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Lipids and Fatty Acids Objectives:
1. What are Lipids?
• properties • glycerolipids vs. isoprenoids • glycerolipid structure • glycerolipid nomenclature
2. Fatty acid biosynthesis
• Cellular localization • Substrate and its origin • Enzymes of de novo fatty acid synthesis • De novo fatty acid elongation (C2->C18)
References:
Buchanan et al. 2000. Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists, Rockville MD. Chapter 10.
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What are lipids?
TAG
Vitamin D2
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What are lipids?
1. Heteropolymers consisting of different subunits
2. Macromolecules defined on the basis of their physical properties:
• Insoluble in water • Soluble in non-polar organic solvents
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Lipids are chemically heterogeneous
TAG
Vitamin D2
Originate from different biosynthetic pathways
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Glycerolipids Fats and oils
H C
H C
H C
H
H
H C
H C
H C
H
H
1
2
3 H C
H C
H C
H
H
H C
H C
H C
H
H
1
2
3
Glycerol backbone
Fatty acyl group
Triacylglycerol (TAG)
sn-1
sn-2
sn-3
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Triacylglycerols form oil droplets
Oilseed embryo
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Glycerolipids
Acyl group
Membrane lipids
HH
OH
H2COH
HH
H
O O H OH
H C
H
H C
H C
H
Glycerol backbone
Fatty acyl group
Head group
Monogalactosyldiacylglycerol (MGDG)
sn-1
sn-3
sn-2
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1. Glycerol backbone
2. Fatty acyl groups on sn-1 and sn-2 positions
• Saturated – no double bonds • Unsaturated – one or multiple double
bonds
3. Head group on sn-3 position
• Sugar moiety -> Glycolipids • Phosphate + alcohol -> Phospholipids • No head group -> Acyl glycerols
Glycerolipid nomenclature
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Oleic acid 18:1 Δ9
Number of Carbon atoms
Number of Double bonds
Trivial name
Double bond position (counting from carboxyl end)
cis configuration unless indicated
Fatty acid nomenclature -
H C C
H - -
H C C
H - -
H C - H C C
H C H
-
trans double bond E configuration Carboxyl end
Methyl end
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
O
HO C - C
H
- C H
- H C H
- H C H -
H C H
- H C H
- H C H
- H C H
- H C H
- H C H
H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
O
HO C - C
H
- C H
- H C H
- H C H -
H C H
- H C H
- H C H
- H C H
- H C H
- H C H
H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
O
HO C - C
H -
H C H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
O
HO C - C
H C H
- C H
- H C H
- H C H -
H C H
- H C H
- H C H
- H C H
- H C H
- H C H
H
- C H - C
H
- H C H
- H C H -
H C H
- H C H
- H C H
- H C H
- H C H
- H C H
H
- H C H
- H C H -
H C H
- H C H
- H C H
- H C H -
H C H
- H C H -
H C H
- H C H
- H C H
- H C H
- H C H
- H C H -
H C H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
- H C H
cis double bond Z configuration
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Common Eukaryotic Fatty Acids
Unsaturated fatty acids
16:0 Hexadecanoic acid Palmitic acid
18:0 Octadecanoic acid Stearic acid
16:1 Δ7 Hexadecenoic acid Palmitoleic acid
16:1 Δ3 t trans-hexadecenoic acid
16:2 Δ7Δ10 Hexadecadienoic acid
16:3 Δ7,Δ10,Δ13 Hexadecatrienoic acid 18:1 Δ9 Octadecenoic acid Oleic acid
18:2 Δ9,Δ12 Octadecadienoic acid Linoleic acid
18:3 Δ9,Δ12,Δ15 Octadecatrienoic acid Linolenic acid
Saturated fatty acids
Symbol Systematic name Trivial name
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• 16 or 18-carbon unbranched acyl chain (even number)
Characteristics of Common Fatty Acids
• Zero to 3 double bonds
• Double bonds are methylene interrupted (Allylic)
Allylic CH2 group
• Components of membrane and storage lipids
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De novo Fatty Acid Synthesis
Fig. 10-9
Site: PLASTID Substrate: acetyl-CoA
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De novo Fatty Acid Synthesis
Fig. 10-11
Acetyl-CoA Carboxylase reaction 1st committed step in fatty acid synthesis
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FAS Enzymes
ACP = Acyl Carrier Protein – primer for FAS, carries the fatty acyl chain
6. ER = 2-3 trans enoyl-ACP reductase
Enzymes:
1. ACCase = Acetyl-CoA Carboxylase
3. CONDENSING ENZYMES – key enzymes of FAS involved in C-C bond formation (CO2 release – IRREVERSIBLE!)
• KAS III – C2 • KAS I – C4-C14 • KAS II – C16
– different substrate specificities:
4. KR = 3-ketoacyl-ACP reductase
5. DH = 3-hydroxyacyl-ACP dehydratase
2. Malonyl-CoA:ACP transacylase
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Organization of FAS Enzymes
TYPE I (Animals and Fungi)
TYPE II (Plants and Bacteria)
Location: Cytosol
Location: Plastid stroma
Organization: A complex of 2 large subunits (250 kDa each) catalyzing all
the reactions of FAS
Organization: Enzyme activities reside on individual proteins – this reflects prokaryotic origin of plastids
KAS KR
DH ER
KAS KR
DH ER
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Fatty Acid Synthesis
Ohlrogge and Browse (1995) Plant Cell 7:795
KASIII
ER
DH
KR
KAS I
ACC
TA
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Fatty Acid Synthesis
FAS
16:0-ACP
18:0-ACP
18:1-ACP
FATB
FATA
LACS
LACS
16:0-CoA
18:1-CoA
AT
AT
16:0
18:1 Chl
orop
last
lipi
ds
DES
FAS = fatty acid synthesis
FAT = acyl-ACP thioesterase
AT = acyl transferase
LACS = acyl-CoA synthetase
DES = stearoyl-ACP desaturase
Main products of FAS are 16:0 and 18:1
AT 18:0 18:0-CoA LACS
FATB KASII
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Main products of FAS are 16:0 and 18:1
Export to the ER Lipid synthesis in the chloroplast
AT FAT
How do we know that?
Acetate labeling of isolated plastids in vitro C16 and C18 fatty acyl chains get labeled
What happens when we add isolated microsomes to the labeled C16 and C18 fatty acids?
Very long chain fatty acids (>C18) get made