Mammalian fatty acid synthase

Wakil, S. J., Fatty acid synthase, a proficient multifunctional enzyme. Biochemistry 28 4523, 1989. Reviews the evidence for the current model of the mammalian fatty acid synthase as depicted in figure 18.14. 

Which catalytic activity of the mammalian fatty acid synthase determines the chain length of the fatty acid product ... 

Fig. 4. X-ray determined protein crystal structures of multienzyme ensembly lines, (a) Mammalian fatty acid synthase at 4.5 A resolution (PDB 2cf2). Domain organization A starter substrate, acetyl-CoA or malonyl-CoA, gets loaded onto the acyl-carrler protein (ACP/absent in the structure) via the malonyl-CoA-/acetyl-CoA-ACP transacylase (MAT). Then, the ketoacyl synthase (KS) catalyzes a decarboxylative condensation reaction and forms the B-ketoacyl-ACP. This is followed from a reduction reaction catalyzed by the B-ketoacyl reductase (KR). Subsequently, the Intermediate gets dehydrated by a dehydratase (DH) and additionally reduced by a B-enoyl reductase (ER). The product gets released from the ACP by a thloesterase (absent in the structure), (b) Module 3 of 6-deoxyerthronolide B synthase at 2.6 A resolution (PDB 2qo3) bound to the inhibitor cerulin. The ketosynthase (KS) - acyltransferase (AT) di-domain is part of the large homodimeric polypeptide involved in biosynthesis of erythromycin from Saccharopolyspora erythraea...

Maier T, Jenni S, Ban N (2006) Architecture of Mammalian Fatty Acid Synthase at 4.5 A Resolution. Science 311 1258... 

We next examine the coordinated functioning of the mammalian fatty acid synthase. Fatty acid synthesis begins with the transfer of the acetyl group of acetyl CoA first to a serine residue in the active site of acetyl transferase and then to the sulfur atom of a cysteine residue in the active site of the condensing enzyme on one chain of the dimeric enzyme. Similarly, the malonyl group is transferred from malonyl CoA first to a serine residue in the active site of malonyl transferase and then to the sulfur atom of the phosphopantetheinyl group of the acyl carrier protein on the other chain in the dimer. Domain 1 of each chain of this dimer interacts with domains 2 and 3 of the other chain. Thus, each of the two functional units of the synthase consists of domains formed by different chains. Indeed, the arenas of catalytic action are... 

Reference Maier Timm, Simon Jermi and Nenad Ban. "Architecture of Mammalian Fatty Acid Synthase at 4.5 A Resolution." Science 311(3 March 2006) 1258 mini review comparing fungal and mammalian enzymes - Smith, Stuart. "Architectural Options for a Fatty Acid Synthase." Science 311(3 March 2006) 1251.]... 

Regulation of metabolic processes can be accomplished by other methods. One is the use of a multienzyme complex (e.g., pyruvate dehydrogenase complex or fatty acid synthase complex) in which various enzymes are organized such that the product of one becomes the substrate for an adjacent enzyme. A single polypeptide chain may contain multiple catalytic centers that carry out a sequence of transformations (e.g., the mammalian fatty acid synthase see Chapter 18). Such multifunctional polypeptides increase catalytic efficiency by abolishing the accumulation of free intermediates and by maintaining a stoichiometry of 1 1 between catalytic centers. 

Mammalian fatty acid synthase is a dimer of identical 272-kd subunits. Each chain is folded into three domains joined by flexible regions that allow domain movenaents that arc required for cooperation between the enzyme s active sites (Figure 22.26). Domain h the substraie-eniry... 

Amy, C. M., Williams-Ahlf, B., Naggert, J. and Smith, S., Molecular cloning of the mammalian fatty acid synthase gene and identification of the promoter region, Biochem J 271 (1990) 675-679. 

Pizer, E. S., Chrest, F. J., DiGiuseppe, J. A. and Han, W. F., Pharmacological inhibitors of mammalian fatty acid synthase suppress DNA replication and induce apoptosis in tumor cell lines. Cancer Res 58 (1998a) 4611-4615. 

These attached acyl carrier protein units are handles that bind the growing fatty acid to the multienzyme complex mammalian fatty acid synthase (mFAS) and guide it through subsequent transformations. The various enzymes occupy seven domains of mFAS. 

Maier, T. et al. (2006) Architecture of mammalian fatty acid synthase at 4.5 angstrom resolution. Science 311, 1258-1262... 

Enzyme complexes performing similar or identical tasks can vary widely between species. An excellent example is the enzyme complex, fatty acid synthase, which catalyzes the synthesis of fatty acids from acetyl-CoA and involves seven catalytic steps (Chap. 13). In E. coli and most bacteria the complex consists of seven different enzymes. In more advanced bacteria and in eukaryotic cells there are fewer types of subunit. For example, the yeast enzyme is a multienzyme complex (Mr = 2.3 x 106) with just two types of subunit (A and B) and a stoichiometry of A Bg. The subunits are multicatalytic. Subunit A (Mr = 185,000) has three catalytic activities and subunit B (Mr = 175,000) has the remaining four. The mammalian liver complex is a dimer, with each subunit... 

The formation of malonyl-CoA signals the beginning of the synthesis of palmitic acid (C16 ()). This occurs on a multifunctional enzyme complex, the fatty acid synthase. In mammalian liver, the enzyme complex consists of two identical polypeptides, each with specific binding sites for malonyl and alkanoyl groups, and eight different enzyme activities. 

What are the characteristics of the fatty acid synthase in the bacterium Escherichia coli that distinguish it from the mammalian multienzyme complex ... 

The mammalian multifunctional fatty acid synthase is a member of a large family of complex enzymes termed megasynthases that participate in step-by-step synthetic pathways. Two important classes of compounds that are synthesized by such enzymes are the polyketides and the nonribosomal peptides. The antibiotic erythromycin is an example of a polyketide, whereas penicillin (Section 8.5.5) is a nonribosomal peptide. 

Most mammalian cells have the capacity to synthesize fatty acids from glucose de novo in a pathway that uses products from glycolysis and two key cytosolic enzymes, acetyl-CoA carboxylase and fatty acid synthase (Chapter 6). This pathway generates long-chain SFA, mainly palmitate (16 0). The de novo synthesized palmitate and the palmitate derived from dietary sources are transported to the ER membranes. In the membranes, two major fatty acid enzymatic modifications of chain elongation and desaturation occur to yield longer chain SFA and unsaturated fatty acids of the n - 9 series. The n - 3 and n - 6 series of PUFA can be synthesized only from dietary fats, as animal cells do not have the... 

The polyketide synthesis chemically and biochemically resembles that of fatty acids. The reaction of fatty acid synthesis is inhibited by the fungal product ceru-lenin [9]. It inhibits all known types of fatty acid synthases, both multifunctional enzyme complex and unassociated enzyme from different sources like that of some bacteria, yeast, plants, and mammalians [10]. Cerulenin also blocks synthesis of polyketides in a wide variety of organisms, including actinomycetes, fungi, and plants [11, 12]. The inhibition of fatty acid synthesis by cerulenin is based on binding to the cysteine residue in the condensation reaction domain [13]. Synthesis of both polyketide and fatty acids is initiated by a Claisen condensation reaction between a starter carboxylic acid and a dicarboxylic acid such as malonic or methylmalonic acid. An example of this type of synthesis is shown in Fig. 1. An acetate and malonate as enzyme-linked thioesters are used as starter and extender, respectively. The starter unit is linked through a thioester linkage to the cysteine residue in the active site of the enzymatic unit, p-ketoacyl ACP synthase (KS), which catalyzes the condensation reaction. On the other hand, the extender... 

This chapter focuses on the catalytic transformations that result in the cyclic biosynthesis and breakdown of fatty acids. These metabolic pathways will serve as a paradigm for three classes of chemical reactions carbon-carbon bond formation and cleavage, oxidation and reduction, and hydration—dehydration. The most extensively studied reactions are those involved in microbial fatty acid biosynthesis (Type II fatty acid synthase (FAS-II)) and mammalian fatty acid /3-oxidation. In both pathways, the reactions are catalyzed by separate enzymes that have been cloned and overexpressed, thus providing a ready source of material for structural and mechanistic studies. In contrast, mammalian fatty acid biosynthesis and microbial fatty acid breakdown are catalyzed by multifunctional enzymes (MFEs) that have historically been less amenable to analysis. 

Prior to about 1982 it was often assumed that the enzymes of the chloroplast fatty acid synthase would be organized into some sort of multienzyme complex even though the absolute requirement of all known plant fatty acid synthase (FAS) preparations for added acyl carrier protein (ACP) precluded the type of complex observed in yeast and mammalian cells. Then in 1982 three laboratories [1-3] independently showed that the FAS consisted of individual proteins that were readily separable by conventional gel filtration techniques i.e. the plant synthase was of the prokaryotic type, similar to that in E. coli. After that, the concept of a multienzyme complex synthesizing fatty acids in chloroplasts was effectively in limbo. 

Prostanoids, consisting of prostaglandins (PGs) and thromboxanes (TXs), are members of the lipid mediators derived enzymatically from fatty acids. Arachidonic acid, a C2o essential fatty acid for most mammalians, is freed from the phospholipid molecule by phospholipase A2, which cleaves off the fatty acid precursor. Prostanoids are produced in a wide variety of cells throughout the body from the sequential oxidation of arachidonic acid by cyclooxygenase, PG hydroperoxidase, and a series of prostaglandin synthases (Figure 2.1). 

The synthesis and breakdown of fatty acids involves a cyclic series of reactions where the enzymes catalyzing these reactions fall into three major superfamilies the thiolase superfamily (synthases and thiolases), the short-chain dehydrogenase reductase superfamily (oxidoreductases), and the crotonase superfamily (hydra-tases and dehydratases). The enzymes comprising these three superfamilies include a significant portion of those found in microbial and mammalian metabolism. Many of the enzymes in microbial metabolism are potential drug targets. 

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