Synthesis of

1
Lecture 2 Synthesis of Fatty Acids,
Phospholipids (glycerophospholipid and
sphingolipid) Cholesterol and Neurosteroids Lipid
rafts (Cholesterol and SM)
2
Acetyl CoA
Acetyl Co-A provides two-carbon acetyl units for
chain elongation in fatty acid synthesis
3
Citrate As Carrier of Acetate Groups (Citrate
shuttle)

• AcetylCoA is synthesized in the mitochondrial
  matrix from ATP and CoA, whereas fatty acids are
  synthesized in the cytosol
• Acetyl CoA cannot be exported from mitochondria
  as such and needs to be shuttled out of the
  mitochondrial matrix as citrate

PDH pyruvate dehydrogenase
In cytosol
4
Palmitate (160) synthesis

• AcetylCoA to MalonylCoA (committed step in
  fatty acid synthesis, catalyzed by acetyl CoA
  carboxylase)
• 7 Acetyl-CoA 7 CO2 7 ATP ?
• 7 Malonyl-CoA 7 ADP 7 Pi 14 H
• MalonylCoA to palmitate
• Acetyl-CoA 7 Malonyl-CoA 14 NADPH 20 H ?
• Palmitate 7 CO2 14 NADP 8CoA 6H2O
• Overall synthesis
• 8 acetyl-CoA 14 NADPH 7 ATP 6 H ?
• Palmitate 14 NADP 8 CoA 7 ADP 7 Pi
  6H2O

Synthesis of 16-C saturated fatty acid palmitate
occurs in the cytosol and is the final product of
the Fatty Acid Synthase complex, which consists
of seven different enzymes and a small protein
called acyl carrier protein (ACP). Further carbon
chain elongation and introduction of double bonds
occurs in the endoplasmic reticulum (ER).
5
Introduction of double bonds occurs in the
endoplasmic reticulum (ER)

• Desaturases introduce double bonds at specific
  positions in a fatty acid chain.
• Mammalian cells are unable to produce double
  bonds at certain locations, e.g., D12. Some
  polyunsaturated fatty acids (PUFA) are dietary
  essentials

6
Arachidonic acid (ArAc, 204) synthesis

• Long chain saturated fatty acids are synthesized
  from 160
• Arachidonic acid C204, n-6, ?5,8,11,14
• synthesized from the essential linolenic acid,
  C183, ?6,9,12 by elongation and desaturation.

7
Membrane phospholipids
Glycerophospholipids often at least have one
fatty acid that is kinked, due to one or more
double bonds.
Sphingolipids usually lack double bonds in their
fatty acid chains.

• Glycerophospholipids
• ( backbone glycerol)

• Sphingolipids
• ( backbone sphingosine)

Ethanolamine PE Choline PC Serine
PS Inositol PI Glycerol
Cardiolipin
Phosphocholine SM Glucose
Cerebroside Di-, tri-, tetra-saccharides Ganglios
ides
8
Synthesis of Membrane Phospholipids

• General assembly of phospholipids
• Synthesis of the backbone molecule,
  glycerol-3-phosphate
• Attachment of fatty acids via fatty acyl CoAs
  yielding phosphatidic acid
• Addition of a hydrophilic head group
• Alteration or exchange of head group (in some
  cases)
• Assembly sites
• Smooth surface of ER
• Inner mitochondrial membrane

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Synthesis of Phospholipids
Two fatty acids linked to coenzyme A (CoA)
carriers are first joined to glycerol-3-phosphate,
yielding phosphatidic acid (PA). A phosphatase
then converts PA to diacylglycerol (DAG).
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Synthesis of Phospholipids (cont..)
The attachment of different polar head groups to
DAG results in formation of PC, PE, and PS. PI is
formed from PA, rather than from DAG.
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Summary of Pathways to PC, PE and PS

• PEMT phosphatidylethanolamine
  N-methyltransferase (primarily in liver) PEMT
  activity is very low in brain.
• PC synthesis is predominantly through Kennedy
  pathway.
• Phosphatidylserine can be converted to PE either
  by decarboxylation of serine to ethanolamine or
  head group transfer.

PEMT
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Phosphatidylcholine Synthesis in Mammals
Kennedy pathway predominant route of PC
synthesis in the brain.
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Cardiolipin (exclusive to Mitochondria)

• The inner membrane is rich in an unusual
  phospholipid, cardiolipin
• Generates an electrochemical potential for
  substrate transport and ATP synthesis.
• Unlike the outer membrane, the inner membrane
  does not contain porins, and is highly
  impermeable.
• Changes in cardiolipin levels with age alter
  mitochondrial function and contribute to the
  aging process.

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Cardiolipin

• Phosphatidylglycerol is a precursor in the
  synthesis of both cardiolipin and
  phosphatidylinositol (another synthetic route to
  PI).In cardiolipin, two phosphatidylglycerol
  units are joined together.
• First identified in the heart, thus its name, and
  is abundant in the heart.
• Exclusive inner mitochondrial membrane
  phospholipids.

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Sphingolipid Synthesis
CO2 CoASH
Serine
OH
Serine palmitoyl transferase

CH2
NH2
HC
Palmitoyl-CoA
Dehydrosphingosine
NADPH
NADP
FAD
FADH2
16
Sphingolipid Synthesis cont..
Fatty acyl CoA Generally saturated FA
R is often a very long chain fatty acid (gt20
carbons)
17
Sphingolipid Synthesis cont..
18
Functional Role of Cholesterol

• Essential component of animal cell membranes
• Precursor of steroid hormones, neurosteroids and
  bile salts
• Precursor of vitamin D
• In the CNS, cholesterol is an important component
  of myelin. Oligodendrocytes make myelin in the
  CNS
• Cholesterol in the brain comes from de novo
  synthesis, no cholesterol is transported from
  blood.
• Most cells can make cholesterol, but liver is the
  most active.

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Cholesterol Biosynthesis

• Cholesterol is synthesized in the cytosol,
  beginning with acetyl-CoA.
• Four stages in synthesis (summary)
• Condensation of 3 acetate units to mevalonate
• Conversion of mevalonate into activated isoprene
• Polymerization of six 5-carbon isoprene units (30
  carbons) to form squalene
• Cyclization of squalene to create the steroid
  nucleus

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Neurosteroid Synthesis

• Niemann-Pick C protein transports cholesterol
  from Endosome/Lysosomes to mitochondria
• Rate limiting step Cholesterol is transported to
  inner mitochondrial membrane via peripheral type
  benzodiazepine receptor (MPTBR) and steroidogenic
  acute regulatory (StAR) protein
• Pregnenolone is synthesized by mitochondrial
  cytochrome P450 cholesterol side chain cleavage
  enzyme (P450scc), then released into cytosol
  where it serves as precursor of all neurosteroids
• Neurosteroids in the brain
• Pregnenolone (PREG) and its sulfate,
• Dehydroepiandrosterone (DHEA) and its sulfate and
  
• Progesterone (PROG)
• Neurosteroids
• affect synaptic functioning
• neuroprotective, and enhance myelinization

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Neurosteroid Synthesis (cont..)

• Cholesterol
• Glucocorticoids
• Mineralocorticoids
• Androgen/estrogen hormones

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Lipid rafts

• Sphingolipids tend to co-localize with
  cholesterol in membrane microdomains called lipid
  rafts.
• Hydrogen bonding between the hydroxyl group of
  cholesterol and the amide group of sphingomyelin
  (SM) accounts for this affinity.
• Act to concentrate certain membrane proteins,
  signal transduction cascades, and ion channels.
• Many channelopathies (ion channel dysfunction)
  are attributed to improper trafficking to the
  membrane, therefore rafts are important to
  assemble the correct signaling molecules in a
  spatially confined manner for efficient
  transduction.
• Have good resistance to solubilization with
  non-ionic detergents (like Triton X-100) and
  therefore proteins are retained in the pellet.

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• Lecture 2 END