Download Chapter 16 (Part 3)

Chapter 16 (Part 3)
Fatty acid Synthesis
Fatty Acid Synthesis
• In mammals fatty acid synthesis occurs
primarily in the liver and adipose tissues
• Also occurs in mammary glands during
lactation.
• Fatty acid synthesis and degradation
go by different routes
• There are four major differences
between fatty acid breakdown and
biosynthesis
The differences between fatty
acid biosynthesis and breakdown
• Intermediates in synthesis are linked to -SH
groups of acyl carrier proteins (as compared
to -SH groups of CoA)
• Synthesis in cytosol; breakdown in
mitochondria
• Enzymes of synthesis are one polypeptide
• Biosynthesis uses NADPH/NADP+; breakdown
uses NADH/NAD+
ACP vs. Coenzyme A
•Intermediates in synthesis are linked to -SH
groups of acyl carrier proteins (as compared to SH groups of CoA)
Fatty Acid Synthesis Occurs in
the Cytosol
• Must have source of acetyl-CoA
• Most acetyl-CoA in mitochondria
• Citrate-malate-pyruvate shuttle provides cytosolic
acetate units and reducing equivalents for fatty
acid synthesis
Citrate synthase
Citrate Lyase
Malate
dehydrogenase
Pyruvate
carboxylase
Malate Enzyme
Fatty Acid Synthesis
• Fatty acids are built from 2-C units derived
from acetyl-CoA
• Acetate units are activated for transfer to
growing FA chain by conversion to malonylCoA
• Decarboxylation of malonyl-CoA and reducing
power of NADPH drive chain growth
• Chain grows to 16-carbons (eight acetylCoAs)
• Other enzymes add double bonds and more
Cs
Acetyl-CoA Carboxylase
Acetyl-CoA + HCO3- + ATP  malonyl-CoA + ADP
• The "ACC enzyme" commits acetate to
fatty acid synthesis
• Carboxylation of acetyl-CoA to form
malonyl-CoA is the irreversible, committed
step in fatty acid biosynthesis
Acetyl-CoA
Carboxylase
Regulation of Acetyl-CoA
Carboxylase (ACCase)
• ACCase forms long, active filamentous
polymers from inactive protomers
• Accumulation of palmitoyl-CoA (product)
leads to the formation of inactive
polymers
• Accumulation of citrate leads to the
formation of the active polymeric form
• Phosphorylation modulates citrate
activation and palmitoyl-CoA inhibition
Regulation of Acetyl-CoA
Carboxylase (ACCase)
• Unphosphorylated ACCase
has low Km for citrate and
is active at low citrate
• Unphosphorylated ACCase
has high Ki for palmitoylCoA and needs high
palmitoyl-CoA to inhibit
• Phosphorylated E has high
Km for citrate and needs
high citrate to activate
• Phosphorylated E has low Ki
for palmitoyl-CoA and is
inhibited at low palmitoylCoA
Fatty Acid Synthesis
• Step 1: Loading – transferring acetyl- and
malonyl- groups from CoA to ACP
• Step 2: Condensation – transferring 2 carbon
unit from malonyl-ACP to acetyl-ACP to form 2
carbon keto-acyl-ACP
• Step 3: Reduction – conversion of keto-acylACP to hydroxyacyl-ACP (uses NADPH)
• Step 4: Dehydration – Elimination of H2O to
form Enoyl-ACP
• Step 5: Reduction – Reduce double bond to
form 4 carbon fully saturated acyl-ACP
Step 1: Loading Reactions
O
H3C C S CoA
acetyl-CoA
acetyl-CoA:ACP
transacylase
O
C
O
HS-ACP
H
O
C
C S CoA
H
malonyl-CoA
HS-ACP
malonyl-CoA:ACP
transacylase
HS-CoA
HS-CoA
O
H3C C S ACP
acetyl-ACP
O
C
O
H
O
C
C S ACP
H
malonyl-ACP
Step 2: Condensation Rxn
O
H3C C S ACP
acetyl-ACP
HS-Ketoacyl-ACP Synthase
HS-ACP
O
C
O
O
H
O
C
C S ACP
H
+
H3C C S
ketoacyl-ACP Synthase
malonyl-ACP
keto-ACP synthase
CO2
O
H
O
H3C C
C
C S ACP
H
acetoacetyl-ACP
Step 3: Reduction
O
H
O
H3C C
C
C S ACP
H
acetoacetyl-ACP
NADPH + H+
Ketoacyl-ACP Reductase
NADP+
OH H
H3C C
C
H
H
O
C S ACP
-hydroxybutyryl-ACP
Step 4: Dehydration
OH H
H3C
C
C
H
H
O
C S ACP
-hydroxyacyl-ACP
-hydroxyacyl-ACP
dehydrase
H20
H3C
C
H
H
O
C
C S ACP
trans-enoyl-ACP
Step 5: Reduction
H3C C
H
O
C
C S ACP
trans-enoyl-ACP
H
NADPH + H+
enoyl-ACP reductase
NADP+
H
H
O
H3C C
C
C S ACP
H
H
trans-enoyl-ACP
Step 6: next condensation
H H O
H3C
C C C S ACP
H H
butyryl-ACP
HS-Ketoacyl-ACP Synthase
HS-ACP
O
C
O
H H O
H
O
C
C S ACP
H
+
H3C
C C C S KAS
H H
malonyl-ACP
keto-ACP synthase
CO2
H3C
H H O
H
O
C C C
C
C S ACP
H H
H
ketoacyl-ACP
Termination
of Fatty
Acid
Synthesis
H O
H3C C C S ACP
H
Palmitoyl-ACP
14
Thioesterase
HS-ACP
H O
H3C C C O
H
Palmitic Acid
14
ATP + HS-CoA
Acyl-CoA
synthetase
AMP + PPi
H O
H3C C C S CoA
H
14
Palmitoyl-CoA
Organization of Fatty Acid
Synthesis Enzymes
• In bacteria and plants, the fatty acid
synthesis reactions are catalyzed
individual soluble enzymes.
• In animals, the fatty acid synthesis
reactions are all present on
multifunctional polypeptide.
• The animal fatty acid synthase is a
homodimer of two identical 250 kD
polypeptides.
Animal Fatty Acid Synthase
Further Processing
of Fatty acids:
Desaturation and
Elongation
Regulation of FA Synthesis
• Allosteric modifiers, phosphorylation and
hormones
• Malonyl-CoA blocks the carnitine
acyltransferase and thus inhibits betaoxidation
• Citrate activates acetyl-CoA carboxylase
• Fatty acyl-CoAs inhibit acetyl-CoA
carboxylase
• Hormones regulate ACC
• Glucagon activates lipases/inhibits ACC
• Insulin inhibits lipases/activates ACC
Allosteric regulation
of fatty acid
synthesis occurs at
ACCase and the
carnitine
acyltransferase
Glucagon inhibits
fatty acid
synthesis while
increasing lipid
breakdown and
fatty acid oxidation
Insulin prevents
action of glucagon