Mammalian fatty acid synthetase is a structurally and functionally symmetrical dimer

Probing the selectivity and protein·protein interactions of a nonreducing fungal polyketide synthase using mechanism-based crosslinkers

Protein·protein interactions, which often involve interactions among an acyl carrier protein (ACP) and ACP partner enzymes, are important for coordinating polyketide biosynthesis. However, the nature of such interactions is not well understood, especially in the fungal nonreducing polyketide synthases (NR-PKSs) that biosynthesize toxic and pharmaceutically important polyketides. Here, we employ mechanism-based crosslinkers to successfully probe ACP and ketosynthase (KS) domain interactions in NR-PKSs. We found that crosslinking efficiency is closely correlated with the strength of ACP·KS interactions and that KS demonstrates strong starter unit selectivity. We further identified positively charged surface residues by KS mutagenesis, which mediates key interactions with the negatively charged ACP surface. Such complementary/matching contact pairs can serve as "adapter surfaces" for future efforts to generate new polyketides using NR-PKSs.

AIMing at metabolic syndrome. -Towards the development of novel therapies for metabolic diseases via apoptosis inhibitor of macrophage (AIM).-

Metabolic syndrome (MetS) is a cascade of metabolic diseases, starting with obesity and progressing to atherosclerosis, and is often fatal because of serious cardiovascular problems such as heart/brain infarction and hemorrhage. Accumulating evidence has revealed a critical involvement of inflammatory responses triggered by lesional macrophages in the pathogenesis of MetS. Importantly, we found that macrophages are associated with disease progression, not only in the induction of inflammation but also in the production of apoptosis inhibitor of macrophages (AIM), which we initially identified as a soluble factor expressed by macrophages. In atherosclerotic plaques, AIM is highly expressed by foam macrophages and inhibits apoptosis of these cells, which results in the accumulation of macrophages, causing inflammatory responses within the lesion, and ultimately disease progression. In adipose tissue, macrophage-derived AIM is incorporated into adipocytes through CD36-mediated endocytosis, thereby reducing the activity of cytosolic fatty acid synthase. This unique response stimulates lipolysis, resulting in a decrease in adipocyte size, which is physiologically relevant to the prevention of obesity. The lipolytic response also stimulates inflammation of adipocytes in association with the induction of metabolic disorders subsequent to obesity. Thus, AIM is involved in the progression of MetS in both an advancing and inhibitory fashion. Regulation of AIM could therefore be therapeutically applicable for MetS.

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.

Macrophage-derived AIM is endocytosed into adipocytes and decreases lipid droplets via inhibition of fatty acid synthase activity

Macrophages infiltrate adipose tissue in obesity and are involved in the induction of inflammation, thereby contributing to the development of obesity-associated metabolic disorders. Here, we show that the macrophage-derived soluble protein AIM is endocytosed into adipocytes via CD36. Within adipocytes, AIM associates with cytosolic fatty acid synthase (FAS), thereby decreasing FAS activity. This decreases lipid droplet size, stimulating the efflux of free fatty acids and glycerol from adipocytes. As an additional consequence of FAS inhibition, AIM prevents preadipocyte maturation. In vivo, the increase in adipocyte size and fat weight induced by high-fat diet (HFD) was accelerated in AIM-deficient (AIM(-)(/-)) mice compared to AIM(+/+) mice. Moreover, injection of recombinant AIM in AIM(-)(/-) mice suppresses the increase in fat mass induced by HFD. Interestingly, metabolic rates are comparable in AIM(-)(/-) and AIM(+/+) mice, suggesting that AIM specifically influences adipocyte status. Thus, this AIM function in adipocytes may be physiologically relevant to obesity progression.

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.

The structure of mammalian fatty acid synthase turned back to front

On page 1667 of this issue, Stuart Smith and colleagues [1] demonstrate that the animal fatty acid synthase is a head-to-head dimer rather than the head-to-tail dimer depicted in textbooks. This has important ramifications for the mechanisms of other multifunctional enzymes such as polyketide synthases [2].

Head-to-Head Coiled Arrangement of the Subunits of the Animal Fatty Acid Synthase

The role of the beta-ketoacyl synthase domains in dimerization of the 2505 residue subunits of the multifunctional animal FAS has been evaluated by a combination of crosslinking and characterization of several truncated forms of the protein. Polypeptides containing only the N-terminal 971 residues can form dimers, but polypeptides lacking only the N-terminal 422 residue beta-ketoacyl synthase domain cannot. FAS subunits can be crosslinked with spacer lengths as short as 6 A, via cysteine residues engineered near the N terminus of the full-length polypeptides. The proximity of the N-terminal beta-ketoacyl synthase domains and their essential role in dimerization is consistent with a revised model for the FAS in which a head-to-head arrangement of two coiled subunits facilitates functional interactions between the dimeric beta-ketoacyl synthase and the acyl carrier protein domains of either subunit.

Experimental Verification of Conformational Variation of Human Fatty Acid Synthase as Predicted by Normal Mode Analysis

Fatty acid synthase (FAS) is a 550 kDa homodimeric enzyme with multiple functional and structural domains. Normal mode analysis of a previously determined 19 A structure of FAS suggested that this enzyme might assume different conformational states with several distinct hinge movements. We have used a simultaneous multiple-model refinement method to search for the presence of the structural conformers from the electron images of FAS. We have demonstrated that the resulting models observed in the electron images are consistent with the predicted conformational changes. This technique demonstrates the potential of the combination of normal mode analysis with multiple model refinement to elucidate the multiple conformations of flexible proteins. Since each of these structures is based on a more homogeneous particle set, this technique has the potential, provided that sufficient references are used, to improve the resolution of the final reconstructions of single particles from electron cryomicroscopy.

Analysis of the biosynthetic gene cluster for the polyether antibiotic monensin in Streptomyces cinnamonensis and evidence for the role of monB and monC genes in oxidative cyclization

The analysis of a candidate biosynthetic gene cluster (97 kbp) for the polyether ionophore monensin from Streptomyces cinnamonensis has revealed a modular polyketide synthase composed of eight separate multienzyme subunits housing a total of 12 extension modules, and flanked by numerous other genes for which a plausible function in monensin biosynthesis can be ascribed. Deletion of essentially all these clustered genes specifically abolished monensin production, while overexpression in S. cinnamonensis of the putative pathway-specific regulatory gene monR led to a fivefold increase in monensin production. Experimental support is presented for a recently-proposed mechanism, for oxidative cyclization of a linear polyketide intermediate, involving four enzymes, the products of monBI, monBII, monCI and monCII. In frame deletion of either of the individual genes monCII (encoding a putative cyclase) or monBII (encoding a putative novel isomerase) specifically abolished monensin production. Also, heterologous expression of monCI, encoding a flavin-linked epoxidase, in S. coelicolor was shown to significantly increase the ability of S. coelicolor to epoxidize linalool, a model substrate for the presumed linear polyketide intermediate in monensin biosynthesis.

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.

Engineering of an active animal fatty acid synthase dimer with only one competent subunit

Animal fatty acid synthases are large polypeptides containing seven functional domains that are active only in the dimeric form. Inactivity of the monomeric form has long been attributed to the obligatory participation of domains from both subunits in catalysis of substrate loading and condensation reactions. However, we have engineered a fatty acid synthase containing one wild-type subunit and one subunit compromised by mutations in all seven functional domains that is active in fatty acid synthesis. This finding indicates that a single subunit, in the context of a dimer, is able to catalyze the entire biosynthetic pathway and suggests that, in the natural complex, each of the two subunits forms a scaffold that optimizes the conformation of the companion subunit.

Quaternary structure of human fatty acid synthase by electron cryomicroscopy

We present the first three-dimensional reconstruction of human fatty acid synthase obtained by electron cryomicroscopy and single-particle image processing. The structure shows that the synthase is composed of two monomers, arranged in an antiparallel orientation, which is consistent with biochemical data. The monomers are connected to each other at their middle by a bridge of density, a site proposed to be the combination of the interdomain regions of the two monomers. Each monomer subunit appears to be subdivided into three structural domains. With this reconstruction of the synthase, we propose a location for the enzyme's two fatty acid synthesis sites.

Mapping the functional topology of the animal fatty acid synthase by mutant complementation in vitro

An in vitro mutant complementation approach has been used to map the functional topology of the animal fatty acid synthase. A series of knockout mutants was engineered, each mutant compromised in one of the seven functional domains, and heterodimers generated by hybridizing all possible combinations of the mutated subunits were isolated and characterized. Heterodimers comprised of a subunit containing either a beta-ketoacyl synthase or malonyl/acetyltransferase mutant, paired with a subunit containing mutations in any one of the other five domains, are active in fatty acid synthesis. Heterodimers in which both subunits carry a knockout mutation in either the dehydrase, enoyl reductase, keto reductase, or acyl carrier protein are inactive. Heterodimers comprised of a subunit containing a thioesterase mutation paired with a subunit containing a mutation in either the dehydrase, enoyl reductase, beta-ketoacyl reductase, or acyl carrier protein domains exhibit very low fatty acid synthetic ability. The results are consistent with a model for the fatty acid synthase in which the substrate loading and condensation reactions are catalyzed by cooperation of an acyl carrier protein domain of one subunit with the malonyl/acetyltransferase or beta-ketoacyl synthase domains, respectively, of either subunit. The beta-carbon-processing reactions, responsible for the complete reduction of the beta-ketoacyl moiety following each condensation step, are catalyzed by cooperation of an acyl carrier protein domain with the beta-ketoacyl reductase, dehydrase, and enoyl reductase domains associated exclusively with the same subunit. The chain-terminating reaction is carried out most efficiently by cooperation of an acyl carrier protein domain with the thioesterase domain of the same subunit. These results are discussed in the context of a revised model for the fatty acid synthase.

Dibromopropanone cross-linking of the phosphopantetheine and active-site cysteine thiols of the animal fatty acid synthase can occur both inter- and intrasubunit. Reevaluation of the side-by-side, antiparallel subunit model

The objective of this study was to test a new model for the homodimeric animal FAS which implies that the condensation reaction can be catalyzed by the amino-terminal beta-ketoacyl synthase domain in cooperation with the penultimate carboxyl-terminal acyl carrier protein domain of either subunit. Treatment of animal fatty acid synthase dimers with dibromopropanone generates three new molecular species with decreased electrophoretic mobilities; none of these species are formed by fatty acid synthase mutant dimers lacking either the active-site cysteine of the beta-ketoacyl synthase domain (C161A) or the phosphopantetheine thiol of the acyl carrier protein domain (S2151A). A double affinity-labeling strategy was used to isolate dimers that carried one or both mutations on one or both subunits; the heterodimers were treated with dibromopropanone and analyzed by a combination of sodium dodecyl sulfate/polyacrylamide gel electrophoresis, Western blotting, gel filtration, and matrix-assisted laser desorption mass spectrometry. Thus the two slowest moving of these species, which accounted for 45 and 15% of the total, were identified as doubly and singly cross-linked dimers, respectively, whereas the fastest moving species, which accounted for 35% of the total, was identified as originating from internally cross-linked subunits. These results show that the two polypeptides of the fatty acid synthase are oriented such that head-to-tail contacts are formed both between and within subunits, and provide the first structural evidence in support of the new model.

Fatty acid synthase dimers containing catalytically active beta-ketoacyl synthase or malonyl/acetyltransferase domains in only one subunit can support fatty acid synthesis at the acyl carrier protein domains of both subunits

A double-tagging, dual affinity chromatographic procedure, which permits isolation of dimers independently mutated in each subunit, has been exploited to probe the functional topology of the animal fatty acid synthase. Dimers were engineered in which the chain-terminating thioesterase reaction was compromised by mutation of the (active-site) serine residue in both subunits; these dimers assembled two long-chain fatty acyl moieties, which remained covalently linked to the 4'-phosphopantetheine residues of the two acyl carrier protein domains. Significantly, dimers that contained an additional mutation that compromised the activity of either the beta-ketoacyl synthase or malonyl/acetyltransferase activity in only one subunit also assembled two long-chain acyl moieties. In contrast, in a control experiment, introduction of an additional mutation that compromised the function of the acyl carrier protein domain in only one subunit resulted in the assembly of only one long-chain acyl moiety per dimer. Because the beta-ketoacyl synthase and malonyl/acetyltransferase domains are located near the amino terminus of the polypeptide and the acyl carrier protein domain near the carboxyl terminus, these results support a modified model for the animal fatty acid synthase in which head-to-tail functional contacts are possible both within as well as between subunits.

Differential affinity labeling of the two subunits of the homodimeric animal fatty acid synthase allows isolation of heterodimers consisting of subunits that have been independently modified

To explore the domain interactions that are required for catalytic activity of the multifunctional, homodimeric fatty acid synthase (FAS), we have formulated a strategy that allows isolation of modified dimers containing independently mutated subunits. Either a hexahistidine or a FLAG octapeptide tag was incorporated into the FAS at either the amino terminus, within an internal noncatalytic domain, or at the carboxyl terminus. The presence of the tags had no effect on the activity of the wild-type FAS. His-tagged dimers were mixed with FLAG-tagged dimers, and the subunits were randomized to produce a mixture of His-tagged homodimers, FLAG-tagged homodimers, and doubly tagged heterodimers. The doubly tagged heterodimers could be purified to homogeneity by chromatography on an anti-FLAG immunoaffinity column followed by a metal ion chelating column. This procedure for isolation of FAS heterodimers was utilized to determine whether the two centers for fatty acid synthesis in the FAS dimer can function independently of each other. Doubly tagged heterodimers, consisting of one wild-type subunit and one subunit in which the thioesterase activity had been eliminated, either by mutation or by treatment with phenylmethanesulfonyl fluoride, have 50% of the wild-type thioesterase activity and, in the presence of substrates, accumulate a long chain fatty acyl moiety on the modified subunit, thus blocking further substrate turnover at this center. Nevertheless, the ability of the heterodimer to synthesize fatty acids is also 50% of the wild-type FAS, demonstrating that an individual center for fatty acid synthesis has the same activity when paired with either a functional or nonfunctional catalytic center.

Fatty acid synthase: in vitro complementation of inactive mutants

The animal fatty acid synthase is a dimer of identical, multifunctional 272 kDa subunits oriented antiparallel such that two centers for fatty acid synthesis are formed at the subunit interface. In order to clarify the interdomain and intersubunit communications necessary for the operation of the two centers, we have explored the possibility of reassembling catalytically-active fatty acid synthase heterodimers from pairs of inactive dimers carrying mutations in different functional domains. To this end, rat fatty acid synthase mutants, defective in either the beta-ketoacyl synthase, C161T or K326A (KS- FAS), or the acyl carrier protein, S2151A (ACP- FAS), domains, were engineered by site-directed mutagenesis, expressed in insect Sf9 cells using a baculovirus expression system, and purified. A novel procedure was devised to facilitate rapid production and isolation of a population of mixed mutant dimers that had undergone randomization of its constituent subunits. Homodimeric mutants (KS- FAS/KS- FAS and ACP- FAS/ACP- FAS) and KS- FAS heterodimers consisting of paired C161T and K326A mutant subunits were unable to synthesize fatty acids, confirming the essential nature of residues C161, K326, and S2151A. However, KS- FAS/ACP- FAS heterodimers regained partial activity. Formation of these heterodimers necessitated prior dissociation and reassociation of the homodimers, indicating that the rate of spontaneous exchange of subunits in the dimer is negligible. The formation of catalytically-active heterodimers from pairs of inactive, complementary homodimers affords a useful method for testing the validity of the current model for the multifunctional complex.

The Escherichia coli malonyl-CoA:acyl carrier protein transacylase at 1.5-A resolution. Crystal structure of a fatty acid synthase component

Endogenous fatty acids are synthesized in all organisms in a pathway catalyzed by the fatty acid synthase complex. In bacteria, where the fatty acids are used primarily for incorporation into components of cell membranes, fatty acid synthase is made up of several independent cytoplasmic enzymes, each catalyzing one specific reaction. The initiation of the elongation step, which extends the length of the growing acyl chain by two carbons, requires the transfer of the malonyl moiety from malonyl-CoA onto the acyl carrier protein. We report here the crystal structure (refined at 1.5-A resolution to an R factor of 0.19) of the malonyl-CoA specific transferase from Escherichia coli. The protein has an alpha/beta type architecture, but its fold is unique. The active site inferred from the location of the catalytic Ser-92 contains a typical nucleophilic elbow as observed in alpha/beta hydrolases. Serine 92 is hydrogen bonded to His-201 in a fashion similar to various serine hydrolases. However, instead of a carboxyl acid typically found in catalytic triads, the main chain carbonyl of Gln-250 serves as a hydrogen bond acceptor in an interaction with His-201. Two other residues, Arg-117 and Glu-11, are also located in the active site, although their function is not clear.

Construction of a cDNA encoding the multifunctional animal fatty acid synthase and expression in Spodoptera frugiperda cells using baculoviral vectors

A cDNA encoding the 2505-residue multifunctional rat fatty acid synthase has been constructed and expressed as a catalytically active protein in Spodoptera frugiperda (Sf9) cells using Autographa californica nuclear polyhedrosis virus (baculovirus). The 7.5 kb cDNA was engineered by the amplification and sequential splicing together of seven fragments contained in overlapping cDNAs that collectively spanned the entire rat fatty acid synthase coding sequence. The full-length cDNA was cloned into a baculoviral transfer vector and used together with linearized baculoviral DNA to co-transfect Sf9 cells. Recombinant viral clones were purified and identified by Western blotting. The recombinant fatty acid synthase was expressed maximally 2 days after infection of the Sf9 cells, constituting up to 20% of the soluble cytoplasm, and could be conveniently separated from the insect host fatty acid synthase by high-performance anion-exchange chromatography. The catalytic properties of the purified recombinant fatty acid synthase are indistinguishable from those of the best preparations of the natural protein obtained from rat liver. These results indicate that, in the insect cell host, all seven catalytic components of the 2505-residue recombinant fatty acid synthase fold correctly, the acyl-carrier-protein domain is appropriately phosphopantetheinylated post-translationally, and the multifunctional polypeptide forms catalytically competent dimers. Thus the baculoviral system appears to be well suited for the expression of specific fatty acid synthase mutants that can be used to explore the mechanism by which the seven domains of this multifunctional homodimer co-operate in the biosynthesis of fatty acids.

Characterization of fatty acid synthase monomers restrained from reassociating by immobilization to a solid support

The controversial question as to whether the ketoreductase activity of the animal fatty acid synthase is lost on dissociation of the homodimer has been addressed by using immobilized subunits which cannot reassociate under the conditions of assay. Ketoreductase activity, assessed with the model substrate S-acetoacetyl-N-acetylcysteamine, was identical in immobilized monomers and dimers, exhibiting normal Michaelis-Menten kinetics with Km values in the millimolar range. When acetoacetyl-CoA was used as a substrate, however, biphasic kinetics were observed in the case of the dimer, with estimated Km values in the micro- and milli-molar ranges, but only the high-Km reaction was observed with the monomer. Thus when the ketoreductase activities of the monomer and dimer are assessed with acetoacetyl-CoA at concentrations sufficient to saturate only the low-Km reaction, it appears that the ketoreductase activity towards acetoacetyl-CoA is lost upon dissociation. Reduction of acetoacetyl-CoA via the low-Km pathway is CoA-dependent, indicating that acetoacetyl-CoA can react with the dimer by two mechanisms: a high-Km pathway analogous to that utilized by model substrates and a low-Km pathway in which substrate and product are transferred between acyl-CoA and acyl-enzyme forms. The results indicate that the ketoreductase activity per se is unaffected by subunit dissociation and are consistent with a model in which the transfer of substrate from CoA ester to the acyl-carrier-protein domain necessitates juxtaposition of the transferase active-site serine residue of one subunit and the phosphopantetheine moiety of the adjacent subunit.

The fatty acid synthase (FAS) gene and its promoter in Rattus norvegicus

Screening of rat liver genomic libraries yielded 5 overlapping clones for rat fatty acid synthase (FAS). From these clones we determined the 18,170 bp sequence of the rat FAS together with 5,028 bp of the 5'-flanking region and 515 bp of the 3'-adjacent genomic sequence. The two FAS transcripts which differ only in the positions of their polyadenylation/termination sites consist of one untranslated and 42 translated exons. Surprisingly, the substrate binding site for enoyl reductase, one of the FAS component functions, is interrupted by an intron. The sizes and the boundaries of the individual domains could be mapped in relation to the exon/intron structure of the gene. These eight partial functions coincide with discrete units of exons. The acyl carrier protein with its prosthetic 4'-phosphopantetheine group is located within a single exon supporting the idea that rat FAS has evolved by gene fusion. Using primer extension the main transcription start site of the FAS mRNA in both hepatic and mammary gland tissues was located at 5,028 bp in the sequence determined. As expected of a gene which is pretranslationally regulated the 5'-flanking region contains, in addition to TATA and CAAT boxes, consensus sequences for several DNA binding proteins.

Structural organization of the multifunctional animal fatty-acid synthase

The amino acid sequence of the multifunctional fatty-acid synthase has been examined to investigate the exact location of the seven functional domains. Good agreement in predicting the location of interdomain boundaries was obtained using three independent methods. First, the sites of limited proteolytic attack that give rise to relatively stable, large polypeptide fragments were identified; cryptic sites for protease attack at the subunit interface were unmasked by first dissociating the dimer into its component subunits. Second, polypeptide regions exhibiting higher-than-average rates of non-conservative mutation were identified. Third, the sizes of putative functional domains were compared with those of related monofunctional proteins that exhibit similar primary or secondary structure. Residues 1-406 were assigned to the oxoacyl synthase, residues 430-802 to the malonyl/acetyl transferase, residues 1630-1850 to the enoyl reductase, residues 1870-2100 to the oxyreductase, residues 2114-2190 to the acyl-carrier protein and residues 2200-2505 to the thioesterase. The 47-kDa transferase and 8-kDa acyl-carrier-protein domains, which are situated at opposite ends of the multifunctional subunit, were nevertheless isolated from tryptic digests as a non-covalently associated complex. Furthermore, a centrally located domain encompassing residues 1160-1545 was isolated as a nicked dimer. These findings, indicating that interactions between the head-to-tail juxtaposed subunits occur in both the polar and equatorial regions, are consistent with previously derived electron-micrograph images that show subunit contacts in these areas. The data permit refinement of the model for the fatty-acid synthase dimer and suggest that the malonyl/acetyl transferase and oxoacyl synthase of one subunit cooperate with the reductases, acyl carrier protein and thioesterase of the companion subunit in the formation of a center for fatty-acid synthesis.

Negative functional interference between two active centers is indicated in animal fatty acid synthetase

When fatty acid synthetase of the Harderian gland of guinea-pig was treated with various amounts of phenylmethanesulfonyl fluoride, the overall activity of the enzyme showed a quadratic decrease with respect to the inhibition degree of the thioesterase activity which was the primary target of inhibition. Moreover, the overall activity per active center of a heterodimer, which was formed between the native monomer and the thioesterase-less monomer, was higher than that of the native enzyme. These results are consistent with the view that the two active centers of the native enzyme exhibit a negative interference to each other.

Rat mammary gland fatty acid synthase: localization of the constituent domains and two functional polyadenylation/termination signals in the cDNA

The rat fatty acid synthase (FAS) is active only as a dimer, although the eight component functions are contained in a single polypeptide chain. Using mRNA from lactating rat mammary glands a cDNA expression library was established. With the overlapping immunologically positive clones we have an 8.9kb cDNA sequence for rat FAS. In the 3'-nontranslated region of the rat FAS cDNA we find a prototype polyadenylation/termination signal and 779 nucleotides upstream, a mutated one. Both of these polyadenylation/termination signals are used and give rise to two equally abundant mRNA species which are coordinately regulated. In the derived amino acid sequence we could locate six of the eight component functions; their order is NH2- beta-ketoacyl synthase - acetyl/malonyl transferases -enoyl reductase - acyl carrier protein - thioesterase -COOH. Comparison of FAS from different sources shows that the primary sequence is conserved only for the active residues and the amino acids in their immediate vicinity.

Molecular cloning and sequencing of a cDNA encoding the acyl carrier protein and its flanking domains in the mammalian fatty acid synthetase

Cloned cDNAs containing coding sequences for domains proximal to the carboxy terminus of the rat fatty acid synthetase have been isolated using an expression vector and domain-specific antibodies. The coding regions were assigned to specific domains of the multifunctional complex by identification of sequences coding for characterized peptide fragments and by recognition of sequences homologous to other monofunctional enzymes. Two clones contain the entire coding region for the acyl carrier protein domain. The sequence is flanked at the 3'-end by a region coding for the thioesterase domain and at the 5'-end by a sequence coding for a reductase, most likely the ketoreductase domain. Thus the ordering of these domain-coding regions in the fatty acid synthetase mRNA is established. The acyl carrier protein domain exhibits about 25% homology with that of the discrete monofunctional acyl carrier proteins of Escherichia coli, spinach and barley, the ketoreductase domain exhibits about 25% homology with bacterial dihydrofolate reductases and the active site of the thioesterase domain exhibits both primary and secondary structural features common to the serine proteases. These findings lend support to the hypothesis that the polyfunctional fatty acid synthetase probably arose by a complex evolutionary process involving fusion of genes coding for seven individual enzymes.

Complete amino acid sequence of the medium-chain S-acyl fatty acid synthetase thio ester hydrolase from rat mammary gland

The complete amino acid sequence of the medium-chain S-acyl fatty acid synthetase thio ester hydrolase (thioesterase II) from rat mammary gland is presented. Most of the sequence was derived by analysis of peptide fragments produced by cleavage at methionyl, glutamyl, lysyl, arginyl, and tryptophanyl residues. A small section of the sequence was deduced from a previously analyzed cDNA clone. The protein consists of 260 residues and has a blocked amino-terminal methionine and calculated Mr of 29,212. The carboxy-terminal sequence, verified by Edman degradation of the carboxy-terminal cyanogen bromide fragment and carboxypeptidase Y digestion of the intact thioesterase II, terminates with a serine residue and lacks three additional residues predicted by the cDNA sequence. The native enzyme contains three cysteine residues but no disulfide bridges. The active site serine residue is located at position 101. The rat mammary gland thioesterase II exhibits approximately 40% homology with a thioesterase from mallard uropygial gland, the sequence of which was recently determined by cDNA analysis [Poulose, A.J., Rogers, L., Cheesbrough, T. M., & Kolattukudy, P. E. (1985) J. Biol. Chem. 260, 15953-15958]. Thus the two enzymes may share similar structural features and a common evolutionary origin. The location of the active site in these thioesterases differs from that of other serine active site esterases; indeed, the enzymes do not exhibit any significant homology with other serine esterases, suggesting that they may constitute a separate new family of serine active site enzymes.

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.