Transcript Fatty Acid Synthesis

Fatty Acid Synthesis
Dr. Sooad Al-Daihan
Biochemistry department
Introduction
There are three systems for the synthesis of fatty acids
1. De novo synthesis of FAs in cytoplasm
2. Chain elongation in mitochondria
3. Chain elongation in microsomes
De novo synthesis of FAs
 In mammals fatty acid synthesis occurs primarily in the cytosol of
the liver and adipose tissues .It also occurs in mammary glands
during lactation.
 Acetyl-CoA is the starting material for FA synthesis. However,
most acetyl-CoA in mitochondria(from the breakdown of sugars,
some amino acids and other fatty acids).
So, acetyl-CoA must be transferred from the mitochondria to the
cytosol
BUT Mitochondria not permeable to acetyl CoA
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 Citrate-malate-pyruvate shuttle provides cytosolic acetyl CoA and
reducing equivalents NADPH for fatty acid synthesis.
 Acetyl–CoA units are shuttled out of the mitochondrial matrix as citrate.
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RULE:
Fatty acid synthesis is a stepwise assembly of acetyl CoA unit
(mostly as malonyl CoA) ending with palmitate (16 C saturated)
 4 Steps repeating cycle, extension 2C:
 Condensation
 Reduction
 Dehydration
 Additional reduction
Formation of Malonyl-coenzyme A
(Activation of acetate)
 Is the committed step in fatty acid synthesis (Rate Limiting Reaction)
 It takes place in two steps:
1. Carboxylation of biotin (involving ATP)
2.Transfer of the carboxyl to acetyl-CoA to form malonyl-CoA
 Reactions are catalyzed by acetyl-CoA carboxylase (multienzyme)
Fatty acid synthase
 It is a multi-enzyme complex
consist of 7 enzymes linked
covalently in a single polypeptide
chain.
 It is a dimer, and each monomer is
identical, consisting of one chain
(250 kD) containing all seven
enzyme activities of fatty acid
synthase and an acyl carrier protein
(ACP)
 ACP contains the vitamin
pantothenic acid in the form of 4'phosphopantetheine (Pant). ACP is
the part that carry the acyl groups
during fatty acid synthesis
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1- A molecule of acetate is transferred from Acetyl CoA to the –SH
group of ACP by acetyl CoA-ACP transacylase (initiation or priming).
2- Next, this 2C fragment is transferred to a cysteine residue in the
active site of the condensing enzyme.
3-The now-empty ACP accepts a 3C malonate unit from malonyl CoA,
malonyl CoA-ACP transacylase catalyzes this reaction
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4- Acetyl unit (on the condensing enzyme) condenses with 2 carbon
portion of malonyl unit on ACP forming acetoacetyl-S- ACP with release
of CO2.
This reaction is catalyzed by β-ketoacyl –ACP synthase
 Active site on the condensing enzyme is free.
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5-The β-ketone is reduced to an alcohol by e- transfer from NADPH.
6- Dehydration yields a trans double bond.
7- Reduction by NADPH yields a saturated chain.
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8- Following transfer of the growing fatty acid from Pant to the
Condensing Enzyme's cysteine sulfhydryl, the cycle begins again, with
another malonyl-CoA.
Note: Acetyl residue successively added is derived from the 2C atoms of
malonyl CoA with the release of the third C as CO2 EXCEPT the 2
donated by the original acetyl CoA which are found at the methyl group
end of the fatty acid.
Product Release
 When the fatty acid is 16 carbon atoms long, a Thioesterase domain
catalyzes hydrolysis of the thioester linking the fatty acid to
phosphopantetheine.
 The16-C saturated fatty acid palmitate is the final product of the
Fatty Acid Synthase complex (but it may produce short chain FAs)
 Further elongation and insertion of double bonds are carried out by
other enzyme system.
 Palmitate, a 16-C saturated fatty acid, is the final product of the Fatty Acid
Synthase reactions.
1- a. How many acetyl-CoA used for initial priming of enzyme? 1
b. How many acetyl-CoA used for synthesis of each malonate? 1
c. How many malonate used (how many reaction cycles) per synthesis of one 16-C
palminate? 7
d. Total acetyl-CoA used for priming & for syntheisis of malonate, a + b(c): 8
2- a. How many ~P bonds of ATP used for synthesis of each malonate? 1
b. Total ~P bonds of ATP used for synthesis of one 16-C palmitate, 2a(1c): 7
3- a. How many NADPH used per reaction cycle? 2
b. Total NADPH used per synthesis of one 16-C palmitate, 3a(1c): 14
No. of cycles = (C/2) – 1
No. of Malonate molecules = (C/2) – 1
No. of Acetyl CoA molecules= [(C/2) – 1] +1
No. of NADPH molecules = [(C/2) – 1] x2
Regulation of FA Synthesis
 Allosteric regulation
• Acetyl CoA carboxylase, which
catalyzes the committed step in fatty
acid synthesis, is a key control site.
• End-product fatty acid is a feedback
inhibitor (palmitoyl-CoA)
• Activated by citrate, which increases
in well-fed state and is an indicator of a
plentiful supply of acetyl-CoA
• Inhibited by long-chain acyl-CoA
Regulation of FA Synthesis
 Glucagon
inhibits fatty acid
synthesis while increasing lipid
breakdown and fatty acid βoxidation.
 Acetyl
CoA cayboxylase is
inactivated by phosphorylation.
 Insulin prevents action of glucagon
Inhibits lipases/activates acetyl
Co A cayboxylase
Further Processing of C16 Fatty Acids
Additional Elongation
In mammalian systems FA elongation can occur either in :
• Microsomes
• Mitochondria
Chain Elongation in Microsomes
 The reactions are similar to that which occurs in the cytosolic FA
synthase in that:
a) The source of the 2 carbon units is malonyl CoA.
b) NADPH is used as reducing power.
 In contrast to denovo synthesis of Fatty Acids, the intermediates in
the subsequent reactions are CoA esters, indicating that the
process is carried out by separate enzymes rather than a complex
of FA synthase type. (uses CoA instead of ACP as the acyl carrier)
 It is the main site for elongation of existing long chain FAs
molecules.
Chain Elongation in Mitochondria
 It differs from the microsomal system in
that acetyl CoA is the source of the added
2C atoms (instead of malonyl CoA)
 NADH and NADPH are sources of
reducing agents
 This system operate by simple reversal of
the pathway of FA oxidation with the
exception that, NADPH-linked α,βunsaturated acyl CoA reductase replaces
FAD linked acyl CoA dehydrogenase.
 The mitochondrial system serves in the
elongation of shorter chain fatty acids to
long chin FAs.
Biosynthesis of Unsaturated Fatty Acids
 Desaturases introduce double bonds at specific positions in a
fatty acid chain.
 Mammalian cells are unable to produce double bonds at
certain locations, e.g., ∆ 12.
 Thus some polyunsaturated fatty acids are dietary essentials,
e.g., linoleic acid, 18:2 cis ∆ 9,12 (18 C atoms long, with cis
double bonds at carbons 9-10 & 12-13)
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 Formation of a double bond in a fatty acid involves the
following endoplasmic reticulum membrane proteins in
mammalian cells:
• NADH-cyt b5 Reductase, a flavoprotein with FAD as
prosthetic group.
• Cytochrome b5, which may be a separate protein or a domain at
one end of the desaturase.
• Desaturase, with an active site that contains two iron atoms
complexed by histidine residues
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 The
∆9 desaturase in the
endoplasmic reticulum catalyzes
the conversion of stearate (18:0)
to oleate (18:1 cis ∆ 9) .
 the overall reaction is:
stearate + NADH + H+ + O2 
oleate + NAD+ + 2H2O
 Synthesis of polyunsaturated fatty
acids involves desaturase and
elongase systems
Differences in the oxidation and
synthesis of FAs