Mechanisms of synthesis of waxy esters in broccoli (Brassica oleracea)

The production of wax esters in transgenic plants: towards a sustainable source of bio-lubricants

Wax esters are high-value compounds used as feedstocks for the production of lubricants, pharmaceuticals, and cosmetics. Currently, they are produced mostly from fossil reserves using chemical synthesis, but this cannot meet increasing demand and has a negative environmental impact. Natural wax esters are also obtained from Simmondsia chinensis (jojoba) but comparably in very low amounts and expensively. Therefore, metabolic engineering of plants, especially of the seed storage lipid metabolism of oil crops, represents an attractive strategy for renewable, sustainable, and environmentally friendly production of wax esters tailored to industrial applications. Utilization of wax ester-synthesizing enzymes with defined specificities and modulation of the acyl-CoA pools by various genetic engineering approaches can lead to obtaining wax esters with desired compositions and properties. However, obtaining high amounts of wax esters is still challenging due to their negative impact on seed germination and yield. In this review, we describe recent progress in establishing non-food-plant platforms for wax ester production and discuss their advantages and limitations as well as future prospects.

Fatty Acid Ethyl Esters of Rhizopus arrhizus

Gas chromatographic and mass spectrometric analyses on selected lipid fractions revealed for the first time the presence of ethyl esters of long-chain fatty acids as biological products. Ethyl esters of oleic, palmitic, and stearic acids were detected in relative concentrations of 21.2, 2.4, and 1.5 percent, respectively, of the total methyl and ethyl ester fraction. Both saturated and unsaturated ethyl esters contain pronounced mass spectral fragments at a mass-to-charge ratio of 88.

A novel in vitro multiple-stress dormancy model for Mycobacterium tuberculosis generates a lipid-loaded, drug-tolerant, dormant pathogen

BACKGROUND

Mycobacterium tuberculosis (Mtb) becomes dormant and phenotypically drug resistant when it encounters multiple stresses within the host. Inability of currently available drugs to kill latent Mtb is a major impediment to curing and possibly eradicating tuberculosis (TB). Most in vitro dormancy models, using single stress factors, fail to generate a truly dormant Mtb population. An in vitro model that generates truly dormant Mtb cells is needed to elucidate the metabolic requirements that allow Mtb to successfully go through dormancy, identify new drug targets, and to screen drug candidates to discover novel drugs that can kill dormant pathogen.

METHODOLOGY/PRINCIPAL FINDINGS

We developed a novel in vitro multiple-stress dormancy model for Mtb by applying combined stresses of low oxygen (5%), high CO(2) (10%), low nutrient (10% Dubos medium) and acidic pH (5.0), conditions Mtb is thought to encounter in the host. Under this condition, Mtb stopped replicating, lost acid-fastness, accumulated triacylglycerol (TG) and wax ester (WE), and concomitantly acquired phenotypic antibiotic-resistance. Putative neutral lipid biosynthetic genes were up-regulated. These genes may serve as potential targets for new antilatency drugs. The triacylglycerol synthase1 (tgs1) deletion mutant, with impaired ability to accumulate TG, exhibited a lesser degree of antibiotic tolerance and complementation restored antibiotic tolerance. Transcriptome analysis with microarray revealed the achievement of dormant state showing repression of energy generation, transcription and translation machineries and induction of stress-responsive genes. We adapted this model for drug screening using the Alamar Blue dye to quantify the antibiotic tolerant dormant cells.

CONCLUSIONS/SIGNIFICANCE

The new in vitro multiple stress dormancy model efficiently generates Mtb cells meeting all criteria of dormancy, and this method is adaptable to high-throughput screening for drugs that can kill dormant Mtb. A critical link between storage-lipid accumulation and development of phenotypic drug-resistance in Mtb was established. Storage lipid biosynthetic genes may be appropriate targets for novel drugs that can kill latent Mtb.

Microsomal preparations from plant and yeast acylate free fatty acids without prior activation to acyl-thioesters

Acylation of fatty acids to hydroxy groups in cells generally require activation to a thioester (ACP or CoA) or transacylation from another oxygen ester. We now show that microsomal membranes from Arabidopsis leaves efficiently acylate free fatty acids to long chain alcohols with no activation of the fatty acids to thioesters prior to acylation. Studies of the fatty alcohol and fatty acids specificities of the reaction in membranes from Arabidopsis leaves revealed that long chain (C18-C24) unsaturated fatty alcohols and C18-C22 unsaturated fatty acids were preferred. Microsomal preparations from Arabidopsis roots and leaves and from yeast efficiently synthesized ethyl esters from ethanol and free fatty acids. This reaction also occurred without prior activation of the fatty acid to a thioester. The results presented strongly suggest that wax ester and ethyl ester formation are carried out by separate enzymes. The physiological significance of the reactions in plants is discussed in connection to suberin and cutin synthesis. The results also have implication regarding the interpretation of lipid metabolic experiments done with microsomal fraction.

Wax constituents on the inflorescence stems of double eceriferum mutants in Arabidopsis reveal complex gene interactions

To shed new light on gene involvement in plant cuticular-wax production, 11 eceriferum (cer) mutants of Arabidopsis having dramatic alterations in wax composition of inflorescence stems were used to create 14 double cer mutants each with two homozygous recessive cer loci. A comprehensive analysis of stem waxes on these double mutants revealed unexpected CER gene interactions and new ideas about individual CER gene functions. Five of the 14 double cer mutants produced significantly more total wax than one of their respective cer parents, indicating from a genetic standpoint a partial bypassing (or complementation) of one cer mutation by the other. Eight of the 14 double cer mutants had alkane amounts lower than both respective cer parents, suggesting that most of these CER gene products play a major additive role in alkane synthesis. Other results suggested that some CER genes function in more than one step of the wax pathway, including those associated with sequential steps in acyl-CoA elongation. Surprisingly, complete epistasis was not observed for any of the cer gene combinations tested. Significant overlap or redundancy of genetic operations thus appears to be a central feature of wax metabolism. Future studies of CER gene product function, as well as the utilization of CER genes for crop improvement, must now account for the complex gene interactions described here.

Cuticular waxes on eceriferum mutants of Arabidopsis thaliana

We present cuticular wax chemical profiles for the leaves and stems of Arabidopsis wildtype Landsberg erecta and eleven isogenic eceriferum mutants: cer5, cer10 to cer15, and cer17 to cer20. These cer mutants have wax profiles that are different from those of wildtype in chemical chain length distribution, amount per chemical class, and/or total wax load. Analyses of detailed leaf and stem wax profiles for these cer mutants have allowed us to place some of these mutants at specific steps in wax production. The cer13 gene is predicted to affect release of the 30 carbon fatty acid from the elongation complex or the reduction of C30 fatty acid to C30 aldehyde. The CER19 gene product is predicted to be involved in C28 to C30 fatty acyl-CoA elongation. The CER20 gene is predicted to affect the oxidation of C29 alkane to C29 secondary alcohol. Several predicted gene products affect only stem specific steps in the wax pathway.

The effect of water on the ion trap analysis of trimethylsilyl derivatives of long-chain fatty acids and alcohols

Gas chromatography/mass spectrometry (GC/MS), with an ion trap mass analyzer, was used to examine the very-long-chain cuticular acid and certain non-acid wax constituents on the leaf sheath surface of Sorghum bicolor before and during 36 hours of light exposure. The mass spectra of the trimethylsilylated acids and alcohols did not match any of those published in searchable mass spectral libraries. The observed differences can be related to the interaction between water and the trimethylsilylated acids and alcohols. Understanding the observed mass spectra of the very-long-chain plant waxes is critical for studies that employ GC/MS with the ion trap mass analyzer to elucidate cuticular wax compositions on plants.

Leaf sheath cuticular waxes on bloomless and sparse-bloom mutants of Sorghum bicolor

Leaf sheath cuticular waxes on wild-type Sorghum bicolor were approximately 96% free fatty acids, with the C28 and C30 acids being 77 and 20% of these acids, respectively. Twelve mutants with markedly reduced wax load were characterized for chemical composition. In all of the 12 mutants, reduction in the amount of C28 and C30 acids accounted for essentially all of the reduction in total wax load relative to wildtype. The bm2 mutation caused a 99% reduction in total waxes. The bm4, bm5, bm6, bm7 and h10 mutations caused more than 91% reduction in total waxes, whereas the remaining six mutants, bm9, bm11, h7, h11, h12 and h13, caused between 35 and 78% reduction in total wax load. Relative to wild-type, bm4 caused a large increase in the absolute amount of C22, C24 and C26 acids, and reduction in the C28 and longer acids, suggesting that bm4 may suppress elongation of C26, acyl-CoA primarily. The h10 mutation increased the absolute amounts of the longest chain length acids, but reduced shorter acids, suggesting that h10 may suppress termination of acyl-CoA elongation. The bm6, bm9, bm11, h7, h11, h12 and h13 mutations increased the relative amounts, but not absolute amounts, of longer chain acids. Based on chemical composition alone, it is still uncertain which genes and their products were altered by these mutations. Nevertheless, these Sorghum cuticular wax mutants should provide a valuable resource for future studies to elucidate gene involvement in the biosynthesis of cuticular waxes, in particular, the very-long-chain fatty acids.

Wax ester-synthesizing activity of lipases

The synthesis/hydrolysis of wax esters was studied in an aqueous solution using purified rat pancreatic lipase, porcine pancreatic carboxylester lipase, and Pseudomonas fluorescens lipase. The equilibrium between wax ester synthesis and hydrolysis favored ester formation at neutral pH. The synthesizing activities were measured using free fatty acid or triacylglycerol as the acyl donor and an equimolar amount of long-chain alcohol as the acyl acceptor. When oleic acid and hexadecanol emulsified with gum arabic were incubated with these lipases, wax ester was synthesized, in a dose- and time-dependent manner, and the apparent equilibrium ratio of palmityl oleate/free oleic acid was about 0.9/0.1. These lipases catalyzed the hydrolysis of palmityl oleate emulsified with gum arabic, and the apparent equilibrium ratio of palmityl oleate/free oleic acid was also about 0.9/0.1. The apparent equilibrium ratio of wax ester/free fatty acid catalyzed by lipase depended on incubation pH and fatty alcohol chain length. When equimolar amounts of trioleoylglycerol and fatty acyl alcohol were incubated with pancreatic lipase, carboxylester lipase, or P. fluorescens lipase, wax esters were synthesized dose-dependently. These results suggest that lipases can catalyze the synthesis of wax esters from free fatty acids or through degradation of triacylglycerol in an aqueous medium.

Resolution and purification of an aldehyde-generating and an alcohol-generating fatty acyl-CoA reductase from pea leaves (Pisum sativum L.)

Higher plant tissues produce both wax esters generated from fatty alcohols and hydrocarbons generated from fatty aldehydes. If two different reductases are responsible for the synthesis of aldehydes and alcohols, both types of reductases may be present in such tissues. To test for this possibility, pea leaves, known to produce both types of wax components, were examined. Subcellular fractionation showed that acyl-CoA reductase activities were localized mainly in the microsomal fraction. Fatty aldehyde formation was rectilinear for 30 min and subsequently decreased, whereas fatty alcohol formation remained linear for 2 h. The two activities in the microsomes were differently affected by pH; alcohol formation was optimal between pH 5 and pH 6, whereas aldehyde formation was optimal at around pH 7.5. With solubilized microsomes, protein concentration dependence of alcohol formation showed a sigmoidal pattern, possibly suggesting inhibition by hexadecanoyl-CoA at low protein concentrations. Bovine serum albumin (BSA) enhanced alcohol formation. In contrast, the aldehyde generation showed a typical protein concentration dependence, and BSA severely inhibited aldehyde generation. Phosphatidylcholine showed over twofold stimulation for alcohol formation, whereas aldehyde formation was only slightly stimulated. All of this biochemical evidence suggested the presence of two different reductases. Confirming this hypothesis, an aldehyde-generating and an alcohol-generating reductase were resolved from the solubilized microsomal proteins using Blue A agarose, gel filtration, and hexadecanoyl-CoA affinity chromatography. SDS-PAGE of the purified proteins showed that the alcohol-generating enzyme was a 58-kDa protein and the aldehyde-forming one was a 28-kDa protein. It is proposed that two different elongating systems are functionally coupled to the alcohol-generating and aldehyde-generating reductases, which in turn are coupled to the transacylase to produce wax esters and to the decarbonylase to produce hydrocarbons, respectively.

Alkane biosynthesis by decarbonylation of aldehyde catalyzed by a microsomal preparation from Botryococcus braunii

The final step in the synthesis of n-hydrocarbons in an animal and a higher plant involves enzymatic decarbonylation of aldehydes to the corresponding alkanes by loss of the carbonyl carbon. Whether such a novel reaction is involved in hydrocarbon synthesis in the colonial microalga, Botryococcus braunii, which is known to produce unusually high levels (up to 32% of dry weight) of n-C27, C29, and C31 alka-dienes and -trienes, was investigated. Dithioerythritol severely inhibited the incorporation of [1-14C]acetate into these hydrocarbons with accumulation of the label in the aldehyde fraction in the B. braunii cells. Microsomal preparations of the alga synthesized alkane from fatty acid and aldehyde in the absence of O2. Conversion of fatty acid to alkane required CoA, ATP, and NADH, whereas conversion of aldehyde to alkane did not require the addition of cofactors. That the alkane synthesis involves a decarbonylation was shown by the production of CO and heptadecane from octadecanal. CO was identified by adsorption to RhCl[(C6H6)3P]3. The decarbonylase had a pH optimum at 7.0, an apparent Km of 65 microM, a Vmax of 1.36 nmol/min/mg and was inhibited by the metal chelators EDTA, O-phenanthroline and 8-hydroxyquinoline. It was stimulated nearly threefold by 2 mM ascorbate and inhibited by the presence of O2. A partial (28%) retention of the aldehydic hydrogen of [1-3H]octadecanal in the heptadecane was observed; the remaining 3H was lost to H2O. The microsomal preparation also catalyzed the oxidation of 14CO to 14CO2, with a pH optimum of 7.0. This accounts for the nonstoichiometry of CO to heptadecane observed. In vivo studies with 14CO showed that the label was incorporated into metabolic products. This metabolic conversion of CO, not found in the previously examined hydrocarbon synthesizing systems, may be necessary for organisms that produce large amounts of hydrocarbons such as the present alga. The mechanism of the decarbonylation and the nature of the decarbonylase remain to be elucidated.

Synthesis of wax esters by a cell-free system from barley (Hordeum vulgare L.)

Experimental evidence for a membranebound microsomal ester synthetase from Bonus barley primary leaves is reported. The results are consistent with at least two mechanisms for the synthesis of barley wax esters: an acyl-CoA-fattyalcohol-transacylase-type reaction and an apparent direct esterification of alcohols with fatty acids. Biosynthesis of wax esters was not specific with regard to the chain length of the tested alcohols. The microsomal preparation readily catalyzed the esterification of C16-, C18-, C22- or C24-labelled alcohols with fatty acids of endogenous origin. Exogenous long-chain alcohols were exclusively incorporated into the alkyl moieties of the esters. Addition of ATP, CoA and-or free fatty acids was not effective in stimulating or depressing the esterifying activity of the microsomal fraction. Partial solubilization of the ester synthetase was obtained using phosphate-buffered saline.

Formation of complex ether lipids from 1-O-alkylglycerols in cell suspension cultures of rape

Photomixotrophic cell suspension cultures of rape, Brassica napus, were incubated with rac-1-O-[1'-(14)C]hexadecylglycerol. Radioactivity was incorporated predominantly into choline glycerophospholipids. Prolonged incubation led also to considerable proportions of labeled ethanolamine glycerophospholipids. In addition to these ionic lipids,isomeric hexadecylacylglycerols as well as hexadecyldiacylglycerols were formed. About a third of the hexadecylglycerol supplied as substrate was cleaved within 48 h incubation. The palmitic acid formed by oxidative cleavage of the substrate was incorporated predominantly into choline glycerophospholipids, ethanolamine glycerophospholipids, and triacylglycerols. Incubation of an equimolar mixture of homologous saturated rac-1-O-[1'(14)C]alkylglycerols (C14, C16, C18, C20) with rape cells showed that alkylglycerols with alkyl moieties having 16 and 18 carbon atoms were incorporated preferentially. Incubation of labeled hexadecyglycerol with a homogenate of rape cells led also predominantly to choline glycerophospholipids; highest yields were obtained at pH 7. Neither the 1-O-alkyl moieties in choline glycerophospholipis nor those in ethanolamine glycerophospholipids were desaturated to 1-O-alk-1'-enylmoieties. The results of these experiments led to the following conclusions: (1) The acylation of 1-O-alkylglycerols to isomeric alkylacylglycerols is catalyzed by two acyltransferases differing in their specificity with regard to the chain length of the alkyl moiety in the substrate. (2) CDP-Choline: diacylglycerol cholinephosphotransferase and CDP-ethanolamine: diacylglycerol ethanolaminephosphotransferase are two enzymes differing in various respects. Cholinephosphotransferase exhibits a much higher affinity for 1-O-alkyl-2-O-acylglycerols than ethanolaminephosphotransferase. The two enzymes show marked differences with regard to their specificity for 1-O-alkyl-2-O-acylglycerols differing in the chain lengths of their alkyl moieties.

Metabolism of long-chain alcohols in cell suspension cultures of soya and rape

Heterotrophic cell suspension cultures of soya (Glycine max) and photomixotrophic cell suspension cultures of rape (Brassica napus) were incubated with cis-9-[1-(14)C]octadecenol for 3-48 h. It was found that under aerobic conditions large proportions of the alcohol are oxidized to oleic acid, which is incorporated predominantly into phospholipids, whereas up to 30% of the substrate is esterified to wax esters. This is true for both the heterotrophic and the photomixotrophic cell suspension cultures, but the metabolic rates are much higher in the latter. Under anaerobic conditions only small proportions of the radioactively labeled alcohol are oxidized to oleic acid, whereas a major portion of the alcohol is esterified to wax esters both in heterotrophic and photomixotrophic cultures. Incubations of homogenates of photomixotrophic rape cells with labeled cis-9-octadecenol showed that pH 6 is optimum for the formation of wax esters. This monounsaturated alcohol is preferred as a substrate over saturated longchain alcohols, whereas short-chain alcohols, cholesterol, and glycerol are not acylated. Incubations of an enzyme concentrate from a homogenate of rape cells with unlabeled cis-9-octadecenol and [1-(14)C]oleic acid, or [1-(14)C]stearoyl-CoA, or di[1-(14)C]palmitoyl-sn-glycero-3-phosphocholine showed that acylation of the longchain alcohol proceeds predominantly through acyl-CoA. Direct esterification of the alcohol with fatty acid as well as acyl transfer from diacylglycerophosphocholine could be demonstrated to occur to a much smaller extent.

Reduction of fatty acids to alcohols in roe of gourami (Trichogaster cosby)

Reduction of fatty acids to alcohols in gourami roe homogenates and fractions thereof was studied. The reducing activity is associated with the microsomal fraction. Activation of the acid and NADPH as reducing cofactor are required. The optimal pH for reduction is between 6.5 and 7.5. Reduction rates were highest for palmitic acid and were about half of that for oleic and linoleic acids. In contrast to the equal reduction rates of the latter acids in vitro, the percentages of oleyl and linoleyl alcohols in wax esters are greatly different in vivo. Very small amounts of aldehyde are found during the reduction and some substrate label is incorporated into the phospholipids. The traces of triacylglycerols in roe lipids are not markedly labelled. In homogenates, newly formed as well as added substrate alcohol is efficiently incorporated into wax esters. Roe homogenate is capable also of oxidizing fatty alcohol to acid. In contrast to reduction, oxidation proceeds with either NADP+ or NAD+ as cofactor. Only a small portion of the newly formed acid is esterified.

Biosynthesis of wax esters in tissues of Sinapis alba L. seeds

The biosynthesis of wax esters has been investigated in maturing seeds of Sinapis alba. Exogenous long-chain alcohols are incorporated exclusively into alkyl moieties of wax esters. Oxidation of the long-chain alcohols is not detected. Exogenous fatty acids are incorporated into acyl moieties of wax esters to a low extent. A reduction of fatty acids to alcohols is not observed. Synthesis of wax esters is localized exclusively in the testa; both outer and inner integument are equally active in wax ester biosynthesis. The biosynthesis of wax esters is specific with regard to both chain length and degree of unsaturation of long-chain alcohols. Exogenous and endogenous sterols are not esterified.

The enzymic deacylation of phospholipids and galactolipids in plants. Purification and properties of a lipolytic acyl-hydrolase from potato tubers

An enzyme preparation that catalyses the deacylation of mono- and di-acyl phospholipids, galactosyl diglycerides, mono- and di-glycerides has been partially purified from potato tubers. The preparation also hydrolyses methyl and p-nitrophenyl esters and acts preferentially on esters of long-chain fatty acids. Triglycerides, wax esters and sterol esters are not hydrolysed. The same enzyme preparation catalyses acyl transfer reactions in the presence of alcohols and also catalyses the synthesis of wax esters from long-chain alcohols and free fatty acids. Gel filtration, DEAE-cellulose chromatography and free-flow electrophoresis failed to achieve any separation of the acyl-hydrolase activities towards different classes of acyl lipids (phosphatidylcholine, monogalactosyl diglyceride, mono-olein, methyl palmitate and p-nitrophenyl palmitate) or any separation of these activities from a major protein component. For each class of lipid the acyl-hydrolase activity was subject to substrate inhibition, was inhibited by relatively high concentrations of di-isopropyl phosphorofluoridate and the pH responses were changed by Triton X-100. The hydrolysis of phosphatidylcholine was stimulated 30-40-fold by Triton X-100. The specific activities of the potato enzyme with galactolipids were at least 70 times higher than those reported for a homogeneous galactolipase enzyme purified from runner bean leaves. The possibility that a single lipolytic acyl-hydrolase enzyme is responsible for the deacylation of several classes of acyl lipid is discussed.