fold between emerged and husk-encased silks. Alkenes accounted for 5-60% of the total hydrocarbon metabolome, and the majority of alkenes were monoenes with a double bond at either the 7th or 9th carbon atom of the alkyl chain. Total hydrocarbon accumulation was impacted to similar degrees by genotype and husk encasement status, whereas genotype predominantly impacted alkene composition. Only minor differences in the metabolome were observed on silks that were emerged into the external environment for 3- versus 6-days. The environmental influence on the metabolome was further investigated by growing inbred lines in 2 years, one of which was warmer and wetter. Inbred lines grown in the drier year accumulated up to 2-fold more hydrocarbons and up to a 22% higher relative abundance of alkenes. In summary, the surface hydrocarbon metabolome of silks is primarily governed by genotype and husk encasement status, with smaller impacts of environment and genotype-by-environment interactions.

CONCLUSIONS

This study reveals that the composition of the cuticular hydrocarbon metabolome on silks is affected significantly by genetic factors, and is therefore amenable to dissection using quantitative genetic approaches. Such studies will clarify the genetic mechanisms responsible for the accumulation of these metabolites, enabling detailed functional investigations of the diverse and complex protective roles of silk surface lipids against environmental stresses.

Conserved Function of ACYL-ACYL CARRIER PROTEIN DESATURASE 5 on Seed Oil and Oleic Acid Biosynthesis between Arabidopsis thaliana and Brassica napus

Previous studies have shown that several ACYL-ACYL CARRIER PROTEIN DESATURASE (AtAAD) members in Arabidopsis thaliana are responsible for oleic acid (C18:1) biosynthesis. Limited research has been conducted on another member, AtAAD5, and its paralog BnAAD5 in the closely related and commercially important plant, Brassica napus. Here, we found that AtAAD5 was predominantly and exclusively expressed in developing embryos at the whole seed developmental stages. The aad5 mutation caused a significant decrease in the amounts of oil and C18:1, and a considerable increase in the content of stearic acid (C18:0) in mature seeds, suggesting that AtAAD5 functioned as an important facilitator of seed oil biosynthesis. We also cloned the full-length coding sequence of BnAAD5-1 from the A3 subgenome of the B. napus inbred line L111. We showed that ectopic expression of BnAAD5-1 in the A. thaliana aad5-2 mutant fully complemented the phenotypes of the mutant, such as lower oil content and altered contents of C18:0 and C18:1. These results help us to better understand the functions of AAD members in A. thaliana and B. napus and provide a promising target for genetic manipulation of B. napus.

Synthetic redesign of plant lipid metabolism

Plant seed lipid metabolism is an area of intensive research, including many examples of transgenic events in which oil composition has been modified. In the selected examples described in this review, progress towards the predictive manipulation of metabolism and the reconstitution of desired traits in a non-native host is considered. The advantages of a particular oilseed crop, Camelina sativa, as a flexible and utilitarian chassis for advanced metabolic engineering and applied synthetic biology are considered, as are the issues that still represent gaps in our ability to predictably alter plant lipid biosynthesis. Opportunities to deliver useful bio-based products via transgenic plants are described, some of which represent the most complex genetic engineering in plants to date. Future prospects are considered, with a focus on the desire to transition to more (computationally) directed manipulations of metabolism.

Amino Acid Change in an Orchid Desaturase Enables Mimicry of the Pollinator's Sex Pheromone

Mimicry illustrates the power of selection to produce phenotypic convergence in biology [1]. A striking example is the imitation of female insects by plants that are pollinated by sexual deception of males of the same insect species [2-4]. This involves mimicry of visual, tactile, and chemical signals of females [2-7], especially their sex pheromones [8-11]. The Mediterranean orchid Ophrys exaltata employs chemical mimicry of cuticular hydrocarbons, particularly the 7-alkenes, in an insect sex pheromone to attract and elicit mating behavior in its pollinators, males of the cellophane bee Colletes cunicularius [11-13]. A difference in alkene double-bond positions is responsible for reproductive isolation between O. exaltata and closely related species, such as O. sphegodes [13-16]. We show that these 7-alkenes are likely determined by the action of the stearoyl-acyl-carrier-protein desaturase (SAD) homolog SAD5. After gene duplication, changes in subcellular localization relative to the ancestral housekeeping desaturase may have allowed proto-SAD5's reaction products to undergo further biosynthesis to both 7- and 9-alkenes. Such ancestral coproduction of two alkene classes may have led to pollinator-mediated deleterious pleiotropy. Despite possible evolutionary intermediates with reduced activity, amino acid changes at the bottom of the substrate-binding cavity have conferred enzyme specificity for 7-alkene biosynthesis by preventing the binding of longer-chained fatty acid (FA) precursors by the enzyme. This change in desaturase function enabled the orchid to perfect its chemical mimicry of pollinator sex pheromones by escape from deleterious pleiotropy, supporting a role of pleiotropy in determining the possible trajectories of adaptive evolution.

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.

Expression of yeast acyl-CoA-∆9 desaturase leads to accumulation of unusual monounsaturated fatty acids in soybean seeds

An acyl-CoA-Δ9 desaturase from Saccharomyces cerevisiae was expressed by subcellular-targeting in soybean (Glycine max) seeds with the goal of increasing palmitoleic acid (16:1Δ9), a high-valued fatty acid (FA), and simultaneously decreasing saturated FA in oil. The expression resulted in the conversion of palmitic acid (16:0) to 16:1Δ9 in soybean seeds. 16:1Δ9 and its elongation product cis-vaccenic acid (18:1Δ11) were increased to 17 % of the total fatty acids by plastid-targeted expression of the enzyme. Other lipid changes include the decrease of polyunsaturated FA and saturated FA, suggesting that a mechanism exists downstream in oil biosynthesis to compensate the FA alternation. This is the first time a cytosolic acyl-CoA-∆9 desaturase is functionally expressed in plastid and stronger activity was achieved than its cytosolic expression. The present study provides a new strategy for converting 16:0 to 16:1Δ9 by engineering acyl-CoA-Δ9 desaturase in commercialized oilseeds.

Biosynthesis and metabolic engineering of palmitoleate production, an important contributor to human health and sustainable industry

Palmitoleate (cis-Δ9-16:1) shows numerous health benefits such as increased cell membrane fluidity, reduced inflammation, protection of the cardiovascular system, and inhibition of oncogenesis. Plant oils containing this unusual fatty acid can also be sustainable feedstocks for producing industrially important and high-demand 1-octene. Vegetable oils rich in palmitoleate are the ideal candidates for biodiesel production. Several wild plants are known that can synthesize high levels of palmitoleate in seeds. However, low yields and poor agronomic characteristics of these plants limit their commercialization. Metabolic engineering has been developed to create oilseed crops that accumulate high levels of palmitoleate or other unusual fatty acids, and significant advances have been made recently in this field, particularly using the model plant Arabidopsis as the host. The engineered targets for enhancing palmitoleate synthesis include overexpression of Δ9 desaturase from mammals, yeast, fungi, and plants, down-regulating KASII, coexpression of an ACP-Δ9 desaturase in plastids and CoA-Δ9 desaturase in endoplasmic reticulum (ER), and optimizing the metabolic flux into triacylglycerols (TAGs). This review will mainly describe the recent progress towards producing palmitoleate in transgenic plants by metabolic engineering along with our current understanding of palmitoleate biosynthesis and its regulation, as well as highlighting the bottlenecks that require additional investigation by combining lipidomics, transgenics and other "-omics" tools. A brief review of reported health benefits and non-food uses of palmitoleate will also be covered.

Oxygen-independent alkane formation by non-heme iron-dependent cyanobacterial aldehyde decarbonylase: investigation of kinetics and requirement for an external electron donor

Cyanobacterial aldehyde decarbonylase (cAD) is, structurally, a member of the di-iron carboxylate family of oxygenases. We previously reported that cAD from Prochlorococcus marinus catalyzes the unusual hydrolysis of aldehydes to produce alkanes and formate in a reaction that requires an external reducing system but does not require oxygen [Das et al. (2011) Angew. Chem. 50, 7148-7152]. Here we demonstrate that cADs from divergent cyanobacterial classes, including the enzyme from N. puntiformes that was reported to be oxygen dependent, catalyze aldehyde decarbonylation at a much faster rate under anaerobic conditions and that the oxygen in formate derives from water. The very low activity (<1 turnover/h) of cAD appears to result from inhibition by the ferredoxin reducing system used in the assay and the low solubility of the substrate. Replacing ferredoxin with the electron mediator phenazine methosulfate allowed the enzyme to function with various chemical reductants, with NADH giving the highest activity. NADH is not consumed during turnover, in accord with the proposed catalytic role for the reducing system in the reaction. With octadecanal, a burst phase of product formation, k(prod) = 3.4 ± 0.5 min(-1), is observed, indicating that chemistry is not rate-determining under the conditions of the assay. With the more soluble substrate, heptanal, k(cat) = 0.17 ± 0.01 min(-1) and no burst phase is observed, suggesting that a chemical step is limiting in the reaction of this substrate.

Acyl-ACP thioesterases from macadamia (Macadamia tetraphylla) nuts: cloning, characterization and their impact on oil composition

The mechanisms by which macadamia nuts accumulate the unusual palmitoleic and asclepic acyl moieties, which constitute up to 20% of the fatty acids in some varieties, are still unknown. Acyl-acyl carrier protein (ACP) thioesterases (EC 3.1.2.14) are intraplastidial enzymes that terminate the synthesis of fatty acids in plants and that facilitate the export of the acyl moieties to the endoplasmic reticulum where they can be used in the production of glycerolipids. Here, we have investigated the possible role of acyl-ACP thioesterase activity in the composition of macadamia kernel oil. Accordingly, two acyl-ACP thioesterases were cloned from developing macadamia kernels, one of the FatA type and the other of the FatB type. These enzymes were heterologously expressed in Escherichia coli, and the recombinant thioesterases were purified, characterized kinetically and assayed with a variety of substrates, demonstrating the high specificity of macadamia FatA towards 16:1-ACP. Acyl-ACP thioesterase activity was also characterized in crude extracts from two different varieties of macadamia, Cate and Beaumont, which accumulate different amounts of n-7 fatty acids. The impact of acyl-ACP thioesterase activities on the oil composition of these kernels is discussed in the light of these results.

Metabolic engineering of seeds can achieve levels of omega-7 fatty acids comparable with the highest levels found in natural plant sources

Plant oils containing ω-7 fatty acids (FAs; palmitoleic 16:1Δ(9) and cis-vaccenic 18:1Δ(11)) have potential as sustainable feedstocks for producing industrially important octene via metathesis chemistry. Engineering plants to produce seeds that accumulate high levels of any unusual FA has been an elusive goal. We achieved high levels of ω-7 FA accumulation by systematic metabolic engineering of Arabidopsis (Arabidopsis thaliana). A plastidial 16:0-ACP desaturase has been engineered to convert 16:0 to 16:1Δ(9) with specificity >100-fold than that of naturally occurring paralogs, such as that from cat's claw vine (Doxantha unguis-cati). Expressing this engineered enzyme (Com25) in seeds increased ω-7 FA accumulation from <2% to 14%. Reducing competition for 16:0-ACP by down-regulating the β-ketoacyl-ACP synthase II 16:0 elongase further increased accumulation of ω-7 FA to 56%. The level of 16:0 exiting the plastid without desaturation also increased to 21%. Coexpression of a pair of fungal 16:0 desaturases in the cytosol reduced the 16:0 level to 11% and increased ω-7 FA to as much as 71%, equivalent to levels found in Doxantha seeds.

An oleic acid-mediated pathway induces constitutive defense signaling and enhanced resistance to multiple pathogens in soybean

Stearoyl-acyl carrier protein-desaturase (SACPD)-catalyzed synthesis of oleic acid (18:1) is an essential step in fatty acid biosynthesis. Arabidopsis mutants (ssi2) with reduced SACPD activity accumulate salicylic acid (SA) and exhibit enhanced resistance to multiple pathogens. We show that reduced levels of 18:1 induce similar defense-related phenotypes in soybean. A Bean pod mottle virus (BPMV)-based vector was employed to effectively silence soybean SACPDs. The silenced plants contained reduced 18:1 and increased stearic acid, developed spontaneous cell death lesions, increased SA accumulation, and constitutively expressed pathogenesis-related genes. These plants also expressed elevated levels of resistance-like genes and showed resistance to bacterial and oomycete pathogens. Exogenous application of glycerol induced similar phenotypes, mimicking the effect of silencing SACPDs in healthy soybean plants. Overexpression of a soybean SACPD increased 18:1 levels in ssi2 but not in wild-type Arabidopsis plants, suggesting that the soybean enzyme was under feedback regulation similar to that of the Arabidopsis isozymes. These results suggest that soybean and Arabidopsis respond similarly to 18:1-derived cues by inducing a novel broad-spectrum resistance-conferring pathway, even though they differ significantly in their lipid biosynthetic pathways. We also demonstrate the efficacy of BPMV-induced gene silencing as a tool for functional studies in soybean.

The crystal structure of the ivy Delta4-16:0-ACP desaturase reveals structural details of the oxidized active site and potential determinants of regioselectivity

The multifunctional acyl-acyl carrier protein (ACP) desaturase from Hedera helix (English ivy) catalyzes the Delta(4) desaturation of 16:0-ACP and the Delta(9) desaturation of 18:0-ACP and further desaturates Delta(9)-16:1 or Delta(9)-18:1 to the corresponding Delta(4,9) dienes. The crystal structure of the enzyme has been solved to 1.95 A resolution, and both the iron-iron distance of approximately 3.2A and the presence of a mu-oxo bridge reveal this to be the only reported structure of a desaturase in the oxidized FeIII-FeIII form. Significant differences are seen between the oxidized active site and the reduced active site of the Ricinus communis (castor) desaturase; His(227) coordination to Fe2 is lost, and the side chain of Glu(224), which bridges the two iron ions in the reduced structure, does not interact with either iron. Although carboxylate shifts have been observed on oxidation of other diiron proteins, this is the first example of the residue moving beyond the coordination range of both iron ions. Comparison of the ivy and castor structures reveal surface amino acids close to the annulus of the substrate-binding cavity and others lining the lower portion of the cavity that are potential determinants of their distinct substrate specificities. We propose a hypothesis that differences in side chain packing explains the apparent paradox that several residues lining the lower portion of the cavity in the ivy desaturase are bulkier than their equivalents in the castor enzyme despite the necessity for the ivy enzyme to accommodate three more carbons beyond the diiron site.

A single mutation in the castor Delta9-18:0-desaturase changes reaction partitioning from desaturation to oxidase chemistry

Sequence analysis of the diiron cluster-containing soluble desaturases suggests they are unrelated to other diiron enzymes; however, structural alignment of the core four-helix bundle of desaturases to other diiron enzymes reveals a conserved iron binding motif with similar spacing in all enzymes of this structural class, implying a common evolutionary ancestry. Detailed structural comparison of the castor desaturase with that of a peroxidase, rubrerythrin, shows remarkable conservation of both identity and geometry of residues surrounding the diiron center, with the exception of residue 199. Position 199 is occupied by a threonine in the castor desaturase, but the equivalent position in rubrerythrin contains a glutamic acid. We previously hypothesized that a carboxylate in this location facilitates oxidase chemistry in rubrerythrin by the close apposition of a residue capable of facilitating proton transfer to the activated oxygen (in a hydrophobic cavity adjacent to the diiron center based on the crystal structure of the oxygen-binding mimic azide). Here we report that desaturase mutant T199D binds substrate but its desaturase activity decreases by approximately 2 x 10(3)-fold. However, it shows a >31-fold increase in peroxide-dependent oxidase activity with respect to WT desaturase, as monitored by single-turnover stopped-flow spectrometry. A 2.65-A crystal structure of T199D reveals active-site geometry remarkably similar to that of rubrerythrin, consistent with its enhanced function as an oxidase enzyme. That a single amino acid substitution can switch reactivity from desaturation to oxidation provides experimental support for the hypothesis that the desaturase evolved from an ancestral oxidase enzyme.

A multifunctional acyl-acyl carrier protein desaturase from Hedera helix L. (English ivy) can synthesize 16- and 18-carbon monoene and diene products

A desaturase with 83% sequence identity to the coriander delta(4)-16:0-ACP desaturase was isolated from developing seeds of Hedera helix (English ivy). Expression of the ivy desaturase in Arabidopsis resulted in the accumulation of 16:1delta(4) and its expected elongation product 18:1delta(6) (petroselinic acid). Expression in Escherichia coli resulted in the accumulation of soluble, active protein that was purified to apparent homogeneity. In vitro assays confirmed delta(4) desaturation with 16:0-ACP; however, with 18:0-acyl acyl carrier protein (ACP) desaturation occurred at the delta(9) position. The ivy desaturase also converted 16:1delta(9)-ACP and 18:1delta(9)-ACP to the corresponding delta(4,9) dienes. These data suggest at least two distinct substrate binding modes, one placing C4 at the diiron active site and the other placing C9 at the active site. In the latter case, 18:0 would likely bind in an extended conformation as described for the castor desaturase with 9-carbons accommodated in the cavity beyond the dirron site. However, delta(4) desaturation would require the accommodation of 12 carbons for C16 substrates or 14 carbons for C18 substrates. The amino acids lining the substrate binding cavity of ivy and castor desaturases are conserved except for T117R and P179I (castor/ivy). Paradoxically, both substitutions, when introduced into the castor desaturase, favored the binding of shorter acyl chains. Thus, it seems likely that delta(4) desaturation would require a non-extended, perhaps U-shaped, substrate conformation. A cis double bond may facilitate the initiation of such a non-extended conformation in the monounsaturated substrates. The multifunctional properties of the ivy desaturase make it well suited for further dissection of the determinants of regiospecificity.

Mutation study of conserved amino acid residues of Spirulina delta 6-acyl-lipid desaturase showing involvement of histidine 313 in the regioselectivity of the enzyme

In the cyanobacterium Spirulina platensis, the desaturation process is carried out by three desaturases: the Delta(9), Delta(12) and Delta(6) desaturases, encoded by desC, desA and desD, respectively. The Delta(6) desaturase is responsible for the catalysis of linoleic acid, yielding gamma-linolenic acid (18:3(Delta 9,12,6)), the end-product of the process. In this study, the desD gene was expressed in Escherichia coli using a pTrcHisA expression system. In order to identify the amino acid residues involved in the enzymatic activity, a sequence comparison was performed using various organisms. The alignment revealed three conserved histidine clusters, a number of conserved residues among all listed organisms and a few conserved residues among cyanobacterial species possibly involved in the desaturation activity. A series of site-directed mutations were generated in the desD gene to evaluate the role of these residues vis-a-vis the enzyme function. This approach revealed that: (1) H313 is involved in the regioselectivity of the enzyme, (2) the three histidine clusters together with H313, H315, D138 and E140 are required for enzymatic activity, most likely as providers of the catalytic Fe center and (3) W294 is also essential for the activity of Delta(6) desaturase, possibly by forming part of the substrate-binding pocket.

Isolation and expression pattern of two putative acyl-ACP desaturase cDNAs from Bassia scoparia

The seed lipids of some higher plants contain unusual fatty acids with potentially valuable non-food uses. Seeds of Bassia scoparia contain one such monounsaturated fatty acid, 16:1Delta5. This fatty acid can be used for the production of an insect oviposition pheromone, which is potentially valuable in the control of the mosquito Culex quinquefasciatus, a vector of West Nile virus. Previous work has established that a number of unusual monounsaturated fatty acids are produced by variant forms of the ubiquitous acyl-ACP desaturases. The isolation and initial characterization of two putative acyl-ACP desaturases from B. scoparia, one of which is seed-specific, suggests that such a variant enzyme occurs in this species.

Azide and acetate complexes plus two iron-depleted crystal structures of the di-iron enzyme delta9 stearoyl-acyl carrier protein desaturase. Implications for oxygen activation and catalytic intermediates

Delta9 stearoyl-acyl carrier protein (ACP) desaturase is a mu-oxo-bridged di-iron enzyme, which belongs to the structural class I of large helix bundle proteins and that catalyzes the NADPH and O2-dependent formation of a cis-double bond in stearoyl-ACP. The crystal structures of complexes with azide and acetate, respectively, as well as the apoand single-iron forms of Delta9 stearoyl-ACP desaturase from Ricinus communis have been determined. In the azide complex, the ligand forms a mu-1,3-bridge between the two iron ions in the active site, replacing a loosely bound water molecule. The structure of the acetate complex is similar, with acetate bridging the di-iron center in the same orientation with respect to the di-iron center. However, in this complex, the iron ligand Glu196 has changed its coordination mode from bidentate to monodentate, the first crystallographic observation of a carboxylate shift in Delta9 stearoyl-ACP desaturase. The two complexes are proposed to mimic a mu-1,2 peroxo intermediate present during catalytic turnover. There are striking structural similarities between the di-iron center in the Delta9 stearoyl-ACP desaturase-azide complex and in the reduced rubrerythrin-azide complex. This suggests that Delta9 stearoyl-ACP desaturase might catalyze the formation of water from exogenous hydrogen peroxide at a low rate. From the similarity in iron center structure, we propose that the mu-oxo-bridge in oxidized desaturase is bound to the di-iron center as in rubrerythrin and not as reported for the R2 subunit of ribonucleotide reductase and the hydroxylase subunit of methane monooxygenase. The crystal structure of the one-iron depleted desaturase species demonstrates that the affinities for the two iron ions comprising the di-iron center are not equivalent, Fe1 being the higher affinity site and Fe2 being the lower affinity site.

Engineering delta 9-16:0-acyl carrier protein (ACP) desaturase specificity based on combinatorial saturation mutagenesis and logical redesign of the castor delta 9-18:0-ACP desaturase

Six amino acid locations in the soluble castor Delta(9)-18:0-acyl carrier protein (ACP) desaturase were identified that can affect substrate specificity. Combinatorial saturation mutagenesis of these six amino acids, in conjunction with selection, using an unsaturated fatty acid auxotroph system, led to the isolation of variants with up to 15-fold increased specific activity toward 16-carbon substrates. The most improved mutant, com2, contained two substitutions (T117R/G188L) common to five of the 19 complementing variants subjected to further analysis. These changes, when engineered into otherwise wild-type 18:0-ACP desaturase to make mutant 5.2, produced a 35-fold increase in specific activity with respect to 16-carbon substrates. Kinetic analysis revealed changes in both k(cat) and K(m) that result in an 82-fold improvement in specificity factor for 16-carbon substrate compared with wild-type enzyme. Improved substrate orientation apparently compensated for loss of binding energy that results from the loss of desolvation energy for 16-carbon substrates. Mutant 5.2 had specific activity for 16-carbon substrates 2 orders of magnitude higher than those of known natural 16-carbon specific desaturases. These data support the hypothesis that it should be possible to reengineer archetypal enzymes to achieve substrate specificities characteristic of recently evolved enzymes while retaining the desired stability and/or turnover characteristics of a parental paralog.

VARIATIONS IN THE BIOSYNTHESIS OF SEED-STORAGE LIPIDS

In many plants lipids represent up to 80% of dry weight of storage tissues. In seeds, lipids accumulate as triacylglycerols (TAGs), which are formed by an extension of the membrane-lipid biosynthetic pathway common to all plant tissues. In contrast to the conserved fatty acid (FA) composition of membrane lipids, the observed divergence in seed oil acyl chains among different species is very high. The acyl groups of seed TAGs can vary in their chain length (from 8 to 24) as well as in their degree of unsaturation. In addition to methylene-interrupted double bonds, many seeds contain TAGs that have unusual functional groups in their FAs, such as hydroxyl, oxirane, or acetylene groups. All of the major steps in the biosynthetic pathway to TAG are now known and sequence information for genes encoding most of the enzymes involved is available. Here we present the current knowledge of the metabolic mechanisms involved in the divergence from the membrane-lipid biosynthetic pathway during storage lipid formation.

Substrate-dependent mutant complementation to select fatty acid desaturase variants for metabolic engineering of plant seed oils

We demonstrate that naturally occurring C(14) and C(16)-specific acyl-acyl carrier protein (ACP) desaturases from plants can complement the unsaturated fatty acid (UFA) auxotrophy of an Escherichia coli fabA/fadR mutant. Under the same growth conditions, C(18)-specific delta(9)-stearoyl (18:0)-ACP desaturases are unable to complement the UFA auxotrophy. This difference most likely results from the presence of sufficient substrate pools of C(14) and C(16) acyl-ACPs but a relative lack of C(18) acyl-ACP pools in E. coli to support the activities of the plant fatty acid desaturase. Based on this, a substrate-dependent selection system was devised with the use of the E. coli UFA auxotroph to isolate mutants of the castor delta(9)-18:0-ACP desaturase that display enhanced specificity for C(14) and C(16) acyl-ACPs. Using this selection system, a number of desaturase variants with altered substrate specificities were isolated from pools of randomized mutants. These included several G188L mutant isolates, which displayed a 15-fold increase in specific activity with 16:0-ACP relative to the wild-type castor delta(9)-18:0-ACP desaturase. Expression of this mutant in Arabidopsis thaliana resulted in the accumulation of unusual monounsaturated fatty acids to amounts of >25% of the seed oil. The bacterial selection system described here thus provides a rapid means of isolating variant fatty acid desaturase activities for modification of seed oil composition.

Stearoyl-acyl carrier protein and unusual acyl-acyl carrier protein desaturase activities are differentially influenced by ferredoxin

Acyl-acyl carrier protein (ACP) desaturases function to position a single double bond into an acyl-ACP substrate and are best represented by the ubiquitous Delta9 18:0-ACP desaturase. Several variant acyl-ACP desaturases have also been identified from species that produce unusual monoenoic fatty acids. All known acyl-ACP desaturase enzymes use ferredoxin as the electron-donating cofactor, and in almost all previous studies the photosynthetic form of ferredoxin rather than the non-photosynthetic form has been used to assess activity. We have examined the influence of different forms of ferredoxin on acyl-ACP desaturases. Using combinations of in vitro acyl-ACP desaturase assays and [(14)C]malonyl-coenzyme A labeling studies, we have determined that heterotrophic ferredoxin isoforms support up to 20-fold higher unusual acyl-ACP desaturase activity in coriander (Coriandrum sativum), Thunbergia alata, and garden geranium (Pelargonium x hortorum) when compared with photosynthetic ferredoxin isoforms. Heterotrophic ferredoxin also increases activity of the ubiquitous Delta9 18:0-ACP desaturase 1.5- to 3.0-fold in both seed and leaf extracts. These results suggest that ferredoxin isoforms may specifically interact with acyl-ACP desaturases to achieve optimal enzyme activity and that heterotrophic isoforms of ferredoxin may be the in vivo electron donor for this reaction.

Characterisation of acyl-ACP desaturases from Macadamia integrifolia Maiden & Betche and Nerium oleander L

The seed oil in Macadamia integrifolia contains about 30% palmitoleic acid (16:1(Delta9)) and Nerium oleander about 12% isoricinoleic acid (Delta9-hydroxy-18:1(Delta12)). It has been shown that palmitoleic acid can be produced by acyl-acyl carrier protein (ACP) desaturases and it has also been shown that fatty acid hydroxylation can occur via direct substitution of a hydrogen atom. Therefore it seemed possible that the enzymes responsible for the making of these unusual fatty acids in M. integrifolia and N. oleander were of acyl-ACP desaturase type. Extracts from developing M. integrifolia developing seeds showed a relative ratio of 16:0-ACP to 18:0-ACP desaturation that was about 13 times higher than in sunflower seeds. N. oleander seed extracts catalysed conversion of 18:0-ACP to 18:1(Delta9) but only trace amounts of Delta9-hydroxy fatty acids were formed. A total of four cDNAs were isolated from developing seeds, of both species, using a fragment isolated with PCR amplification. The M. integrifolia acyl-ACP desaturase cDNA was expressed in Escherichia coli. A partly purified fraction of the enzyme showed a 16:0-ACP to 18:0-ACP desaturation ratio about 90-fold less than that in the Macadamia extracts. Expressed N. oleander acyl-ACP desaturase cDNAs showed predominantly 18:0-ACP desaturase activity and no hydroxylase activity. Thus it is not likely that any of the four acyl-ACP desaturases cloned from M. integrifolia or N. oleander is involved in the production of unusual fatty acids.

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.