Properties of the thioesterase component obtained by limited trypsinization of the fatty acid synthetase multienzyme complex

Lipotype acquisition during neural development is not recapitulated in stem cell-derived neurons

During development, different tissues acquire distinct lipotypes that are coupled to tissue function and homeostasis. In the brain, where complex membrane trafficking systems are required for neural function, specific glycerophospholipids, sphingolipids, and cholesterol are highly abundant, and defective lipid metabolism is associated with abnormal neural development and neurodegenerative disease. Notably, the production of specific lipotypes requires appropriate programming of the underlying lipid metabolic machinery during development, but when and how this occurs is unclear. To address this, we used high-resolution MSALL lipidomics to generate an extensive time-resolved resource of mouse brain development covering early embryonic and postnatal stages. This revealed a distinct bifurcation in the establishment of the neural lipotype, whereby the canonical lipid biomarkers 22:6-glycerophospholipids and 18:0-sphingolipids begin to be produced in utero, whereas cholesterol attains its characteristic high levels after birth. Using the resource as a reference, we next examined to which extent this can be recapitulated by commonly used protocols for in vitro neuronal differentiation of stem cells. Here, we found that the programming of the lipid metabolic machinery is incomplete and that stem cell-derived cells can only partially acquire a neural lipotype when the cell culture media is supplemented with brain-specific lipid precursors. Altogether, our work provides an extensive lipidomic resource for early mouse brain development and highlights a potential caveat when using stem cell-derived neuronal progenitors for mechanistic studies of lipid biochemistry, membrane biology and biophysics, which nonetheless can be mitigated by further optimizing in vitro differentiation protocols.

Peripheral Linker Mediates Acyl Carrier Protein's Recognition of Dehydratase and Stabilizes Type-I Mycobacterium tuberculosis Fatty Acid Synthase

Incomplete structural details of Mycobacterium tuberculosis (Mtb) fatty acid synthase-I (FAS-I) at near-atomic resolution have limited our understanding of the shuttling mechanism of its mobile acyl carrier protein (ACP). Here, we have performed atomistic molecular dynamics simulation of Mtb FAS-I with a homology-modeled structure of ACP stalled at dehydratase (DH) and identified key residues that mediate anchoring of the recognition helix of ACP near DH. The observed distance between catalytic residues of ACP and DH agrees with that reported for fungal FAS-I. Further, the conformation of the peripheral linker is found to be crucial in stabilizing ACP near DH. Correlated interdomain motion is observed between DH, enoyl reductase, and malonyl/palmitoyl transferase, consistent with prior experimental reports of fungal and Mtb FAS-I.

Curiouser and curiouser: progress in understanding the programming of iterative highly-reducing polyketide synthases

Covering: 1996-2022Investigations over the last 2 decades have begun to reveal how fungal iterative highly-reducing polyketide synthases are programmed. Both in vitro and in vivo experiments have revealed the interplay of intrinsic and extrinsic selectivity of the component catalytic domains of these systems. Structural biology has begun to provide high resolution structures of hr-PKS that can be used as the basis for their engineering and reprogramming, but progress to-date remains rudimentary. However, significant opportunities exist for translating the current level of understanding into the ability to rationally re-engineer these highly efficient systems for the production of important biologically active compounds through biotechnology.

Animal Fatty Acid Synthase: A Chemical Nanofactory

Fatty acids are crucial molecules for most living beings, very well spread and conserved across species. These molecules play a role in energy storage, cell membrane architecture, and cell signaling, the latter through their derivative metabolites. De novo synthesis of fatty acids is a complex chemical process that can be achieved either by a metabolic pathway built by a sequence of individual enzymes, such as in most bacteria, or by a single, large multi-enzyme, which incorporates all the chemical capabilities of the metabolic pathway, such as in animals and fungi, and in some bacteria. Here we focus on the multi-enzymes, specifically in the animal fatty acid synthase (FAS). We start by providing a historical overview of this vast field of research. We follow by describing the extraordinary architecture of animal FAS, a homodimeric multi-enzyme with seven different active sites per dimer, including a carrier protein that carries the intermediates from one active site to the next. We then delve into this multi-enzyme's detailed chemistry and critically discuss the current knowledge on the chemical mechanism of each of the steps necessary to synthesize a single fatty acid molecule with atomic detail. In line with this, we discuss the potential and achieved FAS applications in biotechnology, as biosynthetic machines, and compare them with their homologous polyketide synthases, which are also finding wide applications in the same field. Finally, we discuss some open questions on the architecture of FAS, such as their peculiar substrate-shuttling arm, and describe possible reasons for the emergence of large megasynthases during evolution, questions that have fascinated biochemists from long ago but are still far from answered and understood.

About lipid metabolism in Hermetia illucens (L. 1758): on the origin of fatty acids in prepupae

Although increasingly targeted in animal nutrition, black soldier fly larvae or prepupae (BSF, Hermetia illucens L. 1758) require the characterization and modulation of their fatty acid profile to become fully integrated within the feed sector. This improvement will only be possible by the understanding of underlaying biochemical pathways of fatty acid synthesis in BSF. In this study, we hypothesized a labelling of de novo synthesized fatty acids in BSF by the incorporation of deuterated water (D₂O) in their feed. Three batches of fifty larvae were reared on two diets with different polyunsaturated fatty acid profiles moistened with 40% of H₂O or D₂O: chicken feed or 40% of chicken feed and 60% of flax cake. Although the occurrence of D₂O in insect feed increased the larval development time and decreased prepupal weight, it was possible to track the biosynthesis of fatty acids through deuterium labelling. Some fatty acids (decanoic, lauric or myristic acid) were exclusively present in their deuterated form while others (palmitic, palmitoleic or oleic acid) were found in two forms (deuterated or not) indicating that BSF can partially produce these fatty acids via biosynthesis pathways and not only by bioaccumulation from the diet. These results suggest the importance of carbohydrates as a source of acetyl-CoA in the constitution of the BSF fatty acid profile but also the potential importance of specific enzymes (e.g. thioesterase II or Δ12 fat2 desaturase) in BSF fatty acid metabolism. Finally, nearly no deuterated polyunsaturated fatty acids were found in BSF fed with deuterium confirming that BSF is not able to produce these types of fatty acids. Despite the high levels of linolenic acid in flax-enriched diets, BSF will simply bioaccumulate around 13% of this fatty acid and will metabolize approximately two-thirds of it into saturated fatty acids as lauric or myristic acid.

Reengineering the programming of a functional domain of an iterative highly reducing polyketide synthase

A structural model of the enoyl reductase (ER) catalytic domain of the fungal highly-reducing polyketide synthase squalestatin tetraketide synthase (SQTKS) was developed. Simulated docking of substrates and inhibitors allowed the definition of active site residues involved in catalysis and substrate selectivity. These were investigated in silico with the aim of extending the substrate scope. Residues were identified which limit the substrate selectivity of the SQTKS ER, and these were mutated and the engineered ER domain assayed in vitro. Significant changes to the programming of the mutant SQTKS ER domains were observed allowing the processing of longer and more methylated substrates.

MIGA2 Links Mitochondria, the ER, and Lipid Droplets and Promotes De Novo Lipogenesis in Adipocytes

Physical contact between organelles is vital to the function of eukaryotic cells. Lipid droplets (LDs) are dynamic organelles specialized in lipid storage that interact physically with mitochondria in several cell types. The mechanisms coupling these organelles are, however, poorly understood, and the cell-biological function of their interaction remains largely unknown. Here, we discover in adipocytes that the outer mitochondrial membrane protein MIGA2 links mitochondria to LDs. We identify an amphipathic LD-targeting motif and reveal that MIGA2 binds to the membrane proteins VAP-A or VAP-B in the endoplasmic reticulum (ER). We find that in adipocytes MIGA2 is involved in promoting triglyceride (TAG) synthesis from non-lipid precursors. Our data indicate that MIGA2 links reactions of de novo lipogenesis in mitochondria to TAG production in the ER, thereby facilitating efficient lipid storage in LDs. Based on its presence in many tissues, MIGA2 is likely critical for lipid and energy homeostasis in a wide spectrum of cell types.

Substrate selectivity of an isolated enoyl reductase catalytic domain from an iterative highly reducing fungal polyketide synthase reveals key components of programming

A cis-acting enoyl reductase (ER) catalytic domain was isolated from a fungal highly reducing iterative polyketide synthase (HR-iPKS) for the first time and studied in vitro. The ER from the squalestatin tetraketide synthase forms a discrete dimeric protein in solution. The ER shows broad substrate selectivity, reducing enoyl species including both natural and unnatural substrates. Pantetheine-bound substrate thiolesters reacted much faster than the corresponding SNAC thiolesters. The unnatural substrates included Z-olefins, 2-ethyl olefins and pentaketides. Methylation of the substrate modifies the activity of the ER such that the 2,4-dimethyl oct-2-enoyl substrate fits into the active site but cannot be reduced. A new NMR-based assay was developed for the direct observation of the stereochemical preferences at the 4' position of the NADPH cofactor and the C-2 and C-3 positions of the substrates. The assay reveals that the fungal iPKS ER-catalysed reaction is stereochemically identical to that of the vertebrate FAS (vFAS) at the cofactor 4' position and the substrate 3-position, but the high stereoselectivity displayed by intact SQTKS is lost such that reprotonation at the 2-position is unselective by the isolated ER. A 3D model of ER was consistent with these observations and showed that the ER may sequester its final substrate to prevent further chain extension. The results support a developing model for programming by HR-iPKS in which competition for substrates between restrictive and permissive catalytic domains chaperones the growing polyketide to completion, while allowing for errors and evolution.

Genetic effects of FASN, PPARGC1A, ABCG2 and IGF1 revealing the association with milk fatty acids in a Chinese Holstein cattle population based on a post genome-wide association study

Background

A previous genome-wide association study deduced that one (ARS-BFGL-NGS-39328), two (Hapmap26001-BTC-038813 and Hapmap31284-BTC-039204), two (Hapmap26001-BTC-038813 and BTB-00246150), and one (Hapmap50366-BTA-46960) genome-wide significant single nucleotide polymorphisms (SNPs) associated with milk fatty acids were close to or within the fatty acid synthase (FASN), peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PPARGC1A), ATP-binding cassette, sub-family G, member 2 (ABCG2) and insulin-like growth factor 1 (IGF1) genes. To further confirm the linkage and reveal the genetic effects of these four candidate genes on milk fatty acid composition, genetic polymorphisms were identified and genotype-phenotype associations were performed in a Chinese Holstein cattle population.

Results

Nine SNPs were identified in FASN, among which SNP rs41919985 was predicted to result in an amino acid substitution from threonine (ACC) to alanine (GCC), five SNPs (rs136947640, rs134340637, rs41919992, rs41919984 and rs41919986) were synonymous mutations, and the remaining three (rs41919999, rs132865003 and rs133498277) were found in FASN introns. Only one SNP each was identified for PPARGC1A, ABCG2 and IGF1.

Association studies revealed that FASN, PPARGC1A, ABCG2 and IGF1 were mainly associated with medium-chain saturated fatty acids and long-chain unsaturated fatty acids, especially FASN for C10:0, C12:0 and C14:0. Strong linkage disequilibrium was observed among ARS-BFGL-NGS-39328 and rs132865003 and rs134340637 in FASN (D´ > 0.9), and among Hapmap26001-BTC-038813 and Hapmap31284-BTC-039204 and rs109579682 in PPARGC1A (D´ > 0.9). Subsequently, haplotype-based analysis revealed significant associations of the haplotypes encompassing eight FASN SNPs (rs41919999, rs132865003, rs134340637, rs41919992, rs133498277, rs41919984, rs41919985 and rs41919986) with C10:0, C12:0, C14:0, C18:1n9c, saturated fatty acids (SFA) and unsaturated fatty acids (UFA) (P = 0.0204 to P < 0.0001).

Conclusion

Our study confirmed the linkage between the significant SNPs in our previous genome-wide association study and variants in FASN and PPARGC1A. SNPs within FASN, PPARGC1A, ABCG2 and IGF1 showed significant genetic effects on milk fatty acid composition in dairy cattle, indicating their potential functions in milk fatty acids synthesis and metabolism. The findings presented here provide evidence for the selection of dairy cows with healthier milk fatty acid composition by marker-assisted breeding or genomic selection schemes, as well as furthering our understanding of technological processing aspects of cows’ milk.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-016-0418-x) contains supplementary material, which is available to authorized users.

Crystal Structure and Substrate Specificity of Human Thioesterase 2: INSIGHTS INTO THE MOLECULAR BASIS FOR THE MODULATION OF FATTY ACID SYNTHASE

The type I fatty acid synthase (FASN) is responsible for the de novo synthesis of palmitate. Chain length selection and release is performed by the C-terminal thioesterase domain (TE1). FASN expression is up-regulated in cancer, and its activity levels are controlled by gene dosage and transcriptional and post-translational mechanisms. In addition, the chain length of fatty acids produced by FASN is controlled by a type II thioesterase called TE2 (E.C. 3.1.2.14). TE2 has been implicated in breast cancer and generates a broad lipid distribution within milk. The molecular basis for the ability of the TE2 to compete with TE1 for the acyl chain attached to the acyl carrier protein (ACP) domain of FASN is unknown. Herein, we show that human TE1 efficiently hydrolyzes acyl-CoA substrate mimetics. In contrast, TE2 prefers an engineered human acyl-ACP substrate and readily releases short chain fatty acids from full-length FASN during turnover. The 2.8 Å crystal structure of TE2 reveals a novel capping domain insert within the α/β hydrolase core. This domain is reminiscent of capping domains of type II thioesterases involved in polyketide synthesis. The structure also reveals that the capping domain had collapsed onto the active site containing the Ser-101-His-237-Asp-212 catalytic triad. This observation suggests that the capping domain opens to enable the ACP domain to dock and to place the acyl chain and 4'-phosphopantetheinyl-linker arm correctly for catalysis. Thus, the ability of TE2 to prematurely release fatty acids from FASN parallels the role of editing thioesterases involved in polyketide and non-ribosomal peptide synthase synthases.

Regulating effect of β-ketoacyl synthase domain of fatty acid synthase on fatty acyl chain length in de novo fatty acid synthesis

Fatty acid synthase (FAS) is a multifunctional homodimeric protein, and is the key enzyme required for the anabolic conversion of dietary carbohydrates to fatty acids. FAS synthesizes long-chain fatty acids from three substrates: acetyl-CoA as a primer, malonyl-CoA as a 2 carbon donor, and NADPH for reduction. The entire reaction is composed of numerous sequential steps, each catalyzed by a specific functional domain of the enzyme. FAS comprises seven different functional domains, among which the β-ketoacyl synthase (KS) domain carries out the key condensation reaction to elongate the length of fatty acid chain. Acyl tail length controlled fatty acid synthesis in eukaryotes is a classic example of how a chain building multienzyme works. Different hypotheses have been put forward to explain how those sub-units of FAS are orchestrated to produce fatty acids with proper molecular weight. In the present study, molecular dynamic simulation based binding free energy calculation and access tunnels analysis showed that the C16 acyl tail fatty acid, the major product of FAS, fits to the active site on KS domain better than any other substrates. These simulations supported a new hypothesis about the mechanism of fatty acid production ratio: the geometric shape of active site on KS domain might play a determinate role.

De novo lipogenesis in the liver in health and disease: more than just a shunting yard for glucose

Hepatic de novo lipogenesis (DNL) is the biochemical process of synthesising fatty acids from acetyl‐CoA subunits that are produced from a number of different pathways within the cell, most commonly carbohydrate catabolism. In addition to glucose which most commonly supplies carbon units for DNL, fructose is also a profoundly lipogenic substrate that can drive DNL, important when considering the increasing use of fructose in corn syrup as a sweetener. In the context of disease, DNL is thought to contribute to the pathogenesis of non‐alcoholic fatty liver disease, a common condition often associated with the metabolic syndrome and consequent insulin resistance. Whether DNL plays a significant role in the pathogenesis of insulin resistance is yet to be fully elucidated, but it may be that the prevalent products of this synthetic process induce some aspect of hepatic insulin resistance.

In rat hepatocytes, myristic acid occurs through lipogenesis, palmitic acid shortening and lauric acid elongation

The origin of myristic acid in mammalian cells and the regulation of its endogenous cellular low concentration are not known. Another intriguing question is the potential metabolic properties of endogenous myristic acid as compared with exogenous myristic acid. In the present paper, we hypothesised and demonstrated that, in liver cells, in addition to the usual fatty acid synthase (FAS) pathway that produces predominantly palmitic acid and minor amounts of myristic acid, part of endogenous cellular myristic acid also comes from a shortening of palmitic acid, likely by peroxisomal β-oxidation and from lauric acid by elongation. From a nutritional point of view, C16:0 is universally found in natural fats and its shortening to myristic acid could contribute to a non-negligible source of this fatty acid (FA) in the organism. Then, we measured the distribution of endogenously synthesised myristic acid in lipid species and compared it with that of exogenous myristic acid. Our results do not support the hypothesis of different metabolic fates of endogenous and exogenous myristic acid and suggest that whatever the origin of myristic acid, its cellular concentration and lipid distribution are highly regulated.

Biotechnological potential of the ethylmalonyl-CoA pathway

The ethylmalonyl-CoA pathway is central to the carbon metabolism of many α-proteobacteria, like Rhodobacter sphaeroides and Methylobacterium extorquens as well as actinomycetes, like Streptomyces spp. Its function is to convert acetyl-CoA, a central carbon intermediate, to other precursor metabolites for cell carbon biosynthesis. In contrast to the glyoxylate cycle--another widely distributed acetyl-CoA assimilation strategy--the ethylmalonyl-CoA pathway contains many unique CoA-ester intermediates, such as (2R)- and (2S)-ethylmalonyl-CoA, (2S)-methylsuccinyl-CoA, mesaconyl-(C1)-CoA, and (2R, 3S)-methylmalyl-CoA. With this come novel catalysts that interconvert these compounds. Among these unique enzymes is a novel carboxylase that reductively carboxylates crotonyl-CoA, crotonyl-CoA carboxylase/reductase, and (3S)-malyl-CoA thioesterase. The latter represents the first example of a non-Claisen condensation enzyme of the malate synthase superfamily and defines a new class of thioesterases apart from the hotdog-fold and α/β-fold thioesterases. The biotechnological implications of the ethylmalonyl-CoA pathway are tremendous as one looks to tap into the potential of using these new intermediates and catalysts to produce value-added products.

Structure and function of eukaryotic fatty acid synthases

In all organisms, fatty acid synthesis is achieved in variations of a common cyclic reaction pathway by stepwise, iterative elongation of precursors with two-carbon extender units. In bacteria, all individual reaction steps are carried out by monofunctional dissociated enzymes, whereas in eukaryotes the fatty acid synthases (FASs) have evolved into large multifunctional enzymes that integrate the whole process of fatty acid synthesis. During the last few years, important advances in understanding the structural and functional organization of eukaryotic FASs have been made through a combination of biochemical, electron microscopic and X-ray crystallographic approaches. They have revealed the strikingly different architectures of the two distinct types of eukaryotic FASs, the fungal and the animal enzyme system. Fungal FAS is a 2·6 MDa α₆β₆ heterododecamer with a barrel shape enclosing two large chambers, each containing three sets of active sites separated by a central wheel-like structure. It represents a highly specialized micro-compartment strictly optimized for the production of saturated fatty acids. In contrast, the animal FAS is a 540 kDa X-shaped homodimer with two lateral reaction clefts characterized by a modular domain architecture and large extent of conformational flexibility that appears to contribute to catalytic efficiency.

DNA polymorphisms in bovine fatty acid synthase are associated with beef fatty acid composition

The objective of this study was to identify single nucleotide polymorphisms (SNPs) in the thioesterase (TE) domain of the bovine fatty acid synthase (FASN) gene and to evaluate the extent to which they were associated with beef fatty acid composition. The four exons in FASN that encode for the TE domain were sequenced, and three SNPs, AF285607:g.17924A>G, g.18663T>C and g.18727C>T, were identified. Purebred Angus bulls (n = 331) were classified into three genotype groups, g.17924AA (n = 121), g.17924AG (n = 168) and g.17924GG (n = 42). The g.17924A>G genotype was significantly associated with fatty acid composition of longissimus dorsi muscle of Angus bulls. Cattle with the g.17924GG genotype had lower myristic acid (C14:0; P < 0.0001), palmitic acid (C16:0, P < 0.05) and total saturated fatty acid contents (P < 0.01), greater health index (P < 0.001), oleic acid content (C18:1; P < 0.001) and total monounsaturated fatty acid concentration (P < 0.01) in the total lipids and triacylglycerols fraction than did those with the g.17924AA genotype. Because of the linkage disequilibrium between SNPs g.17924A>G and g.18663T>C, similar significant associations of fatty acid contents with the g.18663T>C genotypes were observed. In conclusion, the SNPs g.17924A>G and g.18663T>C may be used as DNA markers to select breeding stock that have a healthier fatty acid composition.

The type I fatty acid and polyketide synthases: a tale of two megasynthases

This review chronicles the synergistic growth of the fields of fatty acid and polyketide synthesis over the last century. In both animal fatty acid synthases and modular polyketide synthases, similar catalytic elements are covalently linked in the same order in megasynthases. Whereas in fatty acid synthases the basic elements of the design remain immutable, guaranteeing the faithful production of saturated fatty acids, in the modular polyketide synthases, the potential of the basic design has been exploited to the full for the elaboration of a wide range of secondary metabolites of extraordinary structural diversity.

Crystal structure of the thioesterase domain of human fatty acid synthase inhibited by Orlistat

Human fatty acid synthase (FAS) is uniquely expressed at high levels in many tumor types. Pharmacological inhibition of FAS therefore represents an important therapeutic opportunity. The drug Orlistat, which has been approved by the US Food and Drug Administration, inhibits FAS, induces tumor cell-specific apoptosis and inhibits the growth of prostate tumor xenografts. We determined the 2.3-A-resolution crystal structure of the thioesterase domain of FAS inhibited by Orlistat. Orlistat was captured in the active sites of two thioesterase molecules as a stable acyl-enzyme intermediate and as the hydrolyzed product. The details of these interactions reveal the molecular basis for inhibition and suggest a mechanism for acyl-chain length discrimination during the FAS catalytic cycle. Our findings provide a foundation for the development of new cancer drugs that target FAS.

Human fatty acid synthase: structure and substrate selectivity of the thioesterase domain

Human fatty acid synthase is a large homodimeric multifunctional enzyme that synthesizes palmitic acid. The unique carboxyl terminal thioesterase domain of fatty acid synthase hydrolyzes the growing fatty acid chain and plays a critical role in regulating the chain length of fatty acid released. Also, the up-regulation of human fatty acid synthase in a variety of cancer makes the thioesterase a candidate target for therapeutic treatment. The 2.6-A resolution structure of human fatty acid synthase thioesterase domain reported here is comprised of two dissimilar subdomains, A and B. The smaller subdomain B is composed entirely of alpha-helices arranged in an atypical fold, whereas the A subdomain is a variation of the alpha/beta hydrolase fold. The structure revealed the presence of a hydrophobic groove with a distal pocket at the interface of the two subdomains, which constitutes the candidate substrate binding site. The length and largely hydrophobic nature of the groove and pocket are consistent with the high selectivity of the thioesterase for palmitoyl acyl substrate. The structure also set the identity of the Asp residue of the catalytic triad of Ser, His, and Asp located in subdomain A at the proximal end of the groove.

Structural and functional organization of the animal fatty acid synthase

The entire pathway of palmitate synthesis from malonyl-CoA in mammals is catalyzed by a single, homodimeric, multifunctional protein, the fatty acid synthase. Each subunit contains three N-terminal domains, the beta-ketoacyl synthase, malonyl/acetyl transferase and dehydrase separated by a structural core from four C-terminal domains, the enoyl reductase, beta-ketoacyl reductase, acyl carrier protein and thiosterase. The kinetics and specificities of the substrate loading reaction catalyzed by the malonyl/acetyl transferase, the condensation reaction catalyzed by beta-ketoacyl synthase and chain-terminating reaction catalyzed by the thioesterase ensure that intermediates do not leak off the enzyme, saturated chains exclusively are elongated and palmitate is released as the major product. Only in the fatty acid synthase dimer do the subunits adopt conformations that facilitate productive coupling of the individual reactions for fatty acid synthesis at the two acyl carrier protein centers. Introduction of a double tagging and dual affinity chromatographic procedure has permitted the engineering and isolation of heterodimeric fatty acid synthases carrying different mutations on each subunit. Characterization of these heterodimers, by activity assays and chemical cross-linking, has been exploited to map the functional topology of the protein. The results reveal that the two acyl carrier protein domains engage in substrate loading and condensation reactions catalyzed by the malonyl/acetyl transferase and beta-ketoacyl synthase domains of either subunit. In contrast, the reactions involved in processing of the beta-carbon atom, following each chain elongation step, together with the release of palmitate, are catalyzed by the cooperation of the acyl carrier protein with catalytic domains of the same subunit. These findings suggest a revised model for the fatty acid synthase in which the two polypeptides are oriented such that head-to-tail contacts are formed both between and within subunits.

Straight-chain acyl-CoA oxidase knockout mouse accumulates extremely long chain fatty acids from alpha-linolenic acid: evidence for runaway carousel-type enzyme kinetics in peroxisomal beta-oxidation diseases

Extremely long chain polyunsaturated fatty acids (ELCPs) with >24 carbons and four or more double bonds are normally found in excitatory tissues but have no known function, and are greatly increased in brain and other tissues of humans with peroxisomal disorders. Straight-chain acyl-CoA oxidase (AOX) catalyzes the first, rate-limiting step of peroxisomal beta-oxidation of very-long-chain saturated and unsaturated fatty acids. We have studied the polyunsaturated fatty acid metabolism of AOX knockout mice (AOX-/- as a model of human AOX deficiency (pseudo-neonatal adrenoleukodystrophy), and as a genetic tool to test the putative peroxisomal beta-oxidation involvement in polyunsaturated fatty acid synthesis. Liver lipids of 26-day-old weanling AOX-/- mice livers accumulate n-3 and n-6 ELCPs from C24 to C30 with 5 and 6 double bonds, have 356 +/- 66 microg/g docosahexaenoic acid (22:6n-3), similar to congenic (AOX -/* = AOX+/+ and AOX+/-) controls (401 +/- 96 microg/g), but increased 22:5n-6 (22.4 +/- 3.7 vs 6.4 +/- 1.5 microg/g). AOX+/* mice injected intraperitoneally at 23 days with [U-(13)C]-18:3n-3 show strong labeling of 22:6n-3 after 72 h, whereas AOX -/- mice display less labeling of 22:6n-3 but strong tracer incorporation into 24:6n-3, 26:6n-3, and 28:6n-3, after the same period. These data suggest that ELCPs are natural runaway elongation by-products of 22:6n-3 and 22:5n-6 synthesis, which are normally disposed of by peroxisomal beta-oxidation. Under conditions with impaired peroxisomal beta-oxidation, such as Zellweger syndrome and adrenoleukodystrophies, ELCPs accumulate due to increased synthesis and impaired disposal. Two mechanisms for the formation of these runaway elongation by-products and the involvement of secondary carnitine deficiency in this process are proposed: n-3 ELCPs are synthesized by a carnitine-dependent multifunctional mitochondrial docosahexaenoic acid synthase (mtDHAS) which normally synthesizes primarily 22:6n-3, while n-6 ELCPs are synthesized by independent elongation enzymes in the endoplasmic reticulum.

The peroxisome proliferator-induced cytosolic type I acyl-CoA thioesterase (CTE-I) is a serine-histidine-aspartic acid alpha /beta hydrolase

Long-chain acyl-CoA thioesterases hydrolyze long-chain acyl-CoAs to the corresponding free fatty acid and CoASH and may therefore play important roles in regulation of lipid metabolism. We have recently cloned four members of a highly conserved acyl-CoA thioesterase multigene family expressed in cytosol (CTE-I), mitochondria (MTE-I), and peroxisomes (PTE-Ia and -Ib), all of which are regulated via the peroxisome proliferator-activated receptor alpha (Hunt, M. C., Nousiainen, S. E. B., Huttunen, M. K., Orii, K. E., Svensson, L. T., and Alexson, S. E. H. (1999) J. Biol. Chem. 274, 34317-34326). Sequence comparison revealed the presence of putative active-site serine motifs (GXSXG) in all four acyl-CoA thioesterases. In the present study we have expressed CTE-I in Escherichia coli and characterized the recombinant protein with respect to sensitivity to various amino acid reactive compounds. The recombinant CTE-I was inhibited by phenylmethylsulfonyl fluoride and diethyl pyrocarbonate, suggesting the involvement of serine and histidine residues for the activity. Extensive sequence analysis pinpointed Ser(232), Asp(324), and His(358) as the likely components of a catalytic triad, and site-directed mutagenesis verified the importance of these residues for the catalytic activity. A S232C mutant retained about 2% of the wild type activity and incubation with (14)C-palmitoyl-CoA strongly labeled this mutant protein, in contrast to wild-type enzyme, indicating that deacylation of the acyl-enzyme intermediate becomes rate-limiting in this mutant protein. These data are discussed in relation to the structure/function of acyl-CoA thioesterases versus acyltransferases. Furthermore, kinetic characterization of recombinant CTE-I showed that this enzyme appears to be a true acyl-CoA thioesterase being highly specific for C(12)-C(20) acyl-CoAs.

Backbone dynamics of Escherichia coli thioesterase/protease I: evidence of a flexible active-site environment for a serine protease

Escherichia coli thioesterase/protease I (TEP-I) is a member of a novel subclass of the lipolytic enzymes with a distinctive GDSLS motif. In addition to possessing thioesterase and protease activities, TEP-I also exhibits arylesterase activity. We have determined the (15)N nuclear magnetic spin relaxation rates, R(1) and R(2), and the steady state (1)H-(15)N heteronuclear Overhauser effect, measured at both 11.74 T and 14.09 T, of (u-(15)N) TEP-I. These data were analyzed using model-free formalism (with axially symmetric rotational diffusion anisotropy) to extract the backbone dynamics of TEP-I. The results reveal that the core structure of the central beta-sheet and the long alpha-helices are rigid, while the binding pocket appears to be rather flexible. The rigid core serves as a scaffold to anchor the essential loops, which form the binding pocket. The most flexible residues display large amplitude fast (ps/ns time-scale) motion and lie on one stripe whose orientation is presumed to be the ligand-binding orientation. We also detected the presence of several residues displaying slow (microseconds/ms time-scale) conformational exchanging processes. These residues lie around the binding pocket and are oriented perpendicularly to the orientation of the flexible stripe. Two of the putative catalytic triads, Ser10 and His157, and their neighbors show motion on the microseconds/ms time-scale, suggesting that their slow motion may have a role in catalysis, in addition to their possible roles in ligand binding. The presence of a flexible substrate-binding pocket may also facilitate binding to a wide range of substrates and confer the versatile functional property of this protein.

Role of type II thioesterases: evidence for removal of short acyl chains produced by aberrant decarboxylation of chain extender units

BACKGROUND

Modular polyketide synthases (PKSs) function as molecular assembly lines in which polyketide chains are assembled by successive addition of chain extension units. At the end of the assembly line, there is usually a covalently linked type I thioesterase domain (TE I), which is responsible for release of the completed acyl chain from its covalent link to the synthase. Additionally, some PKS clusters contain a second thioesterase gene (TE II) for which there is no established role. Disruption of the TE II genes from several PKS clusters has shown that the TE II plays an important role in maintaining normal levels of antibiotic production. It has been suggested that the TE II fulfils this role by removing aberrant intermediates that might otherwise block the PKS complex.

RESULTS

We show that recombinant tylosin TE II behaves in vitro as a TE towards a variety of N-acetylcysteamine and p-nitrophenyl esters. The trends of hydrolytic activity determined by the kinetic parameter k(cat)/K(M) for the analogues tested indicates that simple fatty acyl chains are effective substrates. Analogues that modelled aberrant forms of putative tylosin biosynthetic intermediates were hydrolysed at low rates.

CONCLUSIONS

The behaviour of tylosin TE II in vitro is consistent with its proposed role as an editing enzyme. Aberrant decarboxylation of a malonate-derived moiety attached to an acyl carrier protein (ACP) domain may generate an acetate, propionate or butyrate residue on the ACP thiol. Our results suggest that removal of such groups is a significant role of TE II.

Triacylglycerol synthesis by purified triacylglycerol synthetase of rat intestinal mucosa. Role of acyl-CoA acyltransferase

The activities of the proposed triacylglycerol synthetase complex, acyl-CoA ligase, acyl-CoA acyltransferase (AAT), monoacylglycerol acyltransferase (MGAT), and diacylglycerol acyltransferase (DGAT), coeluted upon Cibacron blue 3GA-agarose affinity chromatography of detergent-solubilized rat intestinal microsomes. The AAT activity is associated with a 54-kDa protein, that binds covalently an acyl group from acyl-CoA via a thiol ester linkage (Lehner, R. and Kuksis, A. (1993) J. Biol. Chem. 268, 24726-24733). Reagents that prevent the acyl-AAT formation inhibit triacylglycerol synthesis as does the removal of AAT from the complex by immunoprecipitation. In the absence of mono- and diacylglycerol acceptors, the acyl group is transferred to water. It is proposed that triacylglycerol synthesis proceeds via a sequential transfer of acyl groups from acyl-CoA ligase to the AAT, from which they are passed to the mono- and diacylglycerol acyltransferases for incorporation into the di- and triacylglycerols depending on the availability of the acyl acceptors.

Crystallization and preliminary diffraction studies of thioesterase II from rat mammary gland

Thioesterase II from rat mammary gland has been crystallized in the presence of decanoic acid by the vapor diffusion method. The crystals belong to the orthorhombic space group P2(1)2(1)2(1), and have cell dimensions, a = 52.7 A, b = 78.0 A, and c = 133.6 A. The asymmetric unit likely consists of two protein monomers based on predictions from its calculated Matthews coefficient. Crystals typically diffract to at least 2.5 A resolution and are suitable for X-ray crystallographic analysis.

Characterization of microsomal long-chain acyl-CoA hydrolase activity in the rat submandibular gland

1. Long-chain fatty acyl-CoA hydrolase in submandibular gland microsomes was characterized and compared to that in liver ones. 2. In rat submandibular gland, the microsomal long-chain acyl-CoA hydrolase showed a higher relative activity for polyunsaturated fatty acyl-CoAs than that of liver microsomes. 3. It was suggested that the hydrolase in rat submandibular gland microsomes may play a role in modulation in the acyl-CoA pool size.

Regulated expression of the rat medium chain hydrolase gene in transgenic rape seed

Medium chain hydrolase (MCH) is an enzyme which regulates the chain length of fatty acid synthesis specifically in the mammary gland of the rat. During lactation, MCH interacts with fatty acid synthase (FAS) to cause premature release of acyl chains, thus providing medium chain fatty acids for synthesis of milk fat. In this study we have investigated the ability of rat MCH to interact with the phylogenetically more distant FAS structure present in plant systems and to cause a perturbation of fatty acid synthesis. In in vitro experiments, addition of purified MCH to rapeseed homogenates was found to cause a significant perturbation of fatty acid synthesis towards medium chain length products. The rat MCH gene was expressed in transgenic oilseed rape using a seed specific rape acyl carrier protein (ACP) promoter and a rape ACP plastid targeting sequence. Western analysis showed MCH protein to be present in transgenic seed and for its expression to be developmentally regulated in concert with storage lipid synthesis. The chimaeric preprotein was correctly processed and immunogold labelling studies confirmed MCH to be localized within plastid organelles. However, fatty acid analysis of oil from MCH-expressing rape seed showed no significant differences to that from control seed.

Human mammary gland function at the onset of lactation: medium-chain fatty acid synthesis

The onset of medium-chain fatty acid synthesis in the human mammary gland was investigated. Colostrum and serum were collected from 31 healthy women and the fatty acid composition of total lipid was analyzed by gas-liquid chromatography. Although colostrum/serum ratios for most fatty acids range from 0.7-2.4, very low levels of 10:0 and 12:0 were present in serum lipids as compared to much higher concentrations of these fatty acids in colostrum lipids (colostrum/serum ratio 16.23 and 17.11 for 10:0 and 12:0, respectively). We have previously found that medium-chain fatty acid levels are very low in prepartum mammary secretions (6-10 wk before term delivery) but are higher and similar in colostrum of women who deliver preterm (3-14 wk) or at full term. The data indicate that parturition, irrespective of length of pregnancy, is the trigger for medium-chain fatty acid synthesis in the human mammary gland.

Cytosolic long-chain acyl-CoA hydrolase, a suitable parameter to measure hepatic response to peroxisome proliferators

The possibility of using cytosolic long-chain acyl-CoA as a parameter to measure the response of liver to peroxisome proliferators was studied. A subcutaneous (s.c.) injection of perfluorooctanoic acid (PFOA) to male Wistar rats caused an increase in activity of cytosolic long-chain acyl-CoA hydrolase. This increase in activity seems to be due to enzyme induction, since it was prevented by simultaneous administration of cycloheximide or actinomycin D with PFOA. The activity of cytosolic long-chain acyl-CoA hydrolase was increased in a dose-dependent manner by the administration of three peroxisome proliferators with diverse chemical structures: alpha-(p-chlorophenoxy)isobutyric acid (clofibric acid), 2,2'-(decamethylenedithio)diethanol (tiadenol) and PFOA. The increased activity produced by clofibric acid lasted throughout a 22-week treatment. A good correlation was found between the activities of cytosolic long-chain acyl-CoA hydrolase and peroxisomal beta-oxidation induced by the administration of the peroxisome proliferators. These results indicate that cytosolic long-chain acyl-CoA hydrolase is a suitable parameter for measuring the response of rat liver to challenges by peroxisome proliferators.

The effect of solvent viscosity on the rate-determining step of fatty acid synthetase

The overall activity of an animal fatty acid synthetase at the saturation level of substrate concentration decreased when the solvent viscosity, eta, of the reaction mixture was increased with viscogens such as glycerol, sucrose, and polyethylene glycol. The activity of the enzyme changed roughly proportional to eta-P, where p = 1.0 for glycerol, p = 0.66 for sucrose, and p less than 0.6 for polyethylene glycol with different molecular sizes. The thioesterase activity, which catalyzes the final partial reaction in the multifunctional enzyme, was not affected by 5-fold increase of solvent viscosity with sucrose. These results suggested that the rate-determining step of the enzyme other than the thioesterase reaction involves a microscopic transport step, the rate of which is influenced by the solvent viscosity. The microscopic transport step may be related to the transfer of the reaction intermediate from one active site to another or to the motion of a larger part of the enzyme requisite for the catalytic reaction. In the solution containing glycerol, the rate-determining motion was primarily diffusion limited since the inverse of the initial rate was proportional to eta, i.e., p = 1. Since the substrate concentration was at a saturation level in this experiment, the viscosity-dependent step cannot be the encounter between the enzyme and substrates, but must be intramolecular in origin, most probably the reaction catalyzed by beta-ketoacyl synthetase. In solutions containing other viscogens, however, p was less than 1.0, indicating a significant involvement of chemical steps in the rate-determining step as well. Bovine serum albumin, when used as a proteinic viscogen, also decreased the initial rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification of long-chain acyl-CoA hydrolase from bovine heart microsomes and regulation of activity by lysophospholipids

Long-chain acyl-CoA hydrolase (EC 3.1.2.2) has been purified 12,000-fold from bovine heart muscle microsomes by extraction with Miranol detergent, followed by column chromatography on Reactive Blue agarose and DEAE-cellulose. The purified enzyme was nearly homogeneous on polyacrylamide gel electrophoresis and had a molecular weight of 41,000 in the presence of dodecyl sulfate. The specificity and kinetic properties of the enzyme were studied using several acyl-CoA derivatives as potential substrates. The enzyme showed a wide degree of specificity with little dependence on either the fatty acyl chain length or the degree of unsaturation of the acyl group. The kinetic properties were in accord with the Michaelis-Menten equation under most conditions, although high concentrations of substrates generally inhibited the enzyme. Arachidonoyl-CoA, which was the most effective substrate, had a Km value of 0.4 microM and a Vmax value of 6.0 mumol min-1 mg-1. The enzyme was strongly and specifically inhibited by constants of 16 and 30 nM, respectively. Other lysolipids and detergents such as deoxycholate and Triton X-100 were weak inhibitors. These properties and others distinguish this enzyme from other acyl-CoA hydrolases and support the idea that lysophospholipids may be important in vivo in the regulation of lipid metabolism.

Amino acid sequence of the serine active-site region of the medium-chain S-acyl fatty acid synthetase thioester hydrolase from rat mammary gland

Medium-chain S-acyl fatty acid synthetase thioester hydrolase (thioesterase II), a discrete monomeric enzyme of 29 kDa, regulates the product specificity of the de novo lipogenic systems in certain specialized mammalian and avian tissues, such as mammary and uropygial glands. The amino acid sequence of a 57-residue region containing the active site of the rat mammary gland enzyme has been established by a combination of amino acid and cDNA sequencing. Thioesterase II was radiolabeled with the serine esterase inhibitor [1,3-14C]diisopropyl-fluorophosphate and digested sequentially with cyanogen bromide, Staphylococcus aureus V8 protease and trypsin. A radiolabeled tryptic peptide was isolated and sequenced by automated Edman degradation and the location of the active-site residue established. The amino acid sequence was confirmed by sequencing an overlapping, unlabeled peptide, obtained by V8 digestion of the whole enzyme, and by dideoxynucleotide sequencing of a thioesterase II cDNA clone isolated from a lambda gt11 expression library. The active center contains the motif Gly-Xaa-Ser-Xaa-Gly, characteristic of the serine esterase family of enzymes. A seven-residue region around the essential serine of the rat mammary thioesterase II, Phe-Gly-Met-Ser-Phe-Gly-Ser, is completely homologous with a region of the mallard uropygial thioesterase, recently analyzed by cDNA sequencing, indicating that this is likely to be the active site of the avian enzyme. Overall homology between the mammalian and avian enzymes for the 57-amino-acid residue region is 47% and suggests that the two enzymes may share a common evolutionary origin.