Organization of Fatty Acid Synthase from Animal Liver

1
Topics
BIOCHEMISTRY 441 Part A Winter 2005
Dept. of Biochemistry
Bill Parson
1. Biosynthesis of fatty acids 2. Phospholipids,
triacylglycerols complex lipids 3. Cholesterol
lipoproteins 4. Photosynthesis antenna
reaction center complexes 5. Photosynthesis
photophosphorylation 6. Photosynthesis carbon
fixation 7. Amino acid catabolism
transamination 8. Amino acid catabolism urea
cycle 9. N2 fixation amino acid
biosynthesis 10. Aromatic amino acids
neurotransmitters 11. One-carbon metabolism 12.
Biosynthesis of pyrimidines purines 13.
Deoxyribonucleotide biosynthesis nucleotide
catabolism
2
Fatty acids have extended hydrocarbon chains
Most natural fatty acids have an even number of
carbons. Unsaturated fatty acids usually have
cis double bonds
3
Fatty acids are components of phospholipids and
triacylglycerols
triacylglycerols
glycerophospholipids
sphingolipids
Triacylglycerols are stored as energy reserves.
Sphingolipids on cell surfaces are sites of cell
recognition. Inositol phospholipids participate
in intracellular signaling.
Fatty acids also are found as cholesterol esters
in lipoproteins, and are attached covalently to
some proteins.
Phospholipid bilayers are the central structural
elements of biological membranes.
4
Triacylglycerols are stored as energy reserves in
adipose tissue and other tissues
capillary
lipid droplets
Cross section of four adipocytes from a guinea
pig. Lipid droplets, consisting mainly of
triacylglycerols, fill most of the volume of the
cells.
5
Fatty acids are synthesized from acetyl-CoA in
adipose tissue the liver
palmitic acid (C160)
palmitoylCoA
stearic acid (C180)
stearoylCoA
9
oleic acid (C181 D9)
1
oleoylCoA
18
H3C
6
MalonylCoA serves as the donor of two-carbon units
malonylCoA
biotin
transcarboxylase site
acetylCoA carboxylase
acetylCoA carboxylase
acetylCoA carboxylase
MalonylCoA is formed from acetylCoA and CO2 by
acetylCoA carboxylase (biotin carboxylase-transcar
boxylase). Biotin is attached covalently to a
Lys residue of the enzyme. In bacteria, the
three domains are in separate subunits in
animals, they are on a single, multifunctional
polypeptide.
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The growing fatty acid chain is attached to
acyl-carrier protein (ACP)
ACP has 4-phosphopantetheine linked to a Ser
residue. Malonyl and acetyl groups are
transferred from CoA to the sulfur atom of the
4-phosphopantetheine.
Pantetheine is vitamin B5
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Acyl-carrier protein from Bacillus subtilis
4-phosphopantetheine
from pdb file 1f80.pdb K. D. Parris et al.
Struct. Fold. Design 8 883 (2000).
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Malonyl-/Acetyl transferase (MAT) loads the
malonyl and acetyl substrates onto ACP
Bacteria have separate malonyl acetyl
transferases
CoA-SH
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The condensation reaction
3-ketoacyl-ACP
malonyl-ACP
S
The acetyl group first is transferred from ACP to
a Cys residue of the synthase. Then it combines
with malonyl-ACP to give a 3-ketoacyl-ACP.
Release of bicarbonate is exothermic and pulls
the reaction in the direction of condensation.
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Reduction by NADPH, dehydration, and a second
reduction generates butyryl-ACP
b-ketoacyl synthase
b-ketoacyl reductase
enoyl reductase
dehydrase
HOH
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To continue the cycle, the fatty acid chain must
move back to the ketoacyl synthase
b-ketoacyl synthase
CO2
H2O
b-ketoacyl reductase
enoyl reductase
dehydrase
HOH
13
A second turn of the cycle generates hexanoyl-ACP
b-ketoacyl synthase
CO2
b-ketoacyl reductase
enoyl reductase
dehydrase
HOH
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The cycle stops when the fatty acid chain reaches
16 carbons
Thioesterase hydrolyzes palmitoyl-ACP, releasing
palmitate (C160)
C140
b-ketoacyl synthase
CO2
thioesterase
C160
HOH
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Chain-length specificity of the
substrate-loading, chain-elongation and
chain-termination activities of mammalian
fatty-acid synthase
S. Smith et al. Prog. Lipid Res. 42 289-317
(2003)
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The enzymes of the fatty acid synthase system
have fused into a single protein during evolution
ACP
CO2-
H3N
O-phosphopantetheine
E. coli 8 separate proteins
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Testing a head-to-tail model of the fatty-acid
synthase dimer
Mix the monomers to form a heterodimer
Is the heterodimer active? The answer depends on
which domain is mutated.
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Suggested model of mammalian fatty-acid synthase
dimer
solid arrows substrate loading condensation.
dotted arrows b-carbon processing
chain-termination
S. Smith et al. Prog. Lipid Res. 42 289-317
(2003)
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Differences between fatty acid synthesis and
oxidation
oxidation
synthesis
H2O
cytosol
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Fatty acids with longer chains (C180 C200)
are synthesized from palmitoylCoA by an
elongation mechanism






These reactions occur in mitochondria the
smooth ER.
NADPH
Different enzymes are involved, and CoA is used
in place of ACP, but the reactions are otherwise
the same as in synthesis of palmitate.
NADP
H2O
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Citrate carries 2-carbon units from mitochondria
to the cytosol
Cytosol
citrate synthase
Mitrochondrion
acetyl-CoA
CoA-SH
CH3CO-S-CoA
oxaloacetate
citrate transporter
CoA-SH ATP
citrate lyase
citrate
ADP Pi
CH3CO-S-CoA
Citrate lyase uses ATP to drive the breakdown of
citrate to acetylCoA oxaloacetate in the
cytosol
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Integration of fatty acid synthesis with
carbohydrate metabolism
fatty acids
citrate
citrate
NADPH
CoA-SH
CoA-SH
ATP
pyruvate
ATP
citrate lyase
acetylCoA
acetylCoA
ADP Pi
TCA cycle
amino acids
oxaloacetate
oxaloacetate
NADH
malate dehydro-genase
NADH
ox. phos.
NAD
ATP
NAD
malate
malate
NADP
pyruvate carboxylase
malic enzyme
NADPH CO2
pyruvate
pyruvate
glucose
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AcetylCoA carboxylase is the main control point
for fatty acid synthesis in animals
the enzyme is regulated by both allosteric
effects and phosphorylation
citrate lyase
insulin stimulates dephosphorylation (activation)
the phosphorylated enzyme is inactive
acetyl-CoA carboxylase
-O--P
AMP-dependent protein kinase
glucagon, epinephrine, and adiponectin stimulate
phosphorylation (inactivation)
malonylCoA inhibits carnitine-acyltransferase I,
blocking oxidation of palmitoylCoA
carnitine-acyltransferase I
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The active (unphosphorylated) form of acetylCoA
carboxylase forms long filaments
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Obesity results from an imbalance between energy
input and output
adipose tissue
In the U.S. 35 of the adult population is
overweight, and 35 is obese. More than 10 of
U.S. children aged 2 to 5 are overweight.
Obesity significantly raises the risk of heart
disease, stroke, type II diabetes, and several
types of cancer.
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Leptin and adiponectin convey signals of
nutritional excess
sympathetic nervous system
neuronal signals
Increase catabolism thermogenesis!
blood
(express gene for uncoupling protein)
Increase blood pressure heart rate!
Decrease fatty acid synthesis switch on
catabolism!
(phosphorylate acetylCoA carboxylase)
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Defects in leptin or its receptor can cause
obesity
weight 35 g
weight 67 g
These mice are the same age. Both are homozygous
for a defective variant of leptin. The mouse on
the right received daily injections of purified
leptin the mouse on the left was not
treated. But most obese humans do not have a
deficiency in leptin.
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Ghrelin and other hormones convey signals of
short-term hunger or satiety
Youre full! Stop eating!
Youre hungry! Eat!
J. Marx, Cellular Warriers at the Battle of the
Bulge Science 299 846 (2003)