Organization of Fatty Acid Synthase from Animal Liver

1

• BIOCHEMISTRY 441 Winter, 2008
• 15. Biosynthesis of fatty acids
• 16. Triacylglycerols, phospholipids complex
  lipids
• 17. Cholesterol lipoproteins
• 18. Photosynthesis antennas reaction centers
• 19. Photosynthesis electron transfer
  photophosphorylation
• 20. Photosynthesis carbon fixation by C3 and C4
  pathways
• 21. Amino acid metabolism transamination and NH3
  transport
• 22. Urea cycle, amino acid catabolism
  biosynthesis
• 23. Aromatic amino acids neurotransmitters
• 24. One-carbon metabolism
• 25. Biosynthesis of pyrimidines purines
• 26. Deoxyribonucleotide biosynthesis nucleotide
  catabolism

2
Fatty acids have extended hydrocarbon chains
1
16
1
18
16
18
Most natural fatty acids have an even number
of carbons. Most natural unsaturated fatty
acids 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.
Phospholipid bilayers are the central structural
elements of biological membranes.
Fatty acids also are found as cholesterol esters
in lipoproteins, and are attached covalently to
some proteins.
4
Triacylglycerols are stored as energy reserves in
adipose tissue and other tissues
capillary
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
Why is CO2 needed?
The labeling patterns suggest that the fatty acid
chain forms by successive addition of two-carbon
units
6
MalonylCoA serves as the donor of two-carbon units
carboxyl-transferase site
biotin-carboxylase site
MalonylCoA is formed from acetylCoA and CO2 by a
multifunctional enzyme, acetylCoA carboxylase
(biotin carboxylase-transcarboxylase). 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.
7
(No Transcript)
8
The fatty acid chain is synthesized attached to a
protein -- acyl-carrier protein (ACP)
Malonyl and acetyl groups are transferred from
CoA to a thiol sulfur atom of ACP.
Bacteria have separate malonyl acetyl
transferases. In animals, one enzyme catalyzes
both reactions.
malonyl/acetyl transferase
9
The functional thiol group of ACP is in a
4-phosphopantetheine group linked to a Ser
residue of the protein
4-phosphopantetheine
CoA-SH
ACP of Bacillus subtilis. 1f80.pdb
In animals, ACP is a domain of a large protein
with multiple catalytic centers in bacteria,
its a separate, small protein.
4-phosphopantetheine also provides the thiol
group of Coenzyme A. Pantetheine is vitamin B5.
10
The condensation reaction
3-ketoacyl-ACP
malonyl-ACP
S
Release of bicarbonate is exothermic and pulls
the reaction in the direction of condensation.
The acetyl group first moves from ACP to a Cys
residue of the keto-synthase, and then combines
with malonyl-ACP to give a 3-ketoacyl-ACP.
11
Reduction by NADPH, followed by dehydration and a
second reduction, generates butyryl-ACP
dehydrase
12
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
The second turn of the cycle generates
hexanoyl-ACP
b-ketoacyl synthase
CO2
b-ketoacyl reductase
enoyl reductase
dehydrase
HOH
14
The cycle continues until the fatty acid chain
reaches 16 carbons
Thioesterase hydrolyzes palmitoyl-ACP, releasing
palmitate (C160)
C140
b-ketoacyl synthase
CO2
thioesterase
C160
HOH
15
(No Transcript)
16
The enzymes of fatty acid synthesis have fused
into a single protein during evolution
ACP
CO2-
H3N
O-phosphopantetheine
E. coli eight separate proteins
How does the growing fatty acid chain bound to
ACP reach all the active sites?
17
(No Transcript)
18
(No Transcript)
19
Differences between fatty acid synthesis and
oxidation
oxidation
synthesis
H2O
cytosol
20
Fatty acids with longer chains (C180 C200)
are synthesized from palmitoylCoA malonylCoA
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
formally the same as in synthesis of palmitate.
NADP
H2O
21
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
22
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
23
AcetylCoA carboxylase (biotin carboxylase/transcar
boxylase) 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
cAMP-dependent protein kinase
glucagon, epinephrine, and adiponectin stimulate
phosphorylation (inactivation)
malonylCoA inhibits carnitine-acyltransferase I,
blocking transport of palmitoylCoA into
mitochondria for oxidation
carnitine-acyltransferase I
24
The active (unphosphorylated) form of acetylCoA
carboxylase forms long filaments
25
An imbalance between energy input and output can
lead to obesity
65 of the adult U.S. population are considered
to be overweight (BMI gt 25) 35 are obese (BMI
gt 30). Obesity raises the risk of heart disease,
stroke, type-II diabetes, and cancer. Individual
susceptibility to obesity is strongly influenced
by heredity.
BMI (weight in kg)/(height in m)2
703x(weight in pounds)/(height in inches)2
26
Leptin and adiponectin convey signals of
nutritional excess
leptin, fatty acids
In obesity, leptin also decreases synthesis of
insulin, which can lead to diabetes.
27
Defects in leptin or its receptor can cause
obesity
weight 67 g
weight 35 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. More than 20 other genes
have been associated with obesity.
28
Ghrelin and PYY convey short-term signals of
hunger or satiety
Suggested further reading on obesity and
regulation of energy balance J. Marx, Cellular
warriers at the battle of the bulge Science 299
846 (2003) E.D. Rosen B.M. Spiegelman,
Adipocytes as regulators of energy balance and
glucose homeostasis Nature 444 847 (2006).