Lipid Metabolism 2: Acetyl-CoA carboxylase, fatty acid synthase reaction, and regulation of fatty acid synthesis

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Lipid Metabolism 2Acetyl-CoA carboxylase, fatty
acid synthase reaction, and regulation of fatty
acid synthesis
Bioc 460 Spring 2008 - Lecture 36 (Miesfeld)
The fatty acid synthase enzyme in eukaryotes is
dimer of two very large polypeptide chains, each
encoding seven functional units
C247 is a fatty acid synthase inhibitor that
reduces breast cancer incidence in mice
AMP-activated kinase (AMPK) is a regulator of
acetyl-CoA carboxylase
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Key Concepts in Lipid Metabolism

• Fatty acid synthesis takes place in the cytosol,
  uses NADPH as coenzyme in redox reactions, and
  the building block is malonyl-CoA.
• Acetyl-CoA carboxylase is the key regulated
  enzyme in fatty acid synthesis and is responsible
  for generating malonyl-CoA in a carboxylation
  reaction using acetyl-CoA.
• The fatty acid synthase protein complex consists
  of six enzymatic activities and the acyl carrier
  protein each reaction cycle adds two carbons
  that are derived from malonyl-CoA following
  decarboxylation.
• The citrate shuttle is responsible for moving
  acetyl-CoA equivalents from the mitochondrial
  matrix to the cytosol.

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• Comparison of fatty acid synthesis and degradation

While the chemistry of the four core reactions
required for the removal or addition of C2 acetyl
groups to the hydrocarbon chain are similar
between fatty acid degradation and synthesis, the
two pathways are in fact quite distinct in terms
of the required enzymes, subcellular location and
source of redox energy.
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Review of Pathway Questions

• 1. What purpose does fatty acid synthesis serve
  in animals?
• Fatty acid synthesis reactions in the cytosol of
  liver and adipose cells convert excess acetyl CoA
  that builds up in the mitochondrial matrix when
  glucose levels are high into fatty acids that can
  be stored or exported as triacylglycerols.
• 2. What is the net reaction in the synthesis C16
  palmitate?
• Fatty acid synthesis8 Acetyl CoA 7 ATP 14
  NADPH 14 H ? Palmitate 8 CoA 7 ADP
  7 Pi 14 NADP 6 H2O

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Review of Pathway Questions

• 3. What are the key enzymes in fatty acid
  synthesis?Acetyl CoA carboxylase - catalyzes
  the commitment step in fatty acid synthesis using
  a biotin-mediated reaction mechanism that
  carboxylates acetyl-CoA to form the C3 compound
  malonyl-CoA.. Fatty acid synthase - this large
  multi-functional enzyme is responsible for
  catalyzing a series of reactions that
  sequentially adds C2 units to a growing fatty
  acid chain covalently attached to the enzyme
  complex.
• 4. What is a real life application of fatty acid
  synthesis?

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Acetyl-CoA carboxylase catalyzes the commitment
step in fatty acid synthesis by converting
acetyl-CoA to malonyl-CoA
C2 acetate unit
Is the energy charge high or low in the cell when
excess acetyl-CoA is available for fatty acid
synthesis?
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The swinging arm mechanism of acetyl-CoA
carboxylase
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The fatty synthesis reaction cycle
ACP
Acetyl-CoA is the priming group only in the first
cycle, after that, only malonyl-CoA is added to
the ACP carrier protein each time. There are
four reaction steps required each cycle to result
in the net addition two carbons to the growing
fatty acid chain.
ACP
ACP
ACP
ACP
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The fatty synthesis reaction cycle
Each cycle of the fatty acid synthase reaction
requires the input of one malonyl-CoA and the
oxidation of 2 NADPH molecules (4 e- total). The
synthesis of C16 palmitate therefore requires 14
NADPH.
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The fatty synthesis reaction cycle
In the final step, the enzyme palmitoyl
thioesterase catalyzes a hydrolysis reaction to
release palmitate.
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The fatty synthesis reaction cycle
Acetyl-CoA is added first to the ACP and then
transferred to the KS subunit (not shown here).
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The ready to go chain is then moved back to KS
to leave the ACP protein open for a new
malonyl-CoA.
The acetyl-CoA is only added in the 1st round.
The chemical reactions take place on the ACP
protein.
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Summary of the fatty synthesis pathway
We begin by forming seven molecules of malonyl
CoA using the acetyl CoA carboxylase reaction 7
Acetyl CoA 7 CO2 7 ATP --gt 7 malonyl 7
ADP 7 Pi We then use these seven malonyl CoA
molecules for seven turns of the reaction cycle
beginning with the priming of fatty acid synthase
by one molecule of acetyl CoA 1 Acetyl CoA 7
malonyl CoA 14 NADPH 14 H --gt
palmitate 7 CO2 8 CoA 14 NADP 6 H2O
There are 7 dehydration steps required for
palmitate synthesis, why only 6 net H2O?
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Summary of the fatty synthesis pathway
The net fatty acid synthesis reaction for
palmitate (C18) can then be written as 8 Acetyl
CoA 7 ATP 14 NADPH 14 H --gt
palmitate 8 CoA 7 ADP 7 Pi 14 NADP 6
H2O
Write the net reaction for the synthesis of C18
stearate.
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The citrate shuttle transports acetyl-CoA
equivalents from the matrix to the cytosol and
generates NADPH
The citrate uses malic enzyme to generate 1 NADPH
for each acetyl-CoA equivalent that is
transported.
The bulk of NADPH needed for fatty acid synthesis
comes from the PPP. Why does this makes sense in
terms of glucose-6P levels?
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Regulation of fatty acid synthesis
Acetyl CoA carboxylase is most active when it
is in a homopolymeric form. Citrate and
palmitoyl CoA are metabolites that bind to an
allosteric site on the enzyme stimulating
polymerization or depolymerization, respectively.

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Metabolic regulation of acetyl-CoA carboxylase
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Hormonal regulation of acetyl-CoA carboxylase
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AMPK is an important metabolic sensor
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Three Metabolic Control Points of FA Synthesis
What is the likely metabolic fate of the
palmitoyl-CoA if this were a liver cell? What
if it were a fat cell?