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

Fatty acid synthase activity in tumor cells

While normal tissues are tightly regulated by nutrition and a carefully balanced system of glycolysis and fatty acid synthesis, tumor cells are under significant evolutionary pressure to bypass many of the checks and balances afforded normally. Cancer cells have high energy expenditure from heightened proliferation and metabolism and often show increased lipogenesis. Fatty acid synthase (FASN), the enzyme responsible for catalyzing the ultimate steps of fatty acid synthesis in cells, is expressed at high levels in tumor cells and is mostly absent in corresponding normal cells. Because of the unique expression profile of FASN, there is considerable interest not only in understanding its contribution to tumor cell growth and proliferation, but also in developing inhibitors that target FASN specifically as an anti-tumor modality. Pharmacological blockade of FASN activity has identified a pleiotropic role for FASN in mediating aspects of proliferation, growth and survival. As a result, a clearer understanding of the role of FASN in tumor cells has been developed.

Structure, mechanism and regulation of pyruvate carboxylase

PC (pyruvate carboxylase) is a biotin-containing enzyme that catalyses the HCO(3)(-)- and MgATP-dependent carboxylation of pyruvate to form oxaloacetate. This is a very important anaplerotic reaction, replenishing oxaloacetate withdrawn from the tricarboxylic acid cycle for various pivotal biochemical pathways. PC is therefore considered as an enzyme that is crucial for intermediary metabolism, controlling fuel partitioning toward gluconeogenesis or lipogenesis and in insulin secretion. The enzyme was discovered in 1959 and over the last decade there has been much progress in understanding its structure and function. PC from most organisms is a tetrameric protein that is allosterically regulated by acetyl-CoA and aspartate. High-resolution crystal structures of the holoenzyme with various ligands bound have recently been determined, and have revealed details of the binding sites and the relative positions of the biotin carboxylase, carboxyltransferase and biotin carboxyl carrier domains, and also a unique allosteric effector domain. In the presence of the allosteric effector, acetyl-CoA, the biotin moiety transfers the carboxy group between the biotin carboxylase domain active site on one polypeptide chain and the carboxyltransferase active site on the adjacent antiparallel polypeptide chain. In addition, the bona fide role of PC in the non-gluconeogenic tissues has been studied using a combination of classical biochemistry and genetic approaches. The first cloning of the promoter of the PC gene in mammals and subsequent transcriptional studies reveal some key cognate transcription factors regulating tissue-specific expression. The present review summarizes these advances and also offers some prospects in terms of future directions for the study of this important enzyme.

Domain architecture of pyruvate carboxylase, a biotin-dependent multifunctional enzyme

Biotin-dependent multifunctional enzymes carry out metabolically important carboxyl group transfer reactions and are potential targets for the treatment of obesity and type 2 diabetes. These enzymes use a tethered biotin cofactor to carry an activated carboxyl group between distantly spaced active sites. The mechanism of this transfer has remained poorly understood. Here we report the complete structure of pyruvate carboxylase at 2.0 angstroms resolution, which shows its domain arrangement. The structure, when combined with mutagenic analysis, shows that intermediate transfer occurs between active sites on separate polypeptide chains. In addition, domain rearrangements associated with activator binding decrease the distance between active-site pairs, providing a mechanism for allosteric activation. This description provides insight into the function of biotin-dependent enzymes and presents a new paradigm for multifunctional enzyme catalysis.

Autonomous folding of interdomain regions of a modular polyketide synthase

Domains within the multienzyme polyketide synthases are linked by noncatalytic sequences of variable length and unknown function. Recently, the crystal structure was reported of a portion of the linker between the acyltransferase (AT) and ketoreductase (KR) domains from module 1 of the erythromycin synthase (6-deoxyerythronolide B synthase), as a pseudodimer with the adjacent ketoreductase (KR). On the basis of this structure, the homologous linker region between the dehydratase (DH) and enoyl reductase (ER) domains in fully reducing modules has been proposed to occupy a position on the periphery of the polyketide synthases complex, as in porcine fatty acid synthase. We report here the expression and characterization of the same region of the 6-deoxyerythronolide B synthase module 1 AT-KR linker, without the adjacent KR domain (termed DeltaN AT1-KR1), as well as the corresponding section of the DH-ER linker. The linkers fold autonomously and are well structured. However, analytical gel filtration and ultracentrifugation analysis independently show that DeltaN AT1-KR1 is homodimeric in solution; site-directed mutagenesis further demonstrates that linker self-association is compatible with the formation of a linker-KR pseudodimer. Our data also strongly indicate that the DH-ER linker associates with the upstream DH domain. Both of these findings are incompatible with the proposed model for polyketide synthase architecture, suggesting that it is premature to allocate the linker regions to a position in the multienzymes based on the solved structure of animal fatty acid synthase.

Architecture of mammalian fatty acid synthase at 4.5 A resolution

The homodimeric mammalian fatty acid synthase is one of the most complex cellular multienzymes, in that each 270-kilodalton polypeptide chain carries all seven functional domains required for fatty acid synthesis. We have calculated a 4.5 angstrom-resolution x-ray crystallographic map of porcine fatty acid synthase, highly homologous to the human multienzyme, and placed homologous template structures of all individual catalytic domains responsible for the cyclic elongation of fatty acid chains into the electron density. The positioning of domains reveals the complex architecture of the multienzyme forming an intertwined dimer with two lateral semicircular reaction chambers, each containing a full set of catalytic domains required for fatty acid elongation. Large distances between active sites and conformational differences between the reaction chambers demonstrate that mobility of the acyl carrier protein and general flexibility of the multienzyme must accompany handover of the reaction intermediates during the reaction cycle.

Directed Mutagenesis Alters the Stereochemistry of Catalysis by Isolated Ketoreductase Domains from the Erythromycin Polyketide Synthase

The ketoreductase (KR) domains eryKR(1) and eryKR(2) from the erythromycin-producing polyketide synthase (PKS) reduce 3-ketoacyl-thioester intermediates with opposite stereospecificity. Modeling of eryKR(1) and eryKR(2) showed that conserved amino acids previously correlated with production of alternative alcohol configurations lie in the active site. eryKR(1) domains mutated at these positions showed an altered stereochemical outcome in reduction of (2R, S)-2-methyl-3-oxopentanoic acid N-acetylcysteamine thioester. The wild-type eryKR(1) domain exclusively gave the (2S, 3R)-3-hydroxy-2-methylpentanoic acid N-acetylcysteamine thioester, while the double mutant (F141W, P144G) gave only the (2S, 3S) isomer, a switch of the alcohol stereochemistry. Mutation of the eryKR(2) domain, in contrast, greatly increased the proportion of the wild-type (2R, 3S)-alcohol product. These data confirm the role of key residues in stereocontrol and suggest an additional way to make rational alterations in polyketide antibiotic structure.

High-Throughput Mutagenesis to Evaluate Models of Stereochemical Control in Ketoreductase Domains from the Erythromycin Polyketide Synthase

Ketoreductase (KR) activities help determine the stereochemistry of the products of modular polyketide synthases (PKSs). For example, domains eryKR(1) and eryKR(2), contained, respectively, in the first and second extension modules of the erythromycin-producing PKS, reduce 3-ketoacyl-thioester intermediates with opposite stereospecificity. Amino acid motifs that correlate with stereochemical outcome have been identified in KRs. We have used saturation mutagenesis of these motifs in eryKR(1) and eryKR(2), and a microplate-based screen of such mutants for activity against (9R, S)-trans-1-decalone, to identify candidate enzymes potentially altered in stereocontrol. Active mutants were reassayed with (2R, S)-2-methyl-3-oxopentanoic acid N-acetylcysteamine thioester, and the alcohol products were analyzed by chiral HPLC. Variant enzymes were found with either altered substrate selectivity for the (2R) or (2S) substrate or altered stereospecificity of reduction, or both, further highlighting the importance of these motifs in stereochemical control.

Molecular Basis of Celmer's Rules: Stereochemistry of Catalysis by Isolated Ketoreductase Domains from Modular Polyketide Synthases

A system is reported for the recombinant expression of individual ketoreductase (KR) domains from modular polyketide synthases (PKSs) and scrutiny of their intrinsic specificity and stereospecificity toward surrogate diketide substrates. The eryKR(1) and the tylKR(1) domains, derived from the first extension module of the erythromycin PKS and the tylosin PKS, respectively, both catalyzed reduction of (2R, S)-2-methyl-3-oxopentanoic acid N-acetylcysteamine thioester, with complete stereoselectivity and stereospecificity, even though the substrate is not tethered to an acyl carrier protein or an intact PKS multienzyme. In contrast, and to varying degrees, the isolated enzymes eryKR(2), eryKR(5), and eryKR(6) exercised poorer control over substrate selection and the stereochemical course of ketoreduction. These data, together with modeling of diketide binding to KR(1) and KR(2), demonstrate the fine energetic balance between alternative modes of presentation of ketoacylthioester substrates to KR active sites.

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.

Evidence for a monomeric structure of nonribosomal Peptide synthetases

Nonribosomal peptide synthetases (NRPS) are multimodular biocatalysts that bacteria and fungi use to assemble many complex peptides with broad biological activities. The same modular enzymatic assembly line principles are found in fatty acid synthases (FAS), polyketide synthases (PKS), and most recently in hybrid NRPS/PKS multienzymes. FAS as well as PKS are known to function as homodimeric enzyme complexes, raising the question of whether NRPS may also act as homodimers. To test this hypothesis, biophysical methods (size exclusion chromatography, analytical equilibrium ultracentrifugation, and chemical crosslinking) and biochemical methods (two-affinity-tag-system and complementation studies with enzymes being inactivated in different catalytic domains) were applied to NRPS subunits from the gramicidin S (GrsA-ATE), tyrocidine (TycB(1)-CAT and TycB(2-3)-AT.CATE), and enterobactin (EntF-CATTe) biosynthetic systems. These methods had revealed the dimeric structure of FAS and PKS previously, but all three NRPS systems investigated are functionally active as monomers.

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.