Fatty Acid Synthesis

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Fatty Acid Synthesis
Dr. Sooad Al-Daihan Biochemistry department
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Introduction

• There are three systems for the synthesis of
  fatty acids
• De novo synthesis of FAs in cytoplasm
• Chain elongation in mitochondria
• Chain elongation in microsomes

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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.
• AcetylCoA 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

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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)

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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 ß-ketoacyl ACP
  synthase
• ? Active site on the condensing enzyme is free.

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• 5-The ß-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.

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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.

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• 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
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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

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Regulation of FA Synthesis

• Glucagon inhibits fatty acid synthesis while
  increasing lipid breakdown and fatty acid
  ß-oxidation.
• Acetyl CoA cayboxylase is inactivated by
  phosphorylation.
• Insulin prevents action of glucagon?Inhibits
  lipases/activates acetyl Co A cayboxylase

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• Further Processing of C16 Fatty Acids
• Additional Elongation
• In mammalian systems FA elongation can occur
  either in
• Microsomes
• Mitochondria

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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.

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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 a,ß-unsaturated 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.

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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, 182 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 (180) to
  oleate (181 cis ? 9) .
• the overall reaction is
• stearate NADH H O2 ? oleate NAD 2H2O
• Synthesis of polyunsaturated fatty acids involves
  desaturase and elongase systems

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Differences in the oxidation and synthesis of FAs