Structure fatty acid synthase

Source Pemble CP IV, Johnson LC, KridelSJ, LowtherWT. Crystal structure of the thioeste-rase domain of human fatty acid synthase inhibited by Orlistat, Nature Structural Molecular Biology 14 704-709 (2007). 

Phosphopantetheine tethering is a posttranslational modification that takes place on the active site serine of carrier proteins - acyl carrier proteins (ACPs) and peptidyl carrier proteins (PCPs), also termed thiolation (T) domains - during the biosynthesis of fatty acids (FAs) (use ACPs) (Scheme 23), polyketides (PKs) (use ACPs) (Scheme 24), and nonribosomal peptides (NRPs) (use T domain) (Scheme 25). It is only after the covalent attachment of the 20-A Ppant arm, required for facile transfer of the various building block constituents of the molecules to be formed, that the carrier proteins can interact with the other components of the different multi-modular assembly lines (fatty acid synthases (FASs), polyketide synthases (PKSs), and nonribosomal peptide synthetases (NRPSs)) on which the compounds of interest are assembled. The structural organizations of FASs, PKSs, and NRPSs are analogous and can be divided into three broad classes the types I, II, and III systems. Even though the role of the carrier proteins is the same in all systems, their mode of action differs from one system to another. In the type I systems the carrier proteins usually only interact in cis with domains to which they are physically attached, with the exception of the PPTases and external type II thioesterase (TEII) domains that act in trans. In the type II systems the carrier proteins selectively interact... 

All the reactions in the synthetic process are catalyzed by a multienzyme complex, fatty acid synthase. Although the details of enzyme structure differ in prokaryotes such as Escherichia coli and in eukaryotes, the four-step process of fatty acid synthesis is the same in all organisms. We first describe the process as it occurs in A1, coli, then consider differences in enzyme structure in other organisms. 

FIGURE 21-7 Structure of fatty acid synthases. The fatty acid synthase of bacteria and plants is a complex of at least seven different polypeptides. In yeast, all seven activities reside in only two polypeptides the vertebrate enzyme is a single large polypeptide. 

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. 

Wang, X., Song, K.S., Guo, Q.X., and Tian, W.X., The galloyl moiety of green tea cate-chins is the critical structural feature to inhibit fatty-acid synthase. Biochem Pharmacol, Nov 15, 66(10), 2039-2047, 2003. 

Although high-resolution structures of type I PKS modules are not available, some insights into the topological organization of modules have been obtained. Electron microscopy on the vertebrate fatty acid synthase (whose domain layout... 

Smith, S., Witkowski, A., Joshi, A.K. 2003. Structural and functional organization of the animal fatty acid synthase. Prog. Lipid Res. 42, 289-317. 

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...

Leibundgut M, Jenni S, Frick C, Ban N (2007) Structural Basis for Substrate Delivery by Acyl Carrier Protein in the Yeast Fatty Acid Synthase. Science 316 288... 

Polyketide synthases, fatty acid synthases, and non-ribosomal peptide synthetases are a structurally and mechanistically related class of enzymes that catalyze the synthesis of biopolymers in the absence of a nucleic acid or other template. These enzymes utilize the common mechanistic feature of activating monomers for condensation via covalently-bound thioesters of phosphopantetheine prosthetic groups. The information for the sequence and length of the resulting polymer appears to be encoded entirely within the responsible proteins. 

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. 

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]. 

Child CJ, Spencer JB, Bhogal P, Shoolingin-Jordan PM. Structural similarities between 6-methylsalicylic acid synthase from Penicillium patulum and vertebrate type 1 fatty acid synthase evidence from thiol modification studies. Biochemistry 1996 35 12267-12274. 

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... 

Although the basic biochemical reactions in fatty acid synthesis are very similar in E. coli and eukaryotes, the structure of the synthase varies considerably. The fatty acid synthases of eukaryotes, in contrast with those of E. coli, have the component enzymes linked in a large polypeptide chain. 

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)... 

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. 

Pantothenic acid is a precursor for the synthesis of coenzyme A (CoA, CoASH) and forms part of the swinging sulfhydryl arm of the fatty acid synthase complex (Chapter 19). Its structure is shown in Figure 38-20. 

Despite their enormous structural diversity, polyketide metabolites are related by their common derivation from highly functionalised carbon chains whose assemblies are controlled by multifunctional enzyme complexes, the polyketide synthases (PKSs) which, like the closely related fatty acid synthases, catalyse repetitious sequences of decarboxylative condensation reactions between simple acyl thioesters and malonate, as shown in Fig. 3 [7]. Each condensation is followed by a cycle of modifying reactions ketoreduction, dehydration and enoyl reduction. In contrast to fatty acid biosynthesis where the full cycle of essentially reductive modifications normally follow each condensation reduction, the PKSs can use this sequence in a highly selective and controlled manner to assemble polyketide intermediates with an enormous number of permutations of functionality along the chain. As shown in Fig. 3, the reduction sequence can be largely or entirely omitted to produce the classical polyketide intermediate which bears a carbonyl on every alternate carbon and which normally cyclises to aromatic polyketide metabolites. On the other hand, the reductive sequence can be used fully or partially after each condensation to produce highly functionalised intermediates such as the Reduced polyketide in Fig. 3. Basic questions to be answered are (i) what is the actual polyketide intermediate... 

---