LIPID BIOSYNTHESIS
• Fatty acid biosynthesis-basic fundamentals
• Fatty acid biosynthesis-elongation and
desaturation
• Triacylglycerols
• Phospholipids
• Cholesterol
• Cholesterol metabolism
Fatty Acid Biosynthesis
Synthesis
•
•
•
•
•
•
Cytosol
Requires NADPH
Acyl carrier protein
D-isomer
CO2 activation
Keto  saturated
Beta Oxidation
•
•
•
•
•
•
Mitochondria
NADH, FADH2
CoA
L-isomer
No CO2
Saturated  keto
Rule:
Fatty acid biosynthesis is a stepwise assembly
of acetyl-CoA units (mostly as malonyl-CoA)
ending with palmitate (C16 saturated)
3 Phases
Activation
Elongation
Termination
Cofactor
ACTIVATION
Biotin
O
CH3C~SCoA
HN
NH
O
CH2CH2CH2CH2CO
S
NHCH2CH2CH2CH2 ENZYME
HCO3-
ATP
Biocytin
ADP + Pi
-OOC-CH
2C~SCoA
O
O
LYS
CO2
O
NH
C N
O
active carbon
S
Acetyl-CoA carboxylase
CH2CH2CH2CH2CO
Carboxybiocytin
Acetyl-CoA Carboxylase
The rate-controlling enzyme of FA synthesis
• In Bacteria -3 proteins
(1) Carrier protein with Biotin
(2) Biotin carboxylase
(3) Transcarboxylase
• In Eukaryotes - 1 protein
(1) Single protein, 2 identical polypeptide
chains
•
(2) Each chain Mwt = 230,000 (230 kDa)
(3) Dimer inactive
(4) Activated by citrate which forms
filamentous form of protein that can be seen in
the electron microscope
Yeast Fatty Acid Synthase Complex
2,500 kDa Multienzyme Complex
6 molecules of 2 peptide chains called A and B
(66)
A: (185,000)
Acyl Carrier protein
-ketoacyl-ACP synthase (condensing enzyme)
-ketoacyl-ACP reductase
B: (175,000)
-hydroxy-ACP dehydrase
enoyl-ACP reductase
palmitoyl thioesterase
Fatty Acid
Synthase
Complex
Acyl Carrier Protein
Phosphopantetheine
H
H HO CH3
O
HS-CH2-CH2-N-C-CH2-CH2-N-C-C-C-CH2-O-P-O-CH2-SerO
OH H
O
Cysteamine
ACP
Acyl carrier protein
10 kDa
H
H HO CH3
O O
HS-CH2-CH2-N-C-CH2-CH2-N-C-C-C-CH2-O-P-O-P-O-CH2
O
O
OH H
O O
Coenzyme A
O
O-P-O
OH
Adenine
H
OH
Initiation
Overall Reaction
Malonyl-CoA + ACP
-OOC-CH
CH3C~SCoA
2C~S-
O
O
CO2
HS-CoA
CH3C-CH2C~SO
ACP
+ HS-CoA
Acyl Carrier
Protein
ACP
O
NOTE:
Malonyl-CoA carbons become new COOH end
Nascent chain remains tethered to ACP
CO2, HS-CoA are released at each condensation
-Carbon
Elongation
CH3C-CH2C~S-
NADPH
D isomer
O
Reduction
O
-Ketoacyl-ACP reductase
H
CH3C-CH2C~SHO
O
-H2O
NADPH
ACP
ACP
Dehydration
 -Hydroxyacyl-ACP dehydrase
H
CH3C-= C- C~S- ACP
H
O
Enoyl-ACP reductase
CH3CH2CH2C~S- ACP
O
Reduction
TERMINATION
-KS
Transfer to Malonyl-CoA
Ketoacyl ACP
Synthase
Transfer to KS
-S-ACP
-CH2CH2CH2C~S- ACP
Free to bind
Malonyl-CoA
O
Split out CO2
CO2
When C16 stage is reached, instead of transferring to KS,
the transfer is to H2O and the fatty acid is released
Fatty Acid Synthase
O
S-C-CH2-CH2-CH3
-Ketoacyl
-ACP synthase
KS
O
CH3-CH2-CH2-C-S
Acetyl-CoA
HS
CoA-SH
NADP+
Enoyl-ACP
reductase NADPH
H+
O
CH3-CH=CH-C-S
-Hydroxyacyl-ACP H2O
KS
ACP
Initiation or
priming
O
S-C-CH3
SH
Malonyl-CoA
Malonyl-CoACoA-SH ACP transacylase
O
O
S -C-CH2-COO-
CH3-CH -CH2-C-S
-Ketoacyl
-ACP reductase
O
S -C-CH3
KS -SH
dehydrase
OH
Acetyl-CoAACP transacylase
KS
NADP+
NADPH
H+
S
C=O
CH2
C=O
CH3
-Keto-ACP
synthase (condensing enzyme)
CO2
KS -SH
Elongation
Substrate Entry
Reduction
Thioesterase
palmitate release
AT
MT
DH KR
ER ACP
CE
CH2
Translocation
HS
HS
TE
SH
SH
Translocation
CH2
CE
TE
Thioesterase
palmitate release
ACP ER KR DH
Reduction
MT
AT
Substrate Entry
Overall Reactions
Acetyl-CoA + 7 malonyl-CoA + 14NADPH + 14H
7H++
Palmitate + 7CO2 + 14NADP+ + 8 HSCoA + 6H2O
7 Acetyl-CoA + 7CO2 + 7ATP
7 malonyl-CoA +7ADP + 7Pi + 7H+
8 Acetyl-CoA + 14NADPH + 7H+ + 7ATP
Palmitate + 14NADP+ + 8 HSCoA + 6H2O + 7ADP + 7Pi
PROBLEM:
Fatty acid biosynthesis takes place in the
cytosol. Acetyl-CoA is mainly in the
Mitochondria
acetyl-CoA
How is acetyl-CoA made available to the cytosolic
fatty acyl synthase?
SOLUTION:
Acetyl-CoA is delivered to cytosol from the
mitochondria as CITRATE
CH2COO
HO-C-COO
mitochondria
CH2COO
CH2COO
HO-C-COO
OAA
CO2
Pyr
Acetyl-CoA
Citrate lyase
COO
C=O
OAA
Malate
CH2
dehydrogenase
COO
NADH
CH2COO
Acetyl-CoA
HS-CoA
L-malate
CO2
COO
HO-C-H L-malate
CH2
COO Malic enzyme
NADP+
NADPH + H+
COO
C=O Pyruvate
CH3
Cytosol
Post-Synthesis Modifications
• C16 satd fatty acid (Palmitate) is the product
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•
•
•
Elongation
Unsaturation
Incorporation into triacylglycerols
Incorporation into acylglycerol phosphates
Elongation of Chain (two systems)
R-CH2CH2CH2C~SCoA Malonyl-CoA*
O
(cytosol)
HS-CoA
OOC-CH2C~SCoA CH3C~SCoA
O
O
CO2
Acetyl-CoA
(mitochondria)
R-CH2CH2CH2CCH2C~SCoA
O
O
1 NADPH
Elongation systems are
NADH
found in smooth ER and
2 - H2O
3 NADPH
mitochondria
R-CH2CH2CH2CH2CH2C~SCoA
O
Desaturation
Rules:
The fatty acid desaturation system is
in the smooth membranes of the endoplasmic
reticulum
There are 4 fatty acyl desaturase enzymes in
mammals designated 9 , 6, 5, and 4 fatty
acyl-CoA desaturase
Mammals cannot incorporate a double bond
beyond 9; plants can.
Mammals can synthesize long chain unsaturated
fatty acids using desaturation and elongation
Rule:
NADH
2
The Desaturase System requires O2 and
resembles an electron transport system
Cyt b5
reductase
O2
Cyt b5
3
(FAD)
Saturated FA-CoA
1
2
NOTE:
1. System is in ER membrane
2. Both NADH and the fatty acid contribute
electrons
3. Fatty acyl desaturase is considered a mixed
function oxidase
Fatty acid desaturation system
C18-stearoly-CoA
C18 9-oleyl-CoA
+ O2 + 2H+
+ 2H2O
Desaturase
Desaturase
2 cyt b5 Fe2+
2 cyt b5 Fe2+
Cyt
b5
2H+ + cyt b5 reductase
FAD
NADH + H+
cyt b5 reductase
FADH2
NAD+
Cyt b5
reductase
Desaturase
Palmitoleate
16:1(9)
Palmitate
16:0
Elongase
Stearate
18:0
Permitted
transitions
in mammals
Desaturase
Essential
fatty acid
Desaturase
-Linolenate
18:3(9,12,15)
Other lipids
Oleate
18:1(9)
Desaturase
Linoleate
18:2(9,12)
Desaturase
-Linolenate
18:3(6,9,12)
Elongase
Eicosatrienoate
20:3(8,11,14)
Desaturase
Arachidonate
20:4(5,8,11,14)