Fatty acid synthase

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Fatty acid synthase
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fatty acid synthase
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Fatty acid synthase (FAS)[1] is an enzyme that in humans is encoded by the FASN gene.[2][3][4][5]

Fatty acid synthase is a multi-enzyme

substrates are handed from one functional domain to the next.[1][6][7][8][9]

Its main function is to catalyze the synthesis of palmitate (C16:0, a long-chain saturated fatty acid) from acetyl-CoA and malonyl-CoA, in the presence of NADPH.[5]

The fatty acids are synthesized by a series of decarboxylative

ketoreductase (KR), dehydratase (DH), and enoyl reductase (ER). The growing fatty acid chain is carried between these active sites while attached covalently to the phosphopantetheine prosthetic group of an acyl carrier protein (ACP), and is released by the action of a thioesterase (TE) upon reaching a carbon chain length of 16 (palmitic acid).[1]

Classes

There are two principal classes of fatty acid synthases.

The mechanism of FAS I and FAS II elongation and reduction is the same, as the domains of the FAS II enzymes are largely homologous to their domain counterparts in FAS I multienzyme polypeptides. However, the differences in the organization of the enzymes - integrated in FAS I, discrete in FAS II - gives rise to many important biochemical differences.[12]

The evolutionary history of fatty acid synthases are very much intertwined with that of polyketide synthases (PKS). Polyketide synthases use a similar mechanism and homologous domains to produce secondary metabolite lipids. Furthermore, polyketide synthases also exhibit a Type I and Type II organization. FAS I in animals is thought to have arisen through modification of PKS I in fungi, whereas FAS I in fungi and the CMN group of bacteria seem to have arisen separately through the fusion of FAS II genes.[10]

Structure

Mammalian FAS consists of a homodimer of two identical protein subunits, in which three

C-terminal domains (enoyl reductase (ER), -ketoacyl reductase (KR), acyl carrier protein (ACP) and thioesterase (TE)).[13][14] The interdomain region allows the two monomeric domains to form a dimer.[13]

The conventional model for organization of FAS (see the 'head-to-tail' model on the right) is largely based on the observations that the bifunctional reagent 1,3-dibromopropanone (DBP) is able to crosslink the active site

heterodimeric FAS containing only one competent monomer is capable of palmitate synthesis.[20]

The above observations seemed incompatible with the classical 'head-to-tail' model for FAS organization, and an alternative model has been proposed, predicting that the KS and MAT domains of both monomers lie closer to the center of the FAS dimer, where they can access the ACP of either subunit (see figure on the top right).[21]

A low resolution X-ray crystallography structure of both pig (homodimer)[22] and yeast FAS (heterododecamer)[23] along with a ~6 Å resolution electron cryo-microscopy (cryo-EM) yeast FAS structure [24] have been solved.

Substrate shuttling mechanism

The solved structures of yeast FAS and mammalian FAS show two distinct organizations of highly conserved catalytic domains/enzymes in this multi-enzyme cellular machine. Yeast FAS has a highly efficient rigid barrel-like structure with 6 reaction chambers which synthesize fatty acids independently, while the mammalian FAS has an open flexible structure with only two reaction chambers. However, in both cases the conserved ACP acts as the mobile domain responsible for shuttling the intermediate fatty acid substrates to various catalytic sites. A first direct structural insight into this substrate shuttling mechanism was obtained by cryo-EM analysis, where ACP is observed bound to the various catalytic domains in the barrel-shaped yeast fatty acid synthase.[24] The cryo-EM results suggest that the binding of ACP to various sites is asymmetric and stochastic, as also indicated by computer-simulation studies[25]

catalytic
domains and their corresponding reactions, visualization by Kosi Gramatikoff. Note that FAS is only active as a homodimer rather than the monomer pictured.
catalytic
domains and their corresponding reactions, visualization by Kosi Gramatikoff.

Regulation

sterol regulatory element binding protein-1c (SREBP-1c) in response to feeding/insulin in living animals.[26][27]

Although

sterol regulatory element binding protein-1c (SREBP-1c) in feeding, regulation of FAS by SREBP-1c is USF-dependent.[27][28][29][30]

Acylphloroglucinols isolated from the fern Dryopteris crassirhizoma show a fatty acid synthase inhibitory activity.[31]

Clinical significance

The FASN gene has been investigated as a possible oncogene.[32] FAS is upregulated in breast and gastric cancers, as well as being an indicator of poor prognosis, and so may be worthwhile as a chemotherapeutic target.[33][34][35] FAS inhibitors are therefore an active area of drug discovery research.[36][37][38][39][40]

FAS may also be involved in the production of an endogenous ligand for the nuclear receptor PPARalpha, the target of the fibrate drugs for hyperlipidemia,[41] and is being investigated as a possible drug target for treating the metabolic syndrome.[42] Orlistat which is a gastrointestinal lipase inhibitor also inhibits FAS and has a potential as a medicine for cancer.[43][44]

In some cancer cell lines, this protein has been found to be fused with estrogen receptor alpha (ER-alpha), in which the N-terminus of FAS is fused in-frame with the C-terminus of ER-alpha.[5]

An association with

uterine leiomyomata has been reported.[45]

See also

References

Further reading

External links