Page 1
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?
Triacylglycerol (TAG)
Vitamin D2
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What are lipids?
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
Vitamin D2
Originate from different biosynthetic pathways
Triacylglycerol (TAG)
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What are lipids?
Macromolecules defined on the basis of their physical properties:
• Insoluble in water • Soluble in non-polar organic solvents
Heteropolymers consisting of different subunits (glycerolipids), or do not form polymers (isoprenoids=terpenoids)
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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
HOH
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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Glycerolipids
Acyl group
Membrane lipids
H C
H
H C
H C
H
Glycerol backbone
Fatty acyl group
Phosphatidylcholine (PC)
sn-1
sn-3
sn-2
O
O
O
O
O
P
Head group
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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 or ω9
Number of Carbon atoms
Number of Double bonds
Trivial name
Double bond position (counting from carboxyl end or methyl 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)
=CH
-CH
-HCH
-CH
-HCH
-HCH
=CH
- =CH
-CH
-HCH-C
H-
HCH
-CH
-HCH
-HCH
=CH
-
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?
From (14C)-acetate labeling experiments of isolated plastids in vitro
C16 and C18 fatty acyl chains get radioactively labeled