Fatty acid synthase, animal

In animals, the enzymes of fatty acid synthesis are components of one long polypeptide chain, the fatty acid synthase, whereas no similar association exists for the degradative enzymes. (Plants and bacteria employ separate enzymes to carry out the biosynthetic reactions.)... 

Rittenberg and Bloch showed in the late 1940s that acetate units are the building blocks of fatty acids. Their work, together with the discovery by Salih Wakil that bicarbonate is required for fatty acid biosynthesis, eventually made clear that this pathway involves synthesis of malonyl-CoA. The carboxylation of acetyl-CoA to form malonyl-CoA is essentially irreversible and is the committed step in the synthesis of fatty acids (Figure 25.2). The reaction is catalyzed by acetyl-CoA carboxylase, which contains a biotin prosthetic group. This carboxylase is the only enzyme of fatty acid synthesis in animals that is not part of the multienzyme complex called fatty acid synthase. 

The enzymes that catalyze formation of acetyl-ACP and malonyl-ACP and the subsequent reactions of fatty acid synthesis are organized quite differently in different organisms. We first discuss fatty acid biosynthesis in bacteria and plants, where the various reactions are catalyzed by separate, independent proteins. Then we discuss the animal version of fatty acid biosynthesis, which involves a single multienzyme complex called fatty acid synthase. 

The processes of fatty acid biosynthesis are catalysed by the enzyme fatty acid synthase. In animals, this is a multifunctional protein containing all of the catalytic activities required, whereas in... 

Fatty acid synthesis is catalysed in animals by the enzyme fatty acid synthase, which is a multifunctional protein containing all of the catalytic activities required. Bearing in mind the necessity to provide a specific binding site for the various substrates involved, and then the fairly complex sequence of reactions carried out, it raises the question of just how it is possible for this process to be achieved at the enzymic level. Nature has devised an elaborate but satisfyingly simple answer to this problem. 

Smith, S. (1994) The animal fatty acid synthase one gene, one polypeptide, seven enzymes. FASEB J. 8, 1248-1259. 

In higher animals as well as in My cobacterium,207 yeast,208 and Euglena, the fatty acid synthase consists of only one or two multifunctional proteins. The synthase from animal tissues has seven catalytic activities in a single 263-kDa 2500-residue protein 209 The protein consists of a series of domains that contain the various catalytic activities needed for the entire synthetic sequence. One domain contains an ACP-like site with a bound 4 -phosphopantetheine as well as a cysteine side chain in the second acylation site. This synthase produces free fatty acids, principally the C16 palmitate. The final step is cleavage of the acyl-CoA by a thioesterase, one of the seven enzymatic activities of the synthase. See Chapter 21 for further discussion. 

It has been shown that the reaction center of multifunctional fatty acid synthases of yeast and animal cells comprises two SH-groups Cys-SH of the condensation enzyme, and pantetheine-SH attached to ACP. The... 

The Organization of the Fatty Acid Synthase Is Different in E. coli and Animals Biosynthesis of Monounsaturated Fatty Acids Follows Distinct Routes in E. coli and Animal Cells... 

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. 

Fatty acid synthesis begins when the substrates, acetyl-CoA and malonyl-CoA, are transferred onto the protein by malonyl-CoA acetyl-CoA-ACP transacylase (MAT, steps 1 and 2 in fig. 18.12a). The numbers in parentheses below the abbreviation of the enzyme in this figure refer to the reactions shown in fig. 18.12. (Whereas E. coli has separate enzymes that catalyze the transfer of acetyl- and malonyl-CoA to ACP, both reactions are catalyzed by the same enzymatic activity (MAT) on the animal fatty acid synthase.) Subsequently, /3-ketobutyryl-ACP and CO2 are formed in a condensation reaction catalyzed by /3-ketoacyl-ACP synthase (KS, step 3 in fig. 18.12a). 

The Organization of the Fatty Acid Synthase Is Different in E. coli and Animals... 

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. 

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). 

Figure 2.5. Reaction sequence for the biosynthesis of fatty acids de novo by the animal FAS. The condensation reaction proceeds with stereochemical inversion of the malonyl C-2, the (3-ketoacyl moiety is reduced by NADPH to D-(3 hydroxyacyl moiety, which then is dehydrated to a trans-enoyl moiety finally, the enoyl moiety is reduced to a saturated acyl moiety by NADPH, with the simultaneous addition of a solvent proton. The two C atoms at the methyl end of the fatty acid are derived from acetyl-CoA, the remainder from malonyl-CoA. The entire series of reactions takes approximately 1 second. PSH, phosphopantetheine. (Reprinted from Prog, in Lipid Res., vol. 42, S. Smith, A. Witkowski and A.K. Joshi, Stuctural and functional organization of the animal fatty acid synthase, pp. 289-317, copyright (2003), with permission from Elsevier).

Joshi, A.K., Witkowski, A., Smith, S. 1997. Mapping of functional interactions between domains of the animal fatty acid synthase by mutant complementation in vitro. Biochemistry 36, 2316-2322. 

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]. 

Although the PKSs of plants have received little attention relative to those of microbial origin, the fatty acid synthases (FASs) of a wide range of plant, animal and microbial sources have been subjected to intensive study and shown to differ in their molecular structure and complexity. Thus type 1 FASs produced by animals, fimgi and yeasts are high molecular weight multienzyme complexes, in which the individual active sites may be covalently linked components of a single polypeptide chain. In the type II FASs of... 

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.]...

Long-term control is mediated by changes in the rates of synthesis and degradation of the enzymes participating in fatty acid synthesis. Animals that have fasted and are then fed high-carbohydrate, low-fat diets show marked increases in their amounts of acetyl CoA carboxylase and fatty acid synthase within a few days. This type of regulation is known as... 

We already mentioned that the enzymes involved in the P-oxidation of fatty acids are located in the mitochondria. The source of two-carbon fragments for the biosynthesis of both fatty acids and isoprenoids like cholesterol is acetyl CoA, which is generated by oxidative metabolism in the mitochondria. Acetyl CoA cannot escape from the mitochondria, but it can be exported to the cyosol as citrate, where it is reconverted to oxaloacete and acetyl CoA. Fatty acid (and cholesterol) biosynthesis takes place in the cyosol, and requires bicarbonate, which is incorporated into acetyl CoA to form malonyl CoA by acetyl CoA carboxylase. The biosynthesis of fatty acids, mostly the Cie palmitate (Chapter 4), requires one molecule of acetyl CoA and seven molecules of malonyl CoA. In animals, the seven enzymatic reactions which are required for fatty acid synthesis are present in a single multifunctional protein complex, known as fatty acid synthase. The synthase also contains an acyl-carrier protein... 

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