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LIPID
BIOSYNTHESIS
Role of lipids
They are the principal form of stored energy in most
organisms & major constituents of cellular
membranes.
Specialised lipids serve as pigments ( Retinol,
carotene); Cofactors ( vitamin K); Detergents ( bile
salts) etc.
Anchors for membrane proteins ( Signalling)
Biosynthesis of fatty acids and
eicosanoids
• Biosynthesis requires the participation of a 3
carbon intermediate, i.e., Malonyl CoA.
Formation of malonyl CoA
 Irreversible process, catalyzed by Acetyl – CoA carboxylase.
 Enzyme contains biotin prosthetic group covalently bound
in amide linkage to the E-Amino group of a lys residue in
any one polypeptide unit of the enzyme.
Process
• Carboxyl grou, derived from bicarbonate,
transferred to Biotin in an ATP dep. Reaction.
• The biotinyl group serves as a temporary carrier of
CO2.
• Transfer of CO2 to acetyl CoA to yield Malonyl CoA.
Fatty acid synthesis
• It is a repeating reaction sequence.
Fatty acid synthase / Multi enzyme
complex
• All the
reactions in
synthetic
processes are
catalyzed by a
multienzyme
complex, Fatty
acid synthase.
• It has 7 separate polypeptides
unit.
ACP – Acyl Carrier Protein
• Small protein
consisting of
the pros. Group
4phosphopantet
heine.
• Serves as a
flexible arm,
tethering the
growing fatty
acyl chain to the
surface of F.A.S
complex
Enzymes And there functions
Beginning of
nd
2
cycle
Net reaction
Structure of fatty acid synthases
Fatty acid synthesis occurs in the cytosol of
many organisms but in the chloroplasts of
plant
• In most higher eukaryotes, fatty acid synthase complex
is found in cytosol.
• Usually, NADPH carries electrons for anabolic reactions,
and NAD+ in catabolic reactions.
• In plants, NADPH is produced in the chloroplast stroma
and hence favouring the reduction of oxygen as in
mitochondria.
Role of citrate
Acetate is shuttled out as citrate
• In nonphotosynt. Eukaryotes, acetyl CoA is formed
in mitochondria.
• Oxidation of fatty acids is not a significant source of
Acetyl CoA for fatty acid biosynthesis in animals,
because the 2 pathways are reciprocally regulated
1) Mitochondrial inner membrane is impermeable to
acetyl CoA
2) So, In the intramembrane acetyl coA reacts with
oxaloacet. To form Citrate by Citrate synthase.
3) Citrate lyase cleaves the Citrate giving back Acetyl
coA
4) OAA is reduced to malate by malate
dehydrogenase and goes back.
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