Download Oxidation and biosynthesis of fatty acids

Oxidation and
biosynthesis of fatty
acids
Stages of fatty acid oxidation
(1) Activation of fatty acids takes place
on the outer mitochondrial membrane
(2) Transport into the mitochondria
(3) Degradation to two-carbon
fragments (as acetyl CoA) in the
mitochondrial matrix (b-oxidation
pathway)
(1) Activation of Fatty Acids
• Fatty acids are converted to CoA thioesters by
acyl-CoA synthetase (ATP dependent)
• The PPi released is hydrolyzed by a
pyrophosphatase to 2 Pi
• Two phosphoanhydride bonds (two ATP equivalents)
are consumed to activate one fatty acid to a
thioester
(2) Transport of Fatty Acyl CoA into Mitochondria
• The carnitine shuttle
system.
• Fatty acyl CoA is first
converted to acylcarnitine
(enzyme carnitine
acyltransferase I (bound to
the outer mitochondrial
membrane).
• Acylcarnitine enters the
mitochondria by a
translocase.
• The acyl group is transferred
back to CoA (enzyme carnitine acyltransferase II).
• Carnitine
shuttle
system
• Path of
acyl group
in red
(3) The Reactions of b oxidation
• The b-oxidation pathway (b-carbon atom (C3)
is oxidized) degrades fatty acids two carbons
at a time
b

1. Oxidation of acyl
CoA by an acyl CoA
dehydrogenase to
give an enoyl CoA
Coenzyme - FAD
2. Hydration of the
double bond between
C-2 and C-3 by enoyl
CoA hydratase with
the 3-hydroxyacyl
CoA (b-hydroxyacyl
CoA) formation
3. Oxidation of
3-hydroxyacyl CoA to
3-ketoacyl CoA by
3-hydroxyacyl CoA
dehydrogenase
Coenzyme – NAD+
4. Cleavage of
3-ketoacyl CoA by
the thiol group of
a second molecule
of CoA with the
formation of
acetyl CoA and an
acyl CoA
shortened by two
carbon atoms.
Enzyme b-ketothiolase.
The shortened acyl
CoA then
undergoes another
cycle of oxidation
The number of
cycles: n/2-1,
where n – the
number of carbon
atoms
b-Oxidation
of
Fatty acyl CoA
saturated fatty
acids
• One round of b oxidation: 4 enzyme steps
produce acetyl CoA from fatty acyl CoA
• Each round generates one molecule each of:
FADH2
NADH
Acetyl CoA
Fatty acyl CoA (2 carbons shorter each round)
Fates of the products of b-oxidation:
- NADH and FADH2 - are used in ETC
- acetyl CoA - enters the citric acid cycle
- acyl CoA – undergoes the next cycle of oxidation
ATP Generation from Fatty Acid Oxidation
Net yield of ATP per one oxidized palmitate
Palmitate (C15H31COOH) - 7 cycles – n/2-1
• The balanced equation for oxidizing one palmitoyl
CoA by seven cycles of b oxidation
Palmitoyl CoA + 7 HS-CoA + 7 FAD+ + 7 NAD+ + 7 H2O
8 Acetyl CoA + 7FADH2 + 7 NADH + 7 H+
ATP generated
8 acetyl CoA
7 FADH2
7 NADH
10x8=80
7x1.5=10.5
7x2.5=17.5
108 ATP
ATP expended to activate palmitate
Net yield:
-2
106 ATP
LIPID METABOLISM:
FATTY ACID
OXIDATION
b-OXIDATION OF ODD-CHAIN FATTY ACIDS
• Odd-chain fatty acids
occur in bacteria and
microorganisms
• Final cleavage product is
propionyl CoA rather
than acetyl CoA
• Three enzymes convert
propionyl CoA to succinyl
CoA (citric acid cycle
intermediate)
Propionyl CoA Is Converted into Succinyl CoA
1. Propionyl CoA is carboxylated to yield the D
isomer of methylmalonyl CoA.
The hydrolysis of an ATP is required.
Enzyme: propionyl CoA carboxylase
Coenzyme: biotin
2. The D isomer of methylmalonyl CoA is
racemized to the L isomer
Enzyme: methylmalonyl-CoA racemase
3. L isomer of methylmalonyl CoA is converted
into succinyl CoA by an intramolecular
rearrangement
Enzyme: methylmalonyl CoA mutase
Coenzyme: vitamin B12 (cobalamin)
OXIDATION OF FATTY ACIDS IN
PEROXISOMES
Peroxisomes - organelles containing
enzyme catalase, which catalyzes
the dismutation of hydrogen
peroxide into water and molecular
oxygen
Acyl CoA
dehydrogenase
transfers electrons
to O2 to yield H2O2
instead of
capturing the highenergy electrons by
ETC, as occurs in
mitochondrial boxidation.
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: b-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-b-ketoacyl
ACP.
Reduction, dehydration,
and a second reduction
convert the C6-bketoacyl 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, b-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)