Mammalian Fatty Acid Synthase: X-ray Structure of a Molecular Assembly Line

Abstract

F atty acids are ubiquitous cellular components with a variety of structural and metabolic functions, from energy storage to modulation of gene expression. In plants and bacteria, de novo synthesis of fatty acids is carried out by individual enzymes. In contrast, in animal cells, the enzymes involved in fatty acid synthesis are organized in a single fatty acid synthase polypeptide (Figure 1). Encompassing all of the activities required to retain and extend an acyl chain from acetyl-CoA and malonyl-CoA precursors and then release a full-length, long-chain fatty acid, mammalian fatty acid synthase (FAS) constitutes a true molecular assembly line (1 ). Human FAS is a target for drug development against obesity and related diseases, and FAS inhibitors have been shown to have antitumor activity. Although the biochemistry of fatty acid synthesis was established over 30 years ago, structural understanding of FAS had, until recently, been limited (high-resolution structures of individual prokaryotic enzymes and a couple of FAS domains were available). The situation has changed dramatically with the recent publication of the first X-ray structure of FAS by Nenad Ban and colleagues (2 ).In mammalian cells, the functional form of FAS is a homodimer (MW ~540 000 Da) with two centers for fatty acid synthesis. The model for FAS organization that prevailed for over 20 years was based on a logical but naïve interpretation of the possible structure of an FAS monomer (3 ). It was observed that the bifunctional compound dibromopropanone could cross-link the thiol in the active site cysteine of the N-terminal b-ketoacyl synthase (KS) domain of one monomer with the thiol in the phosphopantetheine group of the acyl carrier protein (ACP) located near the C-terminus of the other monomer. Therefore, a model with two extended FAS monomers in an antiparallel arrangement was proposed, thereby providing a simple explanation for the complementary interaction of the N-terminal condensing domains of one monomer with the C-terminal reducing and chain terminating domains of the other. However, functional complementation studies of baculovirusexpressed active site FAS mutants revealed a number of intramonomer interactions that could not be explained by the extended, antiparallel model of FAS (4 ). Careful re-examination of the dibromopropanone experiment (5 ) also revealed both intra-and intermonomer cross-links. These investigations eventually resulted in the formulation of an alternative model for FAS organization in which the two monomers were intertwined (6 ) (Box 1).The paucity of structural information on FAS that prevailed until very recently illustrates some of the challenges in structural analysis of large macromolecular complexes. A relatively new technique that can often be used to determine low (10-30 Å)-resolution structures of large macromolecules is molecular electron microscopy (EM). Analysis of FAS by EM was complicated by the flexibility of the molecule, and a first low-resolution 3-D EM reconstruction of FAS was interpreted A B...

The Synthesis of Milk Medium-Chain Fatty Acids in Mammary Gland

“…The synthesis of fatty acid in mammary gland is mainly catalyzed by fatty acid syn thase (FASN) and acetyl CoA carboxylase (ACC). In mammalian cells, the functional form of FASN is a homodimer (MW ~540,000 Da) [6]. FASN is composed of seven domains, including b-ketoacyl synthase (KS), acetyl/malonyl-CoA transferase (MAT), b-hydroxyacyl dehydratase (DH), enoyl reductase (ER), b-ketoacyl reductase (KR), acyl-carrier protein (ACP), and thioesterase I (TE I) (Figure 1).…”

“…The subsequent elongation cycles are performed with the malonyl-CoA as two-carbon units. Lastly, TE I is responsible for the release of fatty acid, with a length of 16 carbons, from ACP [6]. FASN is the crucial enzyme for milk fat synthesis [7].…”

The Lipogenesis Pathway as a Cancer Target

“…(b) Cycle of reactions catalyzed by FASN. Reproduced with permission from ACS Chemical Biology . Copyright 2006 American Chemical Society.…”

An easy, rapid and objective mathematical method to identify fatty acid synthase (oncogenic antigen-519) modulators with potential anticancer value

“…The latter is hydrolysed by TE to free palmitate. A schematic representation of the ultimate architecture of mammalian FASN at 4.5 Å is shown, which demonstrates that mammalian FASN is an intertwined dimer with a large dimerisation interface running through the body of the molecule, perpendicular to the interfaced proposed in the classical scheme [8][9][10][11][12]. Mammalian FASN therefore adopts an X-shape structure with a full set of active sites present in each of two semicircular "reaction chambers" on both sides of the molecule.…”

“…Unfortunately, the clinical relevance of existing FASN inhibitors is limited because of either their chemical instability, which leads to unspecific interactions with other proteins and affects processes other than FASN activity, or their undesirable side effects, including a rapid and profound weight loss and loss of adipose mass by affecting food intake and energy expenditure [7]. Although the combination of FASN structural complexity and until recently the lack of X-ray crystallography data of mammalian FASN created a significant challenge in the exploitation of FASN as a valuable target for drug development, it is hoped that the latest 4.5-Å resolution X-ray crystallographic map of mammalian FASN will form the basis for efforts at structure-based drug design with this target [10][11][12]. While we expect that the improvement in the selectivity and potency of forthcoming novel FASN-targeted small molecule inhibitors may direct the foundation of a new family of chemotherapeutic agents, we herein describe an easy, rapid and objec- Fig.…”