Acetoacyl-acyl carrier protein synthase from avocado: its purification, characterisation and clear resolution from acetyl CoA:ACP transacylase

Molecular cloning and characterization of two β-ketoacyl-acyl carrier protein synthase I genes from Jatropha curcas L

The β-ketoacyl-acyl carrier protein synthase I (KASI) is involved in de novo fatty acid biosynthesis in many organisms. Two putative KASI genes, JcKASI-1 and JcKASI-2, were isolated from Jatropha curcas. The deduced amino acid sequences of JcKASI-1 and JcKASI-2 exhibit around 83.8% and 72.5% sequence identities with AtKASI, respectively, and both contain conserved Cys-His-Lys-His-Phe catalytic active sites. Phylogenetic analysis indicated that JcKASI-2 belongs to a clade with several KASI proteins from dicotyledonous plants. Both JcKASI genes were expressed in multiple tissues, most strongly in filling stage seeds of J. curcas. Additionally, the JcKASI-1 and JcKASI-2 proteins were both localized to the plastids. Expressing JcKASI-1 in the Arabidopsis kasI mutant rescued the mutant's phenotype and restored the fatty acid composition and oil content in seeds to wild-type, but expressing JcKASI-2 in the Arabidopsis kasI mutant resulted in only partial rescue. This implies that JcKASI-1 and JcKASI-2 exhibit partial functional redundancy and KASI genes play a universal role in regulating fatty acid biosynthesis, growth, and development in plants.

Cloning and sequence analysis of putative type II fatty acid synthase genes from Arachis hypogaea L

The cultivated peanut is a valuable source of dietary oil and ranks fifth among the world oil crops. Plant fatty acid biosynthesis is catalysed by type II fatty acid synthase (FAS) in plastids and mitochondria. By constructing a full-length cDNA library derived from immature peanut seeds and homology-based cloning, candidate genes of acyl carrier protein (ACP), malonyl-CoA:ACP transacylase, beta-ketoacyl-ACP synthase (I, II, III), beta-ketoacyl-ACP reductase, beta-hydroxyacyl-ACP dehydrase and enoyl-ACP reductase were isolated. Sequence alignments revealed that primary structures of type II FAS enzymes were highly conserved in higher plants and the catalytic residues were strictly conserved in Escherichia coli and higher plants. Homologue numbers of each type II FAS gene expressing in developing peanut seeds varied from 1 in KASII, KASIII and HD to 5 in ENR. The number of single-nucleotide polymorphisms (SNPs) was quite different in each gene. Peanut type II FAS genes were predicted to target plastids except ACP2 and ACP3. The results suggested that peanut may contain two type II FAS systems in plastids and mitochondria. The type II FAS enzymes in higher plants may have similar functions as those in E. coli.

X-ray crystallographic studies on butyryl-ACP reveal flexibility of the structure around a putative acyl chain binding site

Acyl carrier protein (ACP) is an essential cofactor in biosynthesis of fatty acids and many other reactions that require acyl transfer steps. We have determined the first crystal structures of an acylated form of ACP from E. coli, that of butyryl-ACP. Our analysis of the molecular surface of ACP reveals a plastic hydrophobic cavity in the vicinity of the phosphopantethylated Ser36 residue that is expanded and occupied by the butyryl and beta-mercaptoethylamine moieties of the acylated 4'-phosphopantetheine group in one of our crystal forms. In the other form, the cavity is contracted, and we propose that the protein has adopted the conformation after delivery of substrate into the active site of a partner enzyme.

Purification, characterization, and identification of novel inhibitors of the beta-ketoacyl-acyl carrier protein synthase III (FabH) from Staphylococcus aureus

Staphylococcus aureus is a versatile and dangerous pathogen and one of the major causes of community-acquired and hospital-acquired infections. The rise of multidrug-resistant strains of S. aureus requires the development of new antibiotics with previously unexploited mechanisms of action, such as inhibition of the beta-ketoacyl-acyl carrier protein (ACP) synthase III (FabH). This enzyme initiates fatty acid biosynthesis in a bacterial type II fatty acid synthase, catalyzing a decarboxylative condensation between malonyl-ACP and an acyl coenzyme A (CoA) substrate and is essential for viability. We have identified only one fabH in the genome of S. aureus and have shown that it encodes a protein with 57, 40, and 34% amino acid sequence identity with the FabH proteins of Bacillus subtilis (bFabH1), Escherichia coli (ecFabH), and Mycobacterium tuberculosis (mtFabH). Additional genomic sequence analysis revealed that this S. aureus FabH (saFabH) is not mutated in certain methicillin-resistant S. aureus (MRSA) and vancomycin-resistant S. aureus (VRSA) strains. saFabH was expressed in E. coli with an N-terminal polyhistidine tag and subsequently purified by metal chelate and size exclusion chromatography. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a molecular mass of 37 kDa, while gel filtration demonstrated a mass of 66.7 kDa, suggesting a noncovalent homodimeric structure for saFabH. The apparent K(m) for malonyl-ACP was 1.76 +/- 0.40 microM, and the enzyme was active with acetyl-CoA (k(cat), 16.18 min(-1); K(m), 6.18 +/- 0.9 microM), butyryl-CoA (k(cat), 42.90 min(-1); K(m), 2.32 +/- 0.12 microM), and isobutyryl-CoA (k(cat), 98.0 min(-1); K(m), 0.32 +/- 0.04 microM). saFabH was weakly inhibited by thiolactomycin (50% inhibitory concentration [IC50], >100 microM) yet was efficiently inhibited by two new FabH inhibitors, 5-chloro-4-phenyl-[1,2]-dithiol-3-one (IC50, 1.87 +/- 0.10 microM) and 4-phenyl-5-phenylimino-[1,2,4]dithiazolidin-3-one (IC50, 0.775 +/- 0.08 microM).

Beta-ketoacyl acyl carrier protein synthase III (FabH) is essential for fatty acid biosynthesis in Streptomyces coelicolor A3(2)

The Streptomyces coelicolor fab (fatty acid biosynthesis) gene cluster (fabD-fabH-acpP-fabF) is cotranscribed to produce a leaderless mRNA transcript. One of these genes, fabH, encodes a ketoacyl synthase III that is essential to and is proposed to be responsible for initiation of fatty acid biosynthesis in S. coelicolor.

Development of a scintillation proximity assay for beta-ketoacyl-acyl carrier protein synthase III

Assays of beta-ketoacyl-acyl carrier protein synthases III (KASIII; FabH), a key enzyme initiating bacterial type II fatty acid biosynthesis, usually involve incubation of radiolabeled acetyl-coenzyme A and malonyl-acyl carrier protein (MACP). The radiolabeled acetoacetyl-ACP product is precipitated and separated from the substrate before quantitation. We have developed a scintillation proximity assay (SPA) where use of biotinylated MACP (BMACP) allows the generation of a biotinylated acetoacetyl-ACP. This product, when captured by the streptavidin-coated scintillant-impregnated microspheres, generates an SPA signal. A BMACP K(m) of 7.1 microM was determined using this SPA with the Streptomyces glaucescens FabH. A similar MACP K(m) (6 microM) was determined in a precipitation assay, demonstrating that BMACP is an effective substrate for FabH. IC(50) values of 15.2 microM (SPA) and 24.8 microM were obtained with iodoacetamide and the S. glaucescens FabH. Comparable IC(50) values of 160 microM (SPA) and 125 microM were also obtained with the antibiotic thiolactomycin and the Escherichia coli FabH. These observations demonstrate that FabH inhibitors can be readily detected using a SPA with BMACP and that the effectiveness of inhibitors in the SPA is comparable to that obtained using MACP and a standard TCA precipitation assay. A FabH SPA adaptable to high-throughput screening should facilitate the discovery of potential novel antibiotics.

Fatty-acid biosynthesis in a branched-chain α-keto acid dehydrogenase mutant ofStreptomyces avermitilis

Fatty-acid biosynthesis by a branched-chain alpha-keto acid dehydrogenase (bkd) mutant of Streptomyces avermitilis was analyzed. This mutant is unable to produce the appropriate precursors of branched-chain fatty acid (BCFA) biosynthesis, but unlike the comparable Bacillus subtilis mutant, was shown not to have an obligate growth requirement for these precursors. The bkd mutant produced only straight-chain fatty acids (SCFAs) with membrane fluidity provided entirely by unsaturated fatty acids (UFAs), the levels of which increased dramatically compared to the wild-type strain. The levels of UFAs increased in both the wild-type and bkd mutant strains as the growth temperature was lowered from 37°C to 24°C, suggesting that a regulatory mechanism exists to alter the proportion of UFAs in response either to a loss of BCFA biosynthesis, or a decreased growth temperature. No evidence of a regulatory mechanism for BCFAs was observed, as the types of these fatty acids, which contribute significantly to membrane fluidity, did not alter when the wild-type S. avermitilis was grown at different temperatures. The principal UFA produced by S. avermitilis was shown to be delta9-hexadecenoate, the same fatty acid produced by Escherichia coli. This observation, and the inability of S. avermitilis to convert exogenous labeled palmitate to the corresponding UFA, was shown to be consistent with an anaerobic pathway for UFA biosynthesis. Incorporation studies with theS. avermitilis bkd mutant demonstrated that the fatty acid synthase has a remarkably broad substrate specificity and is able to process a wide range of exogenous branched chain carboxylic acids into unusual BCFAs.Key words: Streptomyces avermitilis, fatty acid biosynthesis, avermectin.

Characterization of beta-ketoacyl-acyl carrier protein synthase III from Streptomyces glaucescens and its role in initiation of fatty acid biosynthesis

The Streptomyces glaucescens fabH gene, encoding beta-ketoacyl-acyl carrier protein (beta-ketoacyl-ACP) synthase (KAS) III (FabH), was overexpressed in Escherichia coli, and the resulting gene product was purified to homogeneity by metal chelate chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the purified protein revealed an Mr of 37,000, while gel filtration analysis determined a native Mr of 72,000 +/- 3,000 (mean +/- standard deviation), indicating that the enzyme is homodimeric. The purified recombinant protein demonstrated both KAS activity and acyl coenzyme A (acyl-CoA):ACP transacylase (ACAT) activity in a 1:0.12 ratio. The KAS and ACAT activities were both sensitive to thiolactomycin inhibition. The KAS activity of the protein demonstrated a Km value of 3.66 microM for the malonyl-ACP substrate and an unusual broad specificity for acyl-CoA substrates, with Km values of 2.4 microM for acetyl-CoA, 0.71 microM for butyryl-CoA, and 0.41 microM for isobutyryl-CoA. These data suggest that the S. glaucescens FabH is responsible for initiating both straight- and branched-chain fatty acid biosynthesis in Streptomyces and that the ratio of the various fatty acids produced by this organism will be dictated by the ratios of the various acyl-CoA substrates that can react with FabH. Results from a series of in vivo directed biosynthetic experiments in which the ratio of these acyl-CoA substrates was varied are consistent with this hypothesis. An additional set of in vivo experiments using thiolactomycin provides support for the role of FabH and further suggests that a FabH-independent pathway for straight-chain fatty acid biosynthesis operates in S. glaucescens.

Condensing enzymes from Cuphea wrightii associated with medium chain fatty acid biosynthesis

Seed oils of most Cuphea species contain > 90% medium chain (C8-C14) fatty acids. Thioesterases with specificity for these substrates are important determinants of the medium chain phenotype. The role of condensing enzymes, however, has not been investigated. cDNA clones encoding beta-ketoacyl-acyl carrier protein (ACP) synthase (KAS) were isolated from C. wrightii, a C10/C12-producing species. Deduced amino acid sequences of four unique clones were approximately 60% identical to plant KAS I sequences and approximately 75% identical to a distinct class of KAS sequences recently identified in castor and barley. A 46 kDa protein that was observed only in developing and mature seed was detected using antiserum directed against recombinant Cuphea KAS protein. The 46 kDa protein was abundant in developing seeds of six medium chain-producing Cuphea species but barely detected in one long chain-producing species. A 48 kDa protein identified immunologically as KAS I was expressed in both medium and long chain-producing Cuphea species and was detected in all tissues tested. In in vitro assays, extracts from C. wrightii and C. viscosissima developing embryos were unable to extend fatty acid chains beyond C10 following treatment with 10 microns cerulenin, a potent inhibitor of KAS I. However, a C. viscosissima mutant, cpr-1, whose seed oils are deficient in caprate relative to wild type, was impaired in extension of C8 to C10 in this assay and Western analysis revealed a specific deficiency in 46 kDa KAS in cpr-1 embryos. These results implicate cerulenin-resistant condensing activity in production of medium chain fatty acids in Cuphea.

In vivo and in vitro effects of thiolactomycin on fatty acid biosynthesis in Streptomyces collinus

A stable-isotope assay was used to analyze the effectiveness of various perdeuterated short-chain acyl coenzyme A (acyl-CoA) compounds as starter units for straight- and branched-chain fatty acid biosynthesis in cell extracts of Streptomyces collinus. In these extracts perdeuterated isobutyryl-CoA was converted to isopalmitate (a branched-chain fatty acid), while butyryl-CoA was converted to palmitate (a straight-chain fatty acid). These observations are consistent with previous in vivo analyses of fatty acid biosynthesis in S. collinus, which suggested that butyryl-CoA and isobutyryl-CoA function as starter units for palmitate and isopalmitate biosynthesis, respectively. Additionally, in vitro analysis demonstrated that acetyl-CoA can function as a starter unit for palmitate biosynthesis. Palmitate biosynthesis and isopalmitate biosynthesis in these cell extracts were both effectively inhibited by thiolactomycin, a known type II fatty acid synthase inhibitor. In vivo experiments demonstrated that concentrations of thiolactomycin ranging from 0.1 to 0.2 mg/ml produced both a dramatic decrease in the cellular levels of branched-chain fatty acids and a surprising three- to fivefold increase in the cellular levels of the straight-chain fatty acids palmitate and myristate. Additional in vivo incorporation studies with perdeuterated butyrate suggested that, in accord with the in vitro studies, the biosynthesis of the palmitate from butyryl-CoA decreases in the presence of thiolactomycin. In contrast, in vivo incorporation studies with perdeuterated acetate demonstrated that the biosynthesis of palmitate from acetyl-CoA increases in the presence of thiolactomycin. These observations clearly demonstrate that isobutyryl-CoA is a starter unit for isopalmitate biosynthesis and that either acetyl-CoA or butyryl-CoA can be a starter unit for palmitate biosynthesis in S. collinus. However, the pathway for palmitate biosynthesis from acetyl-CoA is less sensitive to thiolactomycin, and it is suggested that the basis for this difference is in the initiation step.

Decarboxylation of malonyl-(acyl carrier protein) by 3-oxoacyl-(acyl carrier protein) synthases in plant fatty acid biosynthesis

In order to identify regulatory steps in fatty acid biosynthesis, the influence of intermediate 3-oxoacyl-(acyl carrier proteins) (3-oxoacyl-ACPs) and end-product acyl-ACPs of the fatty acid synthase reaction on the condensation reaction was investigated in vitro, using total fatty acid synthase preparations and purified 3-oxoacyl-ACP synthases (KASs; EC 2.3.1.41) from Cuphea lanceolata seeds. KAS I and II in the fatty acid synthase preparations were assayed for the elongation of octanoyl- and hexadecanoyl-ACP respectively, and the accumulation of the corresponding condensation product 3-oxoacyl-ACP was studied by modulating the content of the reducing equivalentS NADH and NADPH. Complete omission of reducing equivalents resulted with either KAS in the abnormal synthesis of acetyl-ACP from malonyl-ACP by a decarboxylation reaction. Supplementation with NADPH or NADH, separately or in combination with recombinant 3-oxoacyl-ACP reductase (EC 1.1.1.100), led to a decrease in the amount of acetyl-ACP and a simultaneous increase in elongation products. This demonstrates that the accumulation of 3-oxoacyl-ACP inhibits the condensation reaction on the one hand, and induces the decarboxylation of malonyl-ACP on the other. By carrying out similar experiments with purified enzymes, this decarboxylation was attributed to the action of KAS. Our data point to a regulatory mechanism for the degradation of malonyl-ACP in plants which is activated by the accumulation of the fatty acid synthase intermediate 3-oxoacyl-ACP.

Modification of Brassica napus seed oil by expression of the Escherichia coli fabH gene, encoding 3-ketoacyl-acyl carrier protein synthase III

The Escherichia coli fabH gene encoding 3-ketoacyl-acyl carrier protein synthase III (KAS III) was isolated and the effect of overproduction of bacterial KAS III was compared in both E. coli and Brassica napus. The change in fatty acid profile of E. coli was essentially the same as that reported by Tsay et al. (J Biol Chem 267 (1992) 6807-6814), namely higher C14:0 and lower C18:1 levels. In our study, however, an arrest of cell growth was also observed. This and other evidence suggests that in E. coli the accumulation of C14:0 may not be a direct effect of the KAS III overexpression, but a general metabolic consequence of the arrest of cell division. Bacterial KAS III was expressed in a seed- and developmentally specific manner in B. napus in either cytoplasm or plastid. Significant increases in KAS III activities were observed in both these transformation groups, up to 3.7 times the endogenous KAS III activity in mature seeds. Only the expression of the plastid-targeted KAS III gene, however, affected the fatty acid profile of the storage lipids, such that decreased amounts of C18:1 and increased amounts of C18:2 and C18:3 were observed as compared to control plants. Such changes in fatty acid composition reflect changes in the regulation and control of fatty acid biosynthesis. We propose that fatty acid biosynthesis is not controlled by one rate-limiting enzyme, such as acetyl-CoA carboxylase, but rather is shared by a number of component enzymes of the fatty acid biosynthetic machinery.