LIPID METABOLISM
BIOSYNTHESIS or DE NOVO SYNTHESIS OF FATTY ACID
The majority of the fatty acids required supplied through our diet.
Fatty acids are synthesised whenever there is a caloric excess in the our
diet.
The excess carbohydrate and protein obtained through diet can be
converted to fatty acids which are stored as triacylglycerol.
 Fatty acid synthesis involves the similar steps involved
in -oxidation of fatty acid but in a reverse way.
 Mammals can synthesise major portion of the saturated
fatty acid as well as monounsaturated fatty acids.
 The system for the fresh synthesis of fatty acid is
known as de novo synthesis of fatty acid
Takes place in liver and lactating mammary glands and
to a lesser extent in adipose tissue, kidney .
 The enzyme machinery is located in cytoplasm.
 Enzyme system is referred to as extra mitochondrial or
cytoplasmic fatty acid synthase system.
 Palmitic acid is the major fatty acid synthesised
 All the 16 carbon atoms are from acetyl CoA.
The acetyl CoA used as a primer and which forms
carbons 15 and 16 of palmitate. The addition of all the
subsequent 2-C units is through malonyl CoA
formation
 Acetyl CoA and NADPH are the prerequisites for the
fatty acid synthesis.
 Acetyl CoA produced in the mitochondria cannot enter
into cytoplasm through inner mitochondrial membrane.
So acetyl CoA condenses with Oxaloacetate in
mitochondria to form citrate.
 Citrate is freely transported to cytosol where it is
cleaved by citrate lyase to liberate acetyl CoA and
Oxaloacetate.
Mitochondria
Pyruvate
PDH
FA, amino acids
Cytoplasm Glucose
Pyruvate
HMP Shunt
NADPH+H+, CO2
Malic Enzyme
NADP+
Malate
NAD+
Malate Dh
NADH+H+
FA
Oxaloacetate
Acetyl CoA + Oxaloacetate
Citrate synthase
Citrate
AcetylCoA, ADP+Pi
Citrate lyase
CoASH, ATP
Citrate
For FA synthesis, 8 acetyl CoA are transported from the mitochondria
to cytosol, which is linked with the synthesis of 8 NADPH.
14 NADPH are required to synthesise one molecule of Palmitate.
The remaining 6 NADPH supplied from HMP shunt.
*Acetyl CoA carboxylase
1) Acetyl CoA
malonyl CoA
Biotin, ATP, CO2
ADP+Pi
* Regulatory enzyme in FA synthesis.
 The remaining reactions of FA synthesis are catalysed by
multifunctional enzyme known as fatty acid synthase complex [FAS]
 FAS is a dimer with two identical subunits.
 Each monomer possesses the activities of seven different enzymes
and an acyl carrier protein (ACP) bound to 4’phosphopantetheine-SH
group.
 Two subunits lie in antiparallel (head to tail) orientation.
 The-SH group of phosphopantetheine of one subunit is in close
proximity to the –SH of cysteine residue of the other subunit.
 Each monomer of FAS contains all the enzyme activities of fatty
acid synthesis.
Dimer form of enzyme is functionally active.
Because the functional unit consists of half of each subunit
interacting with the complimentary half of the other.
Components of fatty acid synthase complex:
1. Acetyl transferase [AT]
2. Malonyl transferase [MT]
3. -Keto acyl synthase [KS]
4. -Keto acyl reductase [KR]
5. -Hydroxy acyl dehydratase [HD]
6. Enoyl reductase [ER]
7. Thioestarase [TE]
Acyl carrier protein [ACP].
Functional division
KS AT MT HD ER KR ACP
TE
Cys
4’ phosphopantetheine
SH
SH
-----------------------------------------------------------------------Subunit
SH
SH
division
4’ phosphopantetheine
Cys
TE
ACP KR ER HD MT AT
KS
1. Fatty acid synthesis starts with the transfer of an acetyl CoA
to cysteinyl SH group of ACP
Acetyl CoA + (CE)-SH
AT
Acetyl S-(CE) + CoA --1
2. Malonyl CoA-ACP transferase transfers malonate from
malonyl CoA to bind to ACP
Malonyl CoA+ACP-SH
MT Malonyl-S-ACP + CoA—2
3) The acetyl unit attached to cysteine is transferred to
malonyl group attached to ACP. Malonyl moiety loses
CO2,which was added by acetyl CoA carboxylase & form
β-ketoacyl enzyme.
4) -Ketoacyl-enzyme is reduced to -hydroxy butyryl
enzyme complex using NADPH+H+.
5) Molecule of H2O is removed from -OH butyryl enzyme
to form ,  unsaturated acyl enzyme.
6) The unsaturated bond in ,  unsaturated acyl enzyme
is again reduced using NADPH+H+ to form butyryl or
acyl enzyme.The carbon chain attached to ACP is
transferred to cysteine residue and the reactions 2-6
are repeated 6 more times and finally palmitic acid is
synthesised.
7) The completely synthesized fatty acid is released from
the enzyme system by the action of thioesterase enzyme.
Fatty acid chain elongation and desaturation occurs in the
microsomes of endoplasmic reticulum and mitochondria.
Of the 16 carbons present in palmitate, only two come from acetyl CoA directly.
The remaining 14 are from malonyl CoA (produced from acetyl CoA).
During elongation in microsomes palmitate activated to palmitoyl CoA.
Malonyl Co serves as the donor of two carbons at a time in series of reactions.
Major elongation reaction occurs in the body involves the formation of stearyl
CoA [C18] from palmitoyl CoA [C16]
Elongation of this stearyl CoA in brain increases during myelination to provide
C22 and C24 fatty acids of sphingolipids
Mitochondrial elongation is less active and uses acetyl CoA as the source of
two carbon units
8 AcetylCoA+7ATP+14NADPH+14H+  Palmitate+8CoA+7ADP+7Pi+6H2O+14 NADP+
Regulation
Acetyl CoA carboxylase enzyme controls a committed step in fatty acid
synthesis. This enzyme exists as an inactive monomer or an active
polymer. Citrate promotes polymer formation, hence increases FA
synthesis. Palmitoyl CoA and malonyl CoA causes depolymerisation
of the enzyme and inhibit FA synthesis.
Hormonal influence
• Glucagon, epinephrine & norepinephrine inactivate the enzyme by
cAMP dependent phosphorylation and inhibits FA synthesis
• Insulin dephosphorylates & activates the enzyme and promotes FA
synthesis.
Dietary Regulation:
High carbohydrate and fat free diet increases the
synthesis of Acetyl CoA carboxylase and FA synthase,
which promotes FA synthesis.
Fasting and high fat diet decreases FA production.
NADPH influences FA synthesis.
Reference: Essentials of Biochemistry
Dr. S. Nayak
March 2011