Fatty acid synthase, mechanism

Particularly important to the pathways of modular synthases is the incorporation of novel precursors, including nonproteinogenic amino acids in NRP systems [17] and unique CoA thioesters in PK and fatty acid synthases [18]. These building blocks expand the primary metabolism and offer practically unlimited variability applied to natural products. Noteworthy within this context is the contiguous placement of biosynthetic genes for novel precursors within the biosynthetic gene cluster in prokaryotes. Such placement has allowed relatively facile elucidation of biosynthetic pathways and rapid discovery of novel enzyme mechanisms to create such unique building blocks. These new pathways offer a continued expansion of the enzymatic toolbox available for chemical catalysis. 

In contrast to the anaerobic pathway found in E. coli, the aerobic pathway in eukaryotic cells introduces double bonds after the saturated fatty acid has been synthesized. Stearoyl-CoA (18 0) is the major substrate for desaturation. Stearic acid is made by the fatty acid synthase as a minor product, the major product being palmitic acid, and is activated to its CoA derivative by acyl-CoA synthase. In eukaryotic cells an enzyme complex associated with the endoplasmic reticulum desaturates stearoyl-CoA to oleoyl-CoA (18 1A9). This remarkable reaction requires NADH and 02 and results in the formation of a double bond in the middle of an acyl chain with no activating groups nearby. The chemical mechanism for desaturation of long-chain acyl-CoAs remains unclear. 

Menendez, J.A., Ropero, S., Mehmi, I., Atlas, E., Colomer, R., and Lupu, R., Overexpression and hyperactivity of breast cancer-associated fatty acid synthase (oncogenic antigen-519) is insensitive to normal arachidonic fatty acid-induced suppression in lipogenic tissues but it is selectively inhibited by tumoricidal alpha-linolenic and gamma-lrnolenic fatty acids a novel mechanism by which dietary fat can alter mammary tumorigenesis, Int. J. Oncol., 24, 1369, 2004. 

Several architectural paradigms are known for polyketide and fatty acid synthases. While the bacterial enzymes are composed of several monofunctional polypeptides which are used during each cycle of chain elongation, fatty acid and polyketide synthases in higher organisms are multifunctional proteins with an individual set of active sites dedicated to each cycle of condensation and ketoreduction. Peptide synthetases also exhibit a one-to-one correspondence between the enzyme sequence and the structure of the product. Together, these systems represent a unique mechanism for the synthesis of biopolymers in which the template and the catalyst are the same molecule. 

The cofactor of fatty acid synthase is bound to the enzyme at a point near a specific residue of cysteine. This cysteine residue is important in the catalytic mechanism. T he sulfhydryl group of this cysteine is used for temporarily holding the fatty acid moiety each time a new molecule of maIonic acid is transferred to the 4-phosphopantetheine group. One might refer to the diagram of the enzyme in Chapter 5, where the sulfhydryl groups of the cysteine residue and of 4-phosphopantetheine are shown. 

Chang, S.-I. Hammes, G.G. Structure and mechanism of action of a multifunction enzyme fatty acid synthase. Acc. Chem. Res. 1990, 23, 363-369. 

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. 

Fatty-acid synthesis. The pathway and mechanism of fatty-acid synthesis is depicted in this figure. The fatty-acid synthase is a large enzyme complex which contains a number of enzymic activities and acyl carrier proteins. The synthesis is followed only to the formation of butyryl acyl carrier proteins however this, in fact, is repeated until palmytyl acyl carrier protein is formed by the addition of two carbons at a time. The portion added as acetyl CoA, instead of the malonyl CoAs, is indicated in purple to show that the growing chain is added... 

The most abundant membranes in nature are the thylakoids inside chloroplasts of green plants. A surprising amount of lipid traffic is involved in the assembly of these membranes. Almost all the acyl chains that form the core of the photosynthetic membranes are first produced by fatty acid synthase in the chloroplast. In most plants these acyl chains are then exported to the ER where they become esterified to glycerol, desaturated while they are part of phosphatidylcholine and then are returned to the plastid. The exact mechanisms for the export and return of acyl chains are still uncertain although much has been learned (Chapter 17) [10]. The export from plastids across the chloroplast envelope membranes is known to involve a fatty acid intermediate, and probably is a channeled or facilitated process rather than free diffusion because only a tiny pool of free fatty acid is ever detected (A. Koo, 2004). An acyl-CoA synthetase on the envelope membrane is believed to quickly convert the exported fatty acid to a thioester form that is then a substrate for acyltransferases. Transfer of acyl groups to the ER may occur via diffusion of the acyl-CoAs however, recent evidence suggests this initial acyl transfer reaction involves acylation of lyso-phosphatidylcholine and may occur at the chloroplast envelope. 

TNFa and the interferons decrease lipogenesis, by downregulating the mRNA levels of the key lipogenic enzymes acetyl-CoA carboxylase and fatty acid synthase the interferons diminish mRNA levels of fatty acid synthase but not of acetyl-CoA carboxylase. TNF-a, interleukin-1, and some interferons also increase lipolysis, although the mechanism is probably post-transcriptional, since Northern blot analysis shows that TNF-a and the interferons decrease the level of perilipin mRNA. 

Fatty acid biosynthesis from acetyl-CoA to palmitate involves an enzyme complex called fatty acid synthase, which appears to operate by a swinging arm mechanism involving the growing fatty acyl group linked to acyl carrier protein (Figure 18.29). Each of the individual enyzmatic activities below is a part of the fatty acid synthase complex. 

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