Fatty acid synthase dimeric

Diagram of a fatty acid synthase dimer with its head-to-tail association of the two multifunctional polypeptides. 

Rangan, V. S., Joshi, A. K. and Smith, S., 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, J Biol Chem 273 (1998) 34949-34953. 

Figure 21-2. Fatty acid synthase multienzyme complex. The complex is a dimer of two identical polypeptide monomers, 1 and 2, each consisting of seven enzyme activities and the acyl carrier protein (ACP). (Cys— SH, cysteine thiol.) The— SH of the 4 -phosphopantetheine of one monomer is in close proximity to the— SH of the cysteine residue of the ketoacyl synthase of the other monomer, suggesting a "head-to-tail" arrangement of the two monomers. Though each monomer contains all the partial activities of the reaction sequence, the actual functional unit consists of one-half of one monomer interacting with the complementary half of the other. Thus, two acyl chains are produced simultaneously. The sequence of the enzymes in each monomer is based on Wakil.

Each subunit of the enzyme binds acetyl residues as thioesters at two different SH groups at one peripheral cysteine residue (CysSH) and one central 4-phosphopante-theine group (Pan-SH). Pan-SH, which is very similar to coenzyme A (see p. 12), is covalently bound to a protein segment of the synthase known as the acyl-carrier protein (ACP). This part functions like a long arm that passes the substrate from one reaction center to the next. The two subunits of fatty acid synthase cooperate in this process the enzyme is therefore only capable of functioning as a dimer. 

The fatty acid synthases of yeast and of vertebrates are also multienzyme complexes, and their integration is even more complete than in E. coli and plants. In yeast, the seven distinct active sites reside in two large, multifunctional polypeptides, with three activities on the a subunit and four on the /3 subunit. In vertebrates, a single large polypeptide (Afr 240,000) contains all seven enzymatic activities as well as a hydrolytic activity that cleaves the finished fatty acid from the ACP-like part of the enzyme complex. The vertebrate enzyme functions as a dimer (Afr 480,000) in which the two identical subunits lie head-to-tail. The subunits appear to function independently. When all the active sites in one... 

The remaining series of reactions of fatty acid synthesis in eukary-l otes is catalyzed by the multifunctional, dimeric enzyme, fatty acid synthase. Each fatty acid synthase monomer is a multicatalytic polypeptide with seven different enzymic activities plus a domain that covalently binds a molecule of 4 -phosphopantetheine. [Note 4-Phosphopantetheine, a derivative of the vitamin pantothenic add (see p. 379), carries acetyl and acyl units on its terminal thiol (-SH)j group during fatty acid synthesis. It also is a component of 00-enzyme A.] In prokaryotes, fatty acid synthase is a multienzyme complex, and the 4 -phosphopantetheine domain is a separate protein, referred to as the acyl carrier protein (ACP). ACP is used below to refer to the phosphopantetheine-binding domain of the eukaryotic fatty acid synthase molecule. The reaction numbers in1 brackets below refer to Figure 16.9. [Note The enzyme activities listed are actually separate catalytic domains present in each mulf-1 catalytic fatty acid synthase monomer.]... 

Proposed organization of the enzymatic activities of fatty acid synthase from animal liver. Fatty acid synthase exists as a dimer of two giant identical peptides (Mr = 272,000). Each subunit has one copy of acyl carrier protein (ACP) and each of the enzyme activities involved in fatty acid synthesis is covalently linked. The two peptides are organized in a head-to-tail configuration in such a way that it is possible to make two fatty acid molecules at the same time. 

Except for malonyl-CoA formation, all the individual reactions for palmitate synthesis reside on a single multifunctional protein (fatty acid synthase) in animal cells. It has been shown that a dimer of the multifunctional protein is required to catalyze palmitate synthesis. Explain the molecular basis of this observation. 

In prokaryotes, each of the reactions of fatty acid synthesis is catalyzed by a separate enzyme. However, in eukaryotes, the enzymes of the fatty acid synthesis elongation cycle are present in a single polypeptide chain, multifunctional enzyme complex, called fatty acid synthase. The fatty acid synthase complex exists as a dimer, with the ACP moiety shuttling the fatty acyl chain between successive catalytic sites, and from one subunit of the dimer to the other. It is, in effect, a highly efficient production line for fatty acid biosynthesis. 

The animal fatty acid synthase (FAS EC 2.3.1.85) is one of the most complex multifunctional enzymes that have been characterized, as this single polypeptide contains all the catalytic components required for a series of 37 sequential transactions (Smith, 1994). The animal FAS consists of two identical polypeptides of approximately 2500 amino acid residues (MW, ca. 270 kDa), each containing seven catalytic subunits (1) ketoacylsynthase, (2) malonyl/acetyl transferase, (3) dehydrase, (4) enoyl reductase, (5) (3-kcto reductase, (6) acyl carrier protein (ACP), and (7) thioesterase. Although some components of the complex are able to carry out their respective catalytic steps in the monomeric form, only in the FAS dimer do the subunits attain conformations that facilitate coupling of the individual reactions of fatty acid synthesis to occur (Smith et al., 2003). 

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... 

Continued condensation of malonyl-CoA with acetyl-CoA units is catalysed by fatty acid synthase, eventually leading to the 16-carbon palmitic acid (Figme 5.3) this is then released and may undergo separate elongation and/or unsaturation reactions, to yield other fatty acid molecules. The active form of fatty acid synthase is a dimer of identical subunits. 

Animal FASs are functional dimers [76]. While /3-ketoacyl synthase requires dimer formation for activity [77], catalysis of the remaining FAS reactions is carried out by the monomeric enzyme. This behavior is reminiscent of yeast fatty acid synthase, where the -ketoacyl synthase and ACP from different subunits also contribute to the same active site. Electron microscopy and small angle scattering experiments have further defined the structure of the functional complex [34,78]. The overall shape of the molecule, as visualized by electron microscopy, is two side by side cylinders with dimensions of 160x146 x 73 A [34]. 

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... 

Figure 22.23. Schematic Representation of Animal Fatty Acid Synthase. Each of the identical chains in the dimer contains three domains. Domain 1 (blue) contains acetyl transferase (AT), malonyl transferase (MT), and condensing enzyme (CE). Domain 2 (yellow) contains acyl carrier protein (ACP), P-ketoacyl reductase (KR), dehydratase (DH), and enoyl reductase (ER). Domain 3 (red) contains thioesterase (TE). The flexible phosphopantetheinyl group (green) carries the fatty acyl chain from one catalytic site on a chain to another, as well as between chains in the dimer. [After Y. Tsukamoto, H. Wong, J. S. Mattick, and S. J. Wakil. J. Biol. Chem. 258(1983) 15312.]...

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... 

The remaining reactions in fatty acid synthesis take place on the fatty acid synthase multienzyme complex. This complex, the site of seven enzyme activities and ACP, is a 500-kD dimer. Because the enormous polypeptides in the dimer are arranged in a head-to-tail configuration, two fatty acids can be constructed simultaneously. A proposed mechanism for palmitate synthesis is shown in Figure 12.13. 

Malonyl-CoA is then used for fatty acid synthesis. This process requires Fatty Acid Synthase (FAS) (Chirala and Wakil, 2004) that uses acetyl-CoA as a primer, malonyl-CoA as a two-carbon donor, and NADPH as a reducing equivalent (Fig. 1.1). The predominant fatty acid produced by FAS is palmitic acid (Cl 6 0). The structure of FAS has been extensively studied (Asturias et al., 2005 Maier et ah, 2006). FAS is a multifunctional complex consisting of two identical monomers. However, only the dimeric form is active (Chirala et al.,... 

Chirala, S. S., Jayakumar, A., Gu, Z. W., and Wakil, S. J. Human fatty acid synthase role of interdomain in the formation of catalytically active synthase dimer. Proc Natl Acad Sci U S A98 (2001) 3104-3108. 

The fatty acid synthase of mammals is a dimer consisting of identical subunits, each of which contains all the activities necessary to synthesize fatty acids from malonyl CoA and acetyl CoA. Why is a single subunit unable to carry out the reactions ... 

In most bacteria and in chloroplasts, the acyl carrier protein and the enzymes of F.a.b. are discrete proteins in a noncovalently associated multienzyme complex (type II fatty acid synthase), whereas the fatty acid synthase of animals (type I) is a dimer of a single multifunctional protein. Yeast fatty acid synthase is an intermediate type I/type II enzyme (see Table 2). 

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