General Biochemistry II MOLB46105610

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General Biochemistry IIMOLB4610/5610

• Lecture 10
• Biosynthesis of membrane phospholipids

Lehninger Ch. 21 pg. 791 - 799
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Todays lecture

• Membrane behavior
• Distribution of phospholipids
• Phospholipid synthesis in bacteria
• Phospholipid synthesis in eukaryotes
• Plasmalogen synthesis
• Lipid trafficking

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Membrane phospholipids

• PS Phosphatidylserine
• PE Phosphatidylethanolamine
• PC Phosphatidylcholine ( lecithin)
• PG Phosphatidylglycerol
• PI Phosphatidylinositol
• DPG Diphosphatidylglycerol (Cardiolipin)

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Why are there so many different phospholipids ?

• Membranes contain mainly proteins and lipids
• Phospholipid composition dictates membrane
  behavior
• Membranes requiring movement within membrane
• More unsaturated fatty acids (PE and PS often
  have higher amounts of polyunsaturated fatty
  acids)
• Lower packing densities (kinks decrease
  density)
• Shorter fatty acid tails - fewer hydrophobic
  interactions
• Higher membrane fluidity
• Cell compartment specific membrane composition
• cardiolipin found exclusively at the inner
  mitochondrial membrane - decreased leakage

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Distribution of membrane lipids

• Animals
• major components are PE, PS, PC ( lecithin),
  cholesterol
• cell membranes of the central nervous system
  contain additional lipids such as sphingomyelin,
  myelin, cerebrosides, gangliosides
• Plants
• PE and PC predominate
• PI and PG are present as well
• cholesterol is absent but replaced by
  phytosterols
• Bacteria
• major components are PE and PG
• PC is rarely present, sterols are absent

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Phospholipid synthesis in E. coli
pyrophosphate linkage
Phosphatidate

• Requires an activated component
• Both phosphate groups in PPi come from CTP
• CDP-diacylglycerol can be converted to
• Phosphatidylserine (PS) and subsequently to PE
• Phosphatidylglycerol-3-phosphate (PG)

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Synthesis of phosphatidylserine (PS)
phosphatidyl- ethanolamine (PE)

• 1) Phosphatidylserine synthase reaction
• 2) Phosphatidylserine decarboxylase reaction

CMP
PS synthase
CO2
PS decarboxylase
Phosphatidylserine Phosphatidylethanolamine
H
Lehninger pg. 793
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Synthesis of phosphatidylglycerol (PG)

• 1) Phosphatidylglycerol 3-phosphate synthase
• 2) Phosphatidylglycerol 3-P phosphatase

CMP
PG-3-P synthase
CDP-diacyglycerol glycerol-3-P Phosphatidylglyc
erol-3-P
PG-3P phosphatase
Pi
Phosphatidylglycerol-3-P Phosphatidylglycerol
H2O
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Synthesis of cardiolipin

• Cardiolipin synthase reaction
• (Bacteria vs. eukaryotes)

BACTERIA 2 PGs glycerol cardiolipin
EUCARYOTES CDP-diacyglycerol PG CMP
cardiolipin Phosphatidylglycerol is made
exactly as in bacteria
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Eucaryotic synthesis of phosphatidylinositol (PI)

• EUCARYOTES
• CDP-diacylglycerol inositol
• CMP Phosphatidylinositol
• PI and phosphorylated PI derivatives
• play an important role in signal transduction

PI kinase
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Eucaryotic PS synthesis
PS
IN YEAST
A
PE

• In yeast PS is synthesized like in bacteria from
  CDP-diacylglycerol
• In mammals PS is NOT derived CDP-diacylglycerol !
  Instead it is derived from PE via a displacement
  of headgroups (Serine - ethanolamine)
• Q Where does PE come from in mammals?

A
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PE synthesis Mammals vs. Bacteria
MAMMALS
EUCARYOTES BACTERIA
CO2
PS decarboxylase
Phosphatidylserine Phosphatidylethanolamine
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PC synthesis in yeast

• The amino group of the phosphoglyceride is
  methylated 3 times using S-adenosyl-methionine as
  the methyl donor.

PE
This reaction also takes place in bacteria
(Stryer pg 718) and in the liver of mammalian
cells. Although yeast is a eucaryote it shares
share several reactions with bacteria.
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PC synthesis in mammals

• 1) Choline kinase mediated phosphorylation of
  choline
• Activation of phosphocholine by CTP via CTP
  -choline cytidylyl transferase
• Replacement of CMP by diacyglycerol resulting in
  PC.
• This mechanism used in most mammalian cells.

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Plasmalogen

• Ether lipid
• Ether R1-O-R2
• Ether-linked alkene CH2-O-CHCH-(CH2)n-CH3
• 1/2 of heart phospholipids
• produced mainly in peroxisomes
• Other ether lipids (platelet-activating factor)

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Plasmalogen synthesis

• Step-by-step summary of reactions
• backbone DHAP
• Acylation of C1 in DHAP by fatty acyl CoA
• Exchange of an alcohol saturated fatty alcohol
  for the carboxylic group at C1
• Reduction by NADPH
• Acylation by a second acyl CoA at C2
• Headgroup (ethanolamine) attachment at C3
• Oxidation to alkene via mixed function oxidase

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Plasmalogen synthesis
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Platelet activating factor (PAF)

• Ether lipid
• Acetyl group at C2 of DHAP backbone instead of
  fatty acid
• Synthesis similar to plasmalogen
• from DHAP backbone
• subnanomolar concentration of PAF
• induce aggregation of blood platelets
• induce muscle contractions
• activate the immune system
• also the mediator of anaphylactic shocks

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Sphingolipid synthesis

• Four stages
• Synthesis of sphinganine
• Requires palmitoyl-CoA serine (backbone)
• Reduction of keto group with NADPH
• Attachment of a fatty acid to form
  N-acylsphinganine
• Desaturation of sphinganine to yield
  N-acylsphingosine
• Attachment of the headgroup to produce a
  sphingolipid
• Cerebroside (head group e.g. Glucose)
• Glycosidic linkage instead of phosphodiester
  bonds
• Sphingomyelin (head group e.g. choline)

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Lipid trafficking

• Lipids that are synthesized at smooth ER need to
  relocate to target organelle
• Free diffusion is unlikely due to hydrophobic
  nature of lipids
• Specific transport to target compartments
• Requires vesicles that bud from the Golgi complex