Download Lecture 36 - Lipid Metabolism 2

Lipid Metabolism 2:
Acetyl-CoA carboxylase, fatty acid synthase
reaction, and regulation of fatty acid synthesis
Bioc 460 Spring 2008 - Lecture 36 (Miesfeld)
C247
C247 is a fatty acid
synthase inhibitor that
reduces breast cancer
incidence in mice
The fatty acid synthase enzyme
in eukaryotes is dimer of two
very large polypeptide chains,
each encoding seven functional
units
AMP-activated kinase
(AMPK) is a regulator of
acetyl-CoA carboxylase
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.
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.
Difference
subcellular lo cation
carrier protein
enzymes
redox
building block
FA Synthesis
cytosol
acyl carrier protein (ACP)
all acti vities on a single polypeptide chain
reductant is NADPH
malonyl CoA (formed from Acetyl CoA)
FA Degradation
mitochondrial matrix
Coenzyme A (CoA)
multiple enzymes required
oxidants are NAD+ and FAD
acetyl CoA
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 synthesis:
8 Acetyl CoA + 7 ATP + 14 NADPH + 14 H+ →
Palmitate + 8 CoA + 7 ADP + 7 Pi + 14 NADP+ + 6 H2O
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?
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?
The swinging arm mechanism of acetyl-CoA carboxylase
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
The fatty synthesis reaction cycle
Each cycle of the fatty acid synthase
reaction requires the input of one malonylCoA and the oxidation of 2 NADPH
molecules (4 e- total). The synthesis of C16
palmitate therefore requires 14 NADPH.
The fatty synthesis reaction cycle
In the final step, the enzyme
palmitoyl thioesterase
catalyzes a hydrolysis reaction
to release palmitate.
The fatty synthesis reaction cycle
Acetyl-CoA is added first to the
ACP and then transferred to the KS
subunit (not shown here).
The acetyl-CoA is
only added in the
1st round.
The chemical
reactions take
place on the
ACP protein.
The
“ready to
go” chain
is then
moved
back to
KS to
leave the
ACP
protein
open for a
new
malonylCoA.
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 --> 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+ -->
palmitate + 7 CO2 + 8 CoA + 14 NADP+ + 6 H2O
There are 7 dehydration steps required
for palmitate synthesis, why only 6 net H2O?
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+ -->
palmitate + 8 CoA + 7 ADP + 7 Pi + 14 NADP+ + 6 H2O
Write the net reaction for the synthesis of C18 stearate.
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 glucose6P levels?
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.
Metabolic regulation of acetyl-CoA carboxylase
Hormonal regulation of acetyl-CoA carboxylase
AMPK is an important metabolic sensor
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?