Fatty acid synthase, function

USP2 Fatty acid synthase Functions in androgen signaling 166, 167... 

In bacteria and plants, the individual enzymes of the fatty acid synthase system are separate, and the acyl radicals are found in combination with a protein called the acyl carrier protein (ACP). However, in yeast, mammals, and birds, the synthase system is a multienzyme polypeptide complex that incorporates ACP, which takes over the role of CoA. It contains the vitamin pantothenic acid in the form of 4 -phosphopan-tetheine (Figure 45-18). The use of one multienzyme functional unit has the advantages of achieving the effect of compartmentalization of the process within the cell without the erection of permeability barriers, and synthesis of all enzymes in the complex is coordinated since it is encoded by a single gene. 

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.

All the reactions in the pathway take place on a multienzyme complex, fatty acid synthase, which has seven catalytic activities or functional domains. Details of the individual reactions are presented in Appendix 11.1. 

The section of the molecule discussed so far represents a functional unit. In the cell, it is produced from pantothenate. The molecule also occurs in a protein-bound form as 4 -phosphopantetheine in the enzyme fatty acid synthase (see p. 168). In coenzyme A, however, it is bound to 3, 5 -adenosine diphosphate. 

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. 

Zhu G, Li Y, Cai X, Millership JJ, Marchewka MJ, Keithly JS (2004) Expression and functional characterization of a giant Type I fatty acid synthase (CpFASl) gene from Cryptosporidium parvum. Mol Biochem Parasitol 134 127-135 Zhu G, Marchewka M, Keithly JS (2000) Cryptosporidium parvum appears to lack a plastid genome. Microbiol-UK 146 315-321... 

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... 

Pantothenic acid is a component of coenzyme A, which functions in the transfer of acyl groups (Figure 28.17). Coenzyme A contains a thiol group that carries acyl compounds as activated thiol esters. Examples of such structures are succinyl CoA, fatty acyl CoA, and acetyl CoA. Pantothenic acid is also a component of fatty acid synthase (see p. 182). Eggs, liver, and yeast are the most important sources of pan tothenic acid, although the vitamin is widely distributed. Pantothenic acid deficiency is not well characterized in humans, and no RDA has been established. 

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. 

Schuller, H.J., Schorr, R., Hoffman, B., and Schweizer, E., 1992, Regulatory gene IN04 of yeast phospholipid biosynthesis is positively autoregulated and functions as a transactivator of fatty acid synthase genes FAS1 and FAS2 from Saccharomyces cerevisiae. Nucleic Acids Res. 20 5955-5961. 

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]. 

Acyl carrier protein, found in fatty acid synthases and polyketide synthases, functions to carry the elongating fatty acyl chain... 

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... 

J.K. Stoops, S.J. Kolodziej, J.P. Schroeter, J.P. Bretaudiere, and S.J. Wakil. 1992. Structure-function relationships of the yeast fatty acid synthase Negative-stain, cryo-electron microscopy, and image analysis studies of the end views of the structure Proc. Natl. Acad. Sci. USA 89 6585-6589. (PubMed) (Full Text in PMC)... 

The most important functions of pantothenic acid are its incorporation in coenzyme A and acyl carrier protein (AGP). Both CoA and AGP/4-phosphopantetheine function metabolically as carriers of acyl groups. Coenzyme A forms high-eneigy thioester bonds with carboxylic acids. The most important coenzyme is acetyl CoA. Acetic acid is produced during the metabolism of fatty acids, amino acids, or carbohydrates. The active acetate group of acetyl CoA can enter the Krebs cycle and is used in the synthesis of fatty acids or cholesterol. AGP is a component of the fatty acid synthase multienzyme complex. This complex catalyzes several reactions of fatty acid synthesis (condensation and reduction). The nature of the fatty acid synthase complex varies considerably among different species (91). 

Some vitamins undetfio a rather unique transformation prior to becoming functional. They are covalently attached to specific enzymes. Biotin, for example, is covalently bound to the biotin-requiring enzymes. Pantothenic acid, in a modified form, is covalently bound to fatty acid synthase. Riboflavin, following conversion to thecofaclor form, is bound to succinate dehydrogenase, as well as to a few other enzymes requiring riboflavin-based cofactors. 

The reactions of de novo fatty acid biosynthesis are shown in Figure 18-10. They are carried out by two multienzyme systems functioning in sequence. The first is acetyl-CoA carboxylase, which converts acetyl-CoA to malonyl-CoA. The second is fatty acid synthase, which sequentially joins two-carbon units of malonyl-CoA, eventually producing palmitic acid. Both complexes consist of multifunctional subunits. The various catalytic functions can be readily separated in plant cells and prokaryotes, but in yeasts, birds, and mammals, attempts to subdivide catalytic functions lead to loss of activity. Important features of this system are as follows ... 

Synthesis of fatty acid. ACP = Functional unit of acyl-carrier-protein segment of fatty acid synthase. The cysteinyl-SH group of /3-ketoacyI synthase accepts the acetyl group or the acyl group, and its catalytic site, which is adjacent to the -SH group, catalyzes the condensation reaction. 

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