Download acetyl CoA carboxylase

METABOLISM OF
LIPIDS:
SYNTHESIS OF
FATTY ACIDS
Fatty Acid Synthesis
• Occurs mainly in liver and adipocytes, in
mammary glands during lactation
• Occurs in cytoplasm
• FA synthesis and degradation occur by two
completely separate pathways
• When glucose is plentiful, large amounts of
acetyl CoA are produced by glycolysis and
can be used for fatty acid synthesis
Three stages of fatty acid synthesis:
A. Transport of acetyl CoA into cytosol
B. Carboxylation of acetyl CoA
C. Assembly of fatty acid chain
A. Transport of Acetyl CoA to the
Cytosol
• Acetyl CoA from catabolism of
carbohydrates and amino acids is exported
from mitochondria via the citrate transport
system
• Cytosolic NADH also converted to NADPH
• Two molecules of ATP are expended for each
round of this cyclic pathway
Citrate transport
system
Sources of NADPH for Fatty Acid Synthesis
1. One molecule of NADPH is generated for each
molecule of acetyl CoA that is transferred from
mitochondria to the cytosol (malic enzyme).
2. NADPH molecules come from the pentose
phosphate pathway.
B. Carboxylation of Acetyl CoA
Enzyme: acetyl CoA carboxylase
Prosthetic group - biotin
A carboxybiotin intermediate is formed.
ATP is hydrolyzed.
The CO2 group in carboxybiotin is transferred to
acetyl CoA to form malonyl CoA.
Acetyl CoA carboxylase is the regulatory enzyme.
C. The Reactions of Fatty Acid Synthesis
• Five separate stages:
(1) Loading of precursors via thioester
derivatives
(2) Condensation of the precursors
(3) Reduction
(4) Dehydration
(5) Reduction
During the fatty acid synthesis all intermediates are linked
to the protein called acyl carrier protein (ACP-SH), which
is the component of fatty acyl synthase complex.
The pantothenic acid is
a component of ACP.
Intermediates in the
biosynthetic pathway
are attached to the
sulfhydryl terminus of
phosphopantotheine
group.
The elongation phase of fatty acid synthesis starts with
the formation of acetyl ACP and malonyl ACP.
Acetyl transacylase and malonyl transacylase catalyze
these reactions.
Acetyl CoA + ACP  acetyl ACP + CoA
Malonyl CoA + ACP  malonyl ACP + CoA
Condensation
reaction.
Acetyl ACP and
malonyl ACP react to
form acetoacetyl ACP.
Enzyme acyl-malonyl ACP
condensing enzyme.
Reduction.
Acetoacetyl ACP is
reduced to D-3hydroxybutyryl ACP.
NADPH is the
reducing agent
Enzyme: -ketoacyl
ACP reductase
Dehydration.
D-3-hydroxybutyryl
ACP is dehydrated to
form crotonyl ACP
(trans-2-enoyl ACP).
Enzyme:
3-hydroxyacyl ACP
dehydratase
Reduction.
The final step in the cycle
reduces crotonyl ACP to
butyryl ACP.
NADPH is reductant.
Enzyme - enoyl ACP
reductase.
This is the end of first
elongation cycle (first
round).
In the second round
butyryl ACP condenses
with malonyl ACP to
form a C6--ketoacyl
ACP.
Reduction, dehydration,
and a second reduction
convert the C6-ketoacyl ACP into a C6acyl ACP, which is ready
for a third round of
elongation.
Final reaction of FA synthesis
• Rounds of synthesis continue until a
C16 palmitoyl group is formed
• Palmitoyl-ACP is hydrolyzed by a thioesterase
Overall reaction of palmitate synthesis
from acetyl CoA and malonyl CoA
Acetyl CoA + 7 Malonyl CoA + 14 NADPH + 14 H+
Palmitate + 7 CO2 + 14 NADP+ + 8 HS-CoA + 6 H2O
Organization of Multifunctional Enzyme
Complex in Eukaryotes
The synthase is dimer with antiparallel subunits.
Each subunit has three domains.
ACP is located in domain 2.
Domain 1 contains transacylases, ketoacyl-ACP
synthase (condensing enzyme)
Domain 2 contains acyl carrier protein, -ketoacyl
reductase, dehydratase, and enoyl reductase.
Domain 3 contains thioesterase activity.
Fatty Acid Elongation and Desaturation
The common product of fatty acid synthesis is
palmitate (16:0).
Cells contain longer fatty acids and unsaturated
fatty acids they are synthesized in the
endoplasmic reticulum.
The reactions of elongation are similar to the ones
seen with fatty acid synthase (new carbons are
added in the form of malonyl CoA).
For the formation of unsaturated fatty acids there
are various desaturases catalizing the formation of
double bonds.
THE CONTROL OF FATTY ACID METABOLISM
Acetyl CoA carboxylase plays an essential role
in regulating fatty acid synthesis and
degradation.
The carboxylase is controlled by hormones:
 glucagon,
 epinephrine, and
 insulin.
Another regulatory factors:
 citrate,
 palmitoyl CoA, and
 AMP
Global Regulation
is carried out by means of reversible phosphorylation
Acetyl CoA carboxylase is switched off by phosphorylation
and activated by dephosphorylation
Insulin stimulates fatty acid synthesis causing
dephosphorylation of carboxylase.
Glucagon and epinephrine have the reverse effect (keep the
carboxylase in the inactive phosphorylated state).
Protein kinase is
activated by AMP and
inhibited by ATP.
Carboxylase is
inactivated when the
energy charge is low.
Local Regulation
Acetyl CoA carboxylase is allosterically stimulated by
citrate.
The level of citrate is high when both acetyl CoA and ATP
are abundant (isocitrate dehydrogenase is inhibited by
ATP).
Palmitoyl CoA inhibits carboxylase.
Fed state:
Response to Diet
• Insulin level is increased
• Inhibits hydrolysis of stored TGs
• Stimulates formation of malonyl CoA, which inhibits
carnitine acyltransferase I
• FA remain in cytosol (FA oxidation enzymes are in the
mitochondria)
Starvation:
• Epinephrine and glucagon are produced and stimulate
adipose cell lipase and the level of free fatty acids rises
• Inactivate carboxylase, so decrease formation of malonyl
CoA (lead to increased transport of FA into mitochondria
and activate the b-oxidation pathway)