Lipogenesis
Fats not only obtained from the diet but also obtained from lipogenesis in
the body. Lipogenesis means synthesis of neutral fats (TAG) from CHO
and proteins present in excess of body need.
Lipogenesis requires:
1- Synthesis of fatty acids (FA) and glycerol
2- Activation of fatty acids (by CoA) and glycerol (by glycerokinase)
3- the combination of activated fatty acids and activated glycerol to
produce TAG.
1- De novo synthesis of fatty acid (cytoplasmic synthesis):
Occur mainly for the synthesis of palmitic acid (C16:0)
Site: Cytoplasm of liver, mammary glands and adipose tissues.
Acetyl CoA is the precursor of fatty acid synthesis. It is produced
from oxidation of glucose (by oxidative decarboxylation of pyruvate),
and metabolism of ketogenic and mixed amino acids.
In case of high CHO diet, excess acetyl CoA is produced. Some enter
kreb’s and the excess will participate in fatty acid synthesis in
cytoplasm of liver, mammary glands and adipose tissue.
Steps of fatty acid synthesis:
1- Transport of acetyl CoA to cytoplasm:
Acetyl CoA is produced in mitochondria, and FA synthesis occurs in
cytoplasm, so acetyl CoA must be transferred to cytoplasm across
mitochondrial membrane which is impermeable to CoA. The transport
occur through condensation of acetyl CoA inside mitochondria with
oxaloacetate (OAA) to form citrate which can be transferred into
cytoplasm. In cytoplasm, citrate is cleaved by ATP-citrate lyase or
called citrate cleavage system in the presence of ATP and CoA to give
acetyl CoA and OAA.
Conclusion: Acetyl-CoA is delivered to cytosol from the mitochondria
as CITRATE
Transport of acetyl CoA from mitochondria to cytoplasm:
Mitochondria:
OAA + Acetyl CoA
-CoA
↓ citrate synthase
Citrate
Inner mitochondrial membrane
↓
Citrate
+ CoA, ATP
↓ ATP citrate lyase
Cystosol
OAA + Acetyl CoA
Transport of acetyl CoA from mitochondria to cytoplasm:
2-Conversion of acetyl CoA into malonyl CoA by acetyl CoA
carboxylase (ACC)
• The rate limiting step in FA synthesis is the
synthesis of malonyl CoA by combination
of acetyl CoA and CO2 in the presence of ATP
•This step is catalyzed by acetyl CoA carboxylase
(ACC) which is the rate limiting enzyme.
•ACC contains biotin which is the carrier of CO2
[CO2 is covalently linked to biotin which is bound to
lysine residue of ACC].
Reaction catalyzed by Acetyl CoA Carboxylase
3- Remaining series of the pathway is catalyzed by fatty acid synthase (FAS) which is a
multifunctional enzyme.
The pathway catalyzed by FAS involve 7 cycles to produce finally palmitate
Cycle 1: The condensation of acetyl-CoA and two carbons derived from malonyl-CoA,
with elimination of CO2 from the malonyl group forming at the end 4 carbon acyl
(buteryl),
Cycle 2: buteryl + malonyl CoA (with the elimination of CO2 from malonate) gives 6
carbon acyl,
Cycle 3: 6C acyl + malonyl CoA gives 8 C acyl,
and so on for a total 7 cycles ending with the synthesis of 16 C acyl (palmitate).
In each cycle 2 carbon atoms are added. The source of these two carbons is malonyl
CoA.
Fatty acid synthase (FAS) contains a phosphopantetheine residue
called acyl carrier protein (ACP), derived from the vitamin
pantothenic acid, and a cysteine residue;
Both ACP and cystein rediude contain sulfhydryl (thiol) that
can form thioesters with acyl groups.
Steps of FA synthesis by Fatty Acid Synthase (FAS):
a. Initially, acetyl CoA reacts with the thiol of ACP and then the
acetyl group is transferred to the thiol of cysteine residue. This
acetyl group provides the ω-carbon of the fatty acid produced by
the fatty acid synthase complex.
b. A malonyl group from malonyl-CoA forms a thioester with the
thiol of ACP
c. Condensation: The acetyl group on the fatty acid synthase
complex condenses with the malonyl group; the CO2 is released;
and a β-ketoacyl group, containing four carbons, is produced.
d. Reduction: The β-keto group is reduced by NADPH to a βhydroxy group.
e. Dehydration: Then dehydration occurs, producing an enoyl group
with the double bond between carbons 2 and 3
f. Reduction: Finally, the double bond is reduced by NADPH, and a
four-carbon acyl group is generated.
Overall reactions of FA synthesis:
Acetyl-CoA + 7 Malonyl-CoA + 14 NADPH + H+
Palmitic acid (16:0) +7 CO2 + 8 CoA +14 NADP+
The ω carbon and its adjacent carbon are produced from
acetyl CoA (from the first cycle), the reaming 14 carbons are
derived from malonyl CoA (7 molecules; each molecule add 2
carbons in each cycle of the 7 cycles)
Role of NADPH in FA synthesis:
NADPH is important hydrogen carrier for FA synthesis (HOW?)
The condensation of acetyl-CoA and malonyl-CoA form β-keto
acyl (not the acyl itself), so the keto group should be reduced
to give the final product of the cycle which is the saturated
acyl. The reduction steps needs NADPH+H as source of
hydrogen that convert keto group to CH2
Sources of NADPH for fatty acid
synthesis
• From where does NADPH needed
for synthesis come?
– pentose phosphate pathway
(HMP pathway) is the major
source.
– Reduction of OAA (that is
produced from citrate cleave
system in the cytoplasm) to
malate followed by conversion
of malate to pyruvate by malic
enzyme is a source of NADPH
Regulation of acetyl CoA carboxylase (ACC) or regulation of fatty acid synthesis:
A) Allosteric regulation: allosterically activated by citrate & ATP and inhibited by the
product of pathway (palmitic acid).
B) Hormonal regulation: activated by insulin, inhibited by adrenaline and glucagon
(anti-insulin hormone).
Insulin lead to the formation of the active form of the ACC (dephosphorylated form)
because insulin antagonize the action of cAMP/protein kinase activity. cAMP leads to
activation of protein kinase A enzyme leading to phosphorylation of ACC which will
be inactive form. Insulin activates a phosphatase enzyme called protein
phosphatase 2A to dephosphorylate the enzyme; thereby activating it and remove the
inhibitory effect of cAMP/protein kinase.
C) Dietary regulation: prolonged consumption of high claoric diet (e.g CHO diet)
increases the synthesis of acetyl CoA carboxylase and so increase FA synthesis. Fatfree diet, fasting and low CHO reduce enzyme synthesis and so decrease FA synthesis.
Regulation of ACC:
Elongation of fatty acids:
Site: Mitochondria and endoplasmic reticulum
Palmitic acid - the end product of FA synthesis in
cytoplasm can be elongated in endoplasmic
reticulum and mitochondria by the addition of two
carbon atoms from malonyl CoA to give stearic
acid and longer chain saturated FAs
Palmitate is the precursor of stearic and longerchain saturated fatty acids, as well as the
monounsaturated acids (palmitoleic and oleic).
Mammals cannot convert oleic to linoleic or
linolenic (shaded pink) because humans cannot
introduce additional double bonds from the C10 to
the methyl-terminal (ω carbon). So, linoleic or
linolenic are therefore required in the diet as
essential fatty acids.
Arachidonic acid is synthesized in the body from
linolenic acid
Desaturation
-The fatty acid desaturation occurs in the endoplasmic
reticulum
- Humans (mammals) have four desaturase enzymes that add
double bonds at carbons 4, 5, 6, 9 in mammals. And as we
told in previous slide humans can’t add double bond to
carbon 10 to terminal carbon, but plants can.