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Fat mobilization in adipocytes
bR
Note: insulin inhibits TAG
mobilization
PKA
PKA
P
Hormone-sensitive
lipase
TAG
Hormone-sensitive
lipase
DAG
FFA
MAG
FFA
glycerol
FFA
See Fig 16.7 Horton
Fatty acid oxidation
1. Activation
• Acyl CoA synthetase (thiokinase)
2. Transport into mitochondria
• acylcarnitine transferase/acylcarnitine translocase
3. b oxidation cycle
•
•
•
•
acyl CoA dehydrogenase
enoyl CoA hydratase
L-3-hydroxy CoA dehydrogenase
Thiolase
Acyl CoA synthetase
O
R-COO- + CoA-SH + ATP
R-C- S-CoA + AMP + PPi
Note: 4 different enzymes specific for FA of differing chain
length.
location: outer membrane of mitochondria, ER membranes
Transport of fatty acyl CoA into mitochondria
CH3
H
+
H3C N C C
C COOH
H2
CH3 2 OH
Carnitine
Malonyl CoA
Carnitine
acyltransferase I
Fatty AcylCoA + carnitine
acylcarnitine + CoA
translocase
Mito matrix
Fatty AcylCoA + carnitine
acylcarnitine + CoA
Carnitine
acyltransferase II
Net yield of ATP
C16 FA + CoA + ATP
C16 acyl CoA + AMP + PP1
C16 acyl CoA + 7 NAD + 7 FAD
8 AcCoA
31 NADH
15 FADH2
8 GTP
8 AcCoA + 7 NADH + 7 FADH2
24 NADH + 8 FADH2 + 8 GTP + 16 CO2
77.5 ATP
22.5 ATP
8 ATP
108 ATP
- 2 ATP
Net = 106 ATP
b-oxidation of unsaturated fatty acids
• 2,4 dieonyl-CoA reductase: converts cis to trans double bond
• Enoyl-CoA isomerase: converts diene to single double bond
b-oxidation of unsaturated fatty acids
e.g. Linoleic acid: C18 cis,cisDH9,12
C
C
C
C
C
C
C
C
3
18
16
14
C
10
12
C
C
8
C
C
6
C
C
4
3 rounds of b-oxidation
3 Ac-CoA
C12 cis,cis-D3,6
3H C
18
C
C
16
C
C
14
C
C
12
C
g
b
a
C
10
C
C
8
g
b
a
C
4
C
C
2
CO-S-CoA
Enoyl-CoA isomerase
C12 trans,cis-D2,6
3H C
12
C
C
10
C
C
8
C
C
6
C
CO-S-CoA
1round of b-oxidation
Ac-CoA
C10 cis-D4
3H C
12
C
C
8
C
C
6
C
C
4
C
C
2
CO-S-CoA
C
C
2
CO-S-CoA
C10 cis-D4
3H C
C
C
8
12
C
C
6
C
C
4
C
CO-S-CoA
C
2
Acyl-CoA dehydrogenase
C10 trans,cis-D2,4
3HC
C
C
8
12
C
C
6
C
C
4
C
CO-S-CoA
C
2
2,4 dieonyl-CoA reductase
NADP
C10 trans-D3
3HC
C
12
C
8
C
C
6
C
C
4
C
C
2
CO-S-CoA
Enoyl-CoA isomerase
C10 cis-D2
3H C
12
C
C
8
C
C
6
C
C
4
C
C
2
CO-S-CoA
continued b-oxidation
b-oxidation of odd numbered fatty acids
Requires:
• Propionyl CoA carboxylase (biotin)
- adds CO2
• methyl malony CoA racemase
- converts D isomer of methyl malonyl CoA to L isomer
• methyl malonyl CoA mutase (adenosylcobalamin)
- rearranges MMCoA to yield succinyl CoA
Oxidation of odd-numbered fatty acids
O
CH3-CH2-C-S-CoA
CO2
Proprionyl CoA
Biotin, ATP
CH3 O
-OOC-CH
C-S-CoA
D-methylmalonyl CoA
racemase
O
-OOC-CH
C-S-CoA
L-methylmalonyl CoA
CH3
adenosylcobalamin
mutase
-OOC-CH
2
CH2
O
C-S-CoA
succinyl CoA
Cobalamin B12
methylcobalamin
R = CH3
- transfer of methyl groups
adenosylcobalamin
R = 5’-deoxyadenosinyl
- intramolecular rearrangements
See fig 7.24 Horton
Fatty acid biosynthesis
Where: cytoplasm
liver, fat cells
When: good energy charge, insulin
Process:
1. Transfer of AcCoA from mito to cyto
2. Acetyl CoA carboxylase
3. Fatty acid synthase
1. Transfer of AcCoA from mito to cyto
Citrate lyase
Citrate + ATP + + CoA
NADH
NADPH
malate
OAA + AcCoA + ADP + Pi
Citrate lyase
AcCoA
OAA
Pyruvate
citrate
cyto
mito
Antiport with pyr or Pi
OAA
H+
citrate
Pyruvate
AcCoA
Question:
Incubation of tissue using the above pathway with
only one of succinate-2,3-14C or succinate-1,4-14C
will result in the production of radiolabeled fatty
acid. Identify which substrate will yield 14C-fatty
acids.
Acetyl-CoA carboxylase 1 (ACC 1)
AcCoA + CO2 + ATP
biotin
-OOC
malonyl CoA + ADP + Pi
CH2
C - SCoA
O
Regulation:
• Hormonal control
- AMP kinase
- inhibited by PKA mediated phosphorylation
•glucagon (liver)
•adrenalin (adipocytes)
- enhanced by insulin
•
•
Allosteric regulation
- citrate activates phospho form
- inhibited by palmitoyl CoA
Nutritional status
Regulation of FA metabolism by phosphorylation
Adrenalin
Glucagon
activates
inhibits
cAMP
ACT
FA
synthesis
AMPactivated
kinase
PKA
Malonyl CoA
TAG lipase
(inactive)
ACC(active)
PP2A
ACC-PO4(inactive)
phosphatase
TAG lipase-PO4
(active)
Note: insulin activates ACC by stimulating the dephosphorylation reaction
Steps in FA biosynthesis
1. Loading: transfer to ACP and ketoacyl-ACPsynthase
i.
CH3
C - SCoA + ACP-SH
CH3
O
ii.
CH3
O
C - ACP
+ S-Synthase
O
iii.
-OOC-
C – SACP + CoASH
CH3
C – S-Synthase + ACP-SH
O
CH2
C - SCoA
O
-OOC-
ACP-SH
CoASH
CH2
C – S-ACP
O
2. Condensation:
-OOC-
CH2
C – S-ACP
+
CH3
O
C – S-Synthase
O
HS-Synthase
CO2
CH3
C – CH2
C – S-ACP
O
O
3. reduction
H
CH3
C – CH2
C – S-ACP
O
O
CH3
C
C – S-ACP
CH2
OH
NADPH
O
NADP
4. dehydration
H
CH3
H
C – CH2
C – S-ACP
OH
O
CH3
C
C – S-ACP
CH
O
H2O
5. reduction
H
CH3
C
CH
C – S-ACP
CH3
O
CH2
CH
C – S-ACP
O
NADPH
NADP
Subsequent rounds of synthesis
- transfer of growing FA to S-Synthase
- addition of 2 carbon units from malonyl-S-ACP
CH3
CH2
CH2
C – S-ACP
O
Synthase-SH
ACP-SH
CH3
CH2
CH2
C – S-synthase
O
-OOC-
CH2
C-SACP
CO2 + Synthase-SH
O
CH3
CH2
CH2
C
O
CH
C – S-ACP
O
• Chain elongation
• introduction of double bonds
• linoleic and linolenic acids
• synthesis of arachidonic acid – an important precursor of
several biologically active molecules