Friday, 13 February 2015
Sunday, 8 February 2015
Fatty acid synthase (FAS)
Fatty
acid synthase is a multi-enzyme protein that catalyzes fatty acid synthesis. It is not a single enzyme but a whole enzymatic system composed of two
identical 272 kDa multifunctional polypeptides, in which substrates are handed from one functional domain to the
next.[5][6][7][8]
Its
main function is to catalyze the synthesis of palmitate from acetyl-CoA and malonyl-CoA, in the presence of NADPH, into long-chain saturated fatty acids.[4]
Contents
Fatty acids are aliphatic acids fundamental to energy production and
storage, cellular structure and as intermediates in the biosynthesis of hormones and other biologically important molecules. They are synthesized
by a series of decarboxylative Claisen condensation reactions from acetyl-CoA and malonyl-CoA. Following each round of elongation the beta
keto group is reduced to the fully saturated carbon chain by the sequential
action of a ketoreductase (KR),dehydratase (DH), and enol
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 (palmitidic
acid).
There
are two principal classes of fatty acid synthases.
·
Type I systems utilise
a single large, multifunctional polypeptide and are common to both mammals and fungi (although the structural arrangement of fungal
and mammalian synthases differ). A Type I fatty acid synthase system is also
found in the CMN group of bacteria (corynebacteria, mycobacteria, and
nocardia). In these bacteria, the FAS I system produces palmititic acid, and
cooperates with the FAS II system to produce a greater diversity of lipid products.[9]
·
Type II is found in
archaea and bacteria, and is characterized by the use of discrete,
monofunctional enzymes for fatty acid synthesis. Inhibitors of this pathway
(FASII) are being investigated as possible antibiotics.[10]
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.[11]
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.[9]
Mammalian
FAS consists of a homodimer of two identical protein subunits, in which three catalytic domains in the N-terminal section (-ketoacyl synthase (KS),
malonyl/acetyltransferase (MAT), and dehydrase (DH)), are separated by a core
region of 600 residues from four C-terminal domains (enoyl reductase (ER), -ketoacyl reductase (KR), acyl
carrier protein (ACP) and thioesterase (TE)).[12][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 cysteine thiol of the KS domain in one FAS monomer with the phosphopantetheine prosthetic group of the ACP domain in the
other monomer.[14][15] Complementation analysis of FAS dimers carrying different
mutations on each monomer has established that the KS and MAT domains can
cooperate with the ACP of either monomer.[16][17] and a reinvestigation of the DBP crosslinking experiments
revealed that the KS active site Cys161 thiol could be crosslinked to the ACP
4'-phosphopantetheine thiol of either monomer.[18] In addition, it has been recently reported that aheterodimeric FAS containing only one competent monomer is capable of
palmitate synthesis.[19]
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).[20]
A
low resolution X-ray crystallography structure of both pig (homodimer)[21] and yeast FAS (heterododecamer)[22] along with a ~6 Å resolution electron cryo-microscopy (cryo-EM)
yeast FAS structure [23] have been solved.
The
solved structures of yeast FAS and mammalian FAS show two distinct organization
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.[23] The cryo-EM results suggest that the binding of ACP to various
sites is asymmetric and stochastic, as also indicated by computer-simulation
studies[24]
|
FAS revised model with positions of polypeptides, three catalytic domains
and their corresponding reactions, visualization by Kosi Gramatikoff. Note
that FAS is only active as a homodimer rather than the monomer pictured.
|
FAS 'head-to-tail' model with positions of polypeptides,
three catalytic domains
and their corresponding reactions, visualization by Kosi Gramatikoff.
|
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