Fatty acid synthesis

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Fatty acid synthesis
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Fed state
Glycerol-P
Glycerol
Glucose
Triacylglycerol
Fatty acyl CoA
Fatty acid
Malonyl CoA
Pyruvate
Acetyl CoA
TCA cycle
3
Starved state
Glycerol-P
Glycerol
Glucose
Triacylglycerol
Fatty acyl CoA
Fatty acid
gluconeogenesis
Malonyl CoA
Pyruvate
Acetyl CoA
TCA cycle
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Fatty acid biosynthesis
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A three carbon intermediate, malonyl-CoA,
initiates fatty acid synthesis

• FA biosynthesis and breakdown occur by different
  pathways and take place in different parts of the
  cell.
• Biosynthesis requires malonyl-CoA

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The carboxylation of acetyl-CoA yields
malonyl-CoA
Acetyl CoA carboxylase
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Assembly of a long chain fatty acid

• Once malonyl-CoA is synthesized, long carbon FA
  chains may be assembled in a repeating four-step
  sequence.
• With each passage through the cycle the fatty
  acyl chain is extended by two carbons.
• When the chain reaches 16 carbons, the product
  palmitate (160) leaves the cycle.

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The first round of FA biosynthesis

• To initiate FA biosynthesis, malonyl and acetyl
  groups are activated on to the enzyme fatty acid
  synthase.

Malony-CoA

Acetyl-CoA
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Step 1.

• Condensation of an activated acyl group and two
  carbons derived from malonyl-CoA

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Step 2.

• The b-keto group is reduced to an alcohol by
  NADPH

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Step 3.

• The elimination of water creates a double bond.

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Step 4.

• The double bond is reduced to form the
  corresponding saturated fatty acyl group.

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Repetition of these four steps leads to fatty
acid synthesis

• When reaches 16 carbons, the product leaves the
  cycle.
• All the reactions in the synthetic process are
  catalyzed by a multi-enzyme complex, fatty acid
  synthase.

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A more detailed look at fatty acid synthase
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Fatty acyl synthase contains six enzymatic
activities

• Each segment of the disk represents one of the
  six enzymatic activities of the complex.
• At the center is the ACP acyl carrier protein -
  with its phosphopantetheine arm ending in SH.

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The function of the prosthetic group of the ACP

• Serve as a flexible arm, tethering the growing
  fatty acyl chain to the surface of the synthase
  complex
• Carrying the reaction intermediates from one
  enzyme active site to the next.

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Activation of acetyl and malonyl groups

• Before Steps 1-4, the two thiol groups on the
  enzyme complex must be charged with the correct
  acyl groups.

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The activation of the acetyl group

• The acetyl group from acetyl-CoA is transferred
  to the Cys-SH group of the b-ketoacyl ACP
  synthase.
• This reaction is catalyzed by acetyl-CoA
  transacetylase.

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The activation of the malonyl group

• Transfer of the malonyl group to the SH group of
  the ACP is catalyzed by malonyl-CoA ACP
  transferase.
• The charged acetyl and malonyl groups are now in
  close proximity to each other

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Step 1.

• Condensation of the activated acetyl and malonyl
  groups to form acetoacetyl-ACP, catalyzed by
  b-ketoacyl-ACP synthase.

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Step 2.

• Reduction. The acetoacetyl-ACP is reduced to
  b-hydroxybutyryl-ACP, catalyzed by b-ketoacyl-ACP
  reductase (needs NADPH H)

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Step 3.

• Dehydration to yield a double bond in the
  product, trans-D2-butenoyl-ACP, catalyzed by
  b-hydroxyacyl-ACP dehydratase.

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Step 4.

• Reduction of the double bond to form butyryl-ACP,
  catalyzed by enoyl-reductase.
• Another NADPH dependent reaction.

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The growing chain is transferred from the acyl
carrier protein

• This reaction makes way for the next incoming
  malonyl group.
• The enzyme involved is acetyl-CoA transacetylase.

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Beginning of the second round of the FA synthesis
cycle

• The butyryl group is on the Cys-SH group.
• The incoming malonyl group is first attached to
  ACP.
• In the condensation step, the entire butyryl
  group is exchanged for the carboxyl group on the
  malonyl residue.

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The result of fatty acyl synthase activity

• Seven cycles of condensation and reduction
  produce the 16-carbon saturated palmitoyl group,
  still bound to ACP.
• Chain elongation usually stops at this point, and
  free palmitate is released from the ACP molecule
  by hydrolytic activity in the synthase complex.
• Smaller amounts of longer fatty acids such as
  stearate (180) are also formed.

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The overall reaction for the synthesis of
palmitate from acetyl-CoA can be considered in
two parts.
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Part 1.

• First, the formation of seven malonyl-CoA
  molecules
• 7Acetyl-CoA 7CO2 7ATP 7malonyl-CoA 7ADP
  7Pi

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Part 2.

• Then the seven cycles of condensation and
  reduction
• Acetyl-CoA 7malonyl-CoA 14NADPH 14H
• palmitate 7CO2 8CoA 14NADP 6H2O
• The biosynthesis of FAs requires acetyl-CoA and
  the input of energy in the form of ATP and
  reducing power of NADPH.

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Location of FA synthesis

• FA synthase complex is found exclusively in the
  cytosol.
• The location segregates synthetic processes from
  degradative reactions.

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• In hepatocytes
• the NADPH/NAD ratio is very high (75)
  in the cytosol, furnishing a strongly reducing
  environment for the reductive synthesis of fatty
  acids and other biomolecules.

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Fatty acid synthesis requires considerable
amounts of NADPH H

• Acetyl-CoA 7malonyl-CoA 14NADPH 14H
• palmitate 7CO2 8CoA 14NADP 6H2O
• In hepatocytes and adipocytes, cytosolic NADPH is
  largely generated by the malic enzyme and by the
  pentose phosphate pathway.

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1. The malic enzyme

• The pyruvate produced in the reaction reenters
  the mitochondrion.

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2. The pentose phosphate pathway

• In hepatocytes and the mammary gland of lactating
  animals, the NADPH is supplied primarily by the
  pentose phosphate pathway.

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Fatty acid synthesis requires considerable
amounts of acetyl-CoA

• 7Acetyl-CoA 7CO2 7ATP 7malonyl-CoA 7ADP
  7Pi
• Nearly all acetyl-CoA used in fatty acid
  synthesis is formed in mitochondria from pyruvate
  oxidation.
• So acetate must go from the mitochondria to the
  cytosol

Cytosol site of acetate utilization
Mitochondria site of acetate manufacture
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Acetate is shuttled out of mitochondria as citrate

• The mitochondrial inner membrane is impermeable
  to acetyl-CoA
• Intra-mitochondrial acetyl-CoA first reacts with
  oxaloacetate to form citrate, in the TCA cycle
  catalyzed by citrate synthase.

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• Citrate then passes into the cytosol through the
  mitochondrial inner membrane on the citrate
  transporter.
• In the cytosol, citrate is cleaved by citrate
  lyase regenerating acetyl-CoA.

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• The other product --oxaloacetate cannot return to
  the mitochondrial matrix directly.
• Instead, oxaloacetate is reduced to malate

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• Malate returns to the mitochondrial matrix on the
  malate-a-ketoglutarate transporter in exchange
  for citrate.

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Regulation of fatty acid synthesis

• When a cell has more energy, the excess is
  generally converted to FAs and stored as lipids
  such as triacylglycerol.
• The reaction catalyzed by acetyl-CoA carboxylase
  is the rate limiting step in the biosynthesis of
  fatty acids.

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The carboxylation of acetyl-CoA yields
malonyl-CoA
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Regulation of acetyl-CoA carboxylase(1)

• Palmitoyl-CoA acts as a feedback inhibitor of the
  enzyme, and citrate is an activator.
• When there is an increase in mitochondrial
  acetyl-CoA and ATP, citrate is transported out of
  mitochondria,
• Citrate becomes both the precursor of cytosolic
  acetyl-CoA and a signal for the activation of
  acetyl-CoA carboxylase.

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Regulation of acetyl-CoA carboxylase (2)
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Regulation of acetyl-CoA carboxylase (3)

• Additionally, these pathways are regulated at the
  level of gene expression.
• For example, when animals ingest an excess of
  certain polyunsaturated fatty acids, the
  expression of genes encoding a wide range of
  lipogenic enzymes in the liver is suppressed.

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Additional modification to the newly synthesized
fatty acid

• Extended to form longer fatty acids
• Converted to monounsaturated and polyunsaturated
  fatty acids

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Fatty acid elongation

• Palmitate in animal cells is the precursor of
  other long-chained FAs.
• By further additions of acetyl groups, through
  the action of FA elongation systems present in
  the smooth endoplasmic reticulum and the
  mitochondria.

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The desaturation of FAs

• Palmitate and stearate serve as precursors of the
  two most common monosaturated fatty acids of
  animal cells palmitoleate (161D9), and oleate
  (181D9).
• The double bond is introduced by fatty acyl-CoA
  desaturase in the smooth endoplasmic reticulum.

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Essential fatty acids

• Mammalian hepatocytes readily introduce double
  bonds at the D9 position of FAs but cannot
  between C-10 and the methyl-terminal end.
• Linoleate, 182D9,12 and linolenate 183D9,12,15
  cannot be synthesized by mammals, but plants can
  synthesize both.

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The fate of fatty acids

• Most of the FAs synthesized or ingested by an
  organism have one of two fates
• incorporated into triacylglycerols for the
  storage of metabolic energy
• incorporation into the phospholipid components of
  membranes.

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The formation of phosphatidic acid

• Fatty acyl groups are first activated by
  formation of fatty acyl-CoA molecules.
• then transferred to ester linkage with L-glycerol
  3-phosphate.

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Phosphatidic acid may be converted to
triacylglycerols or phospholipids

• Triacylglycerols and phosholipids are both
  synthesized from phosphatidic acid

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Lecithin (phosphatidyl choline)
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Partitioning of the fates of fatty acids

• Depends on the needs of the organism
• During rapid growth, synthesis of new membranes
  requires membrane phospholipid synthesis
• Organisms that have a plentiful supply of food
  but are not actively growing shunt most of their
  fatty acids into storage fats.

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Summary of lipid metabolism

• FA biosynthesis requires malonyl-CoA formation
• The long carbon chains of FA acids are assembled
  in a repeating four-step sequence catalyzed by
  the multifunctional enzyme fatty acid synthase.
• With each passage through the cycle, the fatty
  acyl chain is extended by two carbons
• When the chain length reaches 16 carbons, the
  product (palmitate 160) leaves the cycle.

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• Cytosolic NADPH is largely generated by the malic
  enzyme and by the pentose phosphate pathway.
• FA biosynthesis occurs in the cytosol
• FA biosynthesis is regulated by the activity of
  acetyl-CoA carboxylase
• Synthesized FA are either stored as TG or made
  into membrane lipids