fatty acid oxidation
DESCRIPTION
fatty acid oxidationTRANSCRIPT
-
Fatty Acid Oxidation
Molecular Biochemistry II
-
A 16-C fatty acid with numbering conventions is shown.
Most naturally occurring fatty acids have an even number of carbon atoms.
The pathway for catabolism of fatty acids is referred to as the b-oxidation pathway, because oxidation occurs at the b-carbon (C-3).
C
O
O
-
1
2
3
4
a
b
g
C
O
O
-
1
2
3
4
a
b
g
EMBED ChemDraw.Document.4.5
fatty acid with a cis-9
double bond
_1008576813.cdx_1008576919.cdx_965825676.cdx -
Triacylglycerols (triglycerides) are the most abundant dietary lipids. They are the form in which we store reduced C for energy.
Each triacylglycerol has a glycerol backbone to which are esterified 3 fatty acids
Most triacylglycerols are mixed. The 3 fatty acids differ in chain length & number of double bonds.
H
2
C
H
C
H
2
C
O
H
O
H
O
H
H
2
C
H
C
H
2
C
O
O
O
C
R
O
C
C
R
O
R
O
H
O
C
R
O
H
2
C
H
C
H
2
C
O
H
O
H
O
H
H
2
C
H
C
H
2
C
O
O
O
C
R
O
C
C
R
O
R
O
H
O
C
R
O
glycerol fatty acid triacylglycerol
EMBED ChemDraw.Document.4.5
_977227313.cdx_977227727.cdx_1008575693.cdx_977227399.cdx_977163222.cdx -
Lipid digestion, absorption, transport will be covered separately.
Lipases hydrolyze triacylglycerols, releasing 1 fatty acid at a time, yielding diacylglycerols, & eventually glycerol.
H
2
C
H
C
H
2
C
O
H
O
H
O
H
H
2
C
H
C
H
2
C
O
O
O
C
R
O
C
C
R
O
R
O
H
O
C
R
O
H
2
C
H
C
H
2
C
O
H
O
H
O
H
H
2
C
H
C
H
2
C
O
O
O
C
R
O
C
C
R
O
R
O
H
O
C
R
O
glycerol fatty acid triacylglycerol
EMBED ChemDraw.Document.4.5
_977227313.cdx_977227727.cdx_1008575693.cdx_977227399.cdx_977163222.cdx -
Glycerol, arising from hydrolysis of triacylglycerols, is converted to the Glycolysis intermediate dihydroxyacetone phosphate, by reactions catalyzed by:
1 Glycerol Kinase
2 Glycerol Phosphate Dehydrogenase.
C
H
2
C
H
C
H
2
O
H
H
O
O
P
O
3
-
C
H
2
C
H
C
H
2
O
H
H
O
O
H
C
H
2
C
C
H
2
O
H
O
P
O
3
-
O
C
H
2
C
H
C
H
2
O
H
H
O
O
P
O
3
-
C
H
2
C
H
C
H
2
O
H
H
O
O
H
C
H
2
C
C
H
2
O
H
O
P
O
3
-
O
glycerol glycerol-3-P dihydroxyacetone-P
EMBED ChemDraw.Document.4.5
ATP ADP
H+ +
NAD+ NADH
1
2
_977171398.cdx -
Free fatty acids, which in solution have detergent properties, are transported in the blood bound to albumin, a serum protein produced by the liver.
Several proteins have been identified that facilitate transport of long chain fatty acids into cells, including the plasma membrane protein CD36.
C
O
O
-
1
2
3
4
a
b
g
C
O
O
-
1
2
3
4
a
b
g
EMBED ChemDraw.Document.4.5
fatty acid with a cis-9
double bond
_1008576813.cdx_1008576919.cdx_965825676.cdx -
Fatty acid activation:
Fatty acids must be esterified to Coenzyme A before they can undergo oxidative degradation, be utilized for synthesis of complex lipids, or be attached to proteins as lipid anchors.
Acyl-CoA Synthases (Thiokinases) of ER & outer mitochondrial membranes catalyze activation of long chain fatty acids, esterifying them to coenzyme A.
This process is ATP-dependent, & occurs in 2 steps.
There are different Acyl-CoA Synthases for fatty acids of different chain lengths.
-
Acyl-CoA Synthases
Exergonic PPi (P~P) hydrolysis, catalyzed by Pyrophosphatase, makes the coupled reaction spontaneous.
2 ~P bonds of ATP are cleaved.
The acyl-CoA product includes one "~" thioester linkage.
N
N
N
N
N
H
2
O
O
H
O
H
H
H
H
C
H
2
H
O
P
O
P
O
P
-
O
O
O
-
O
-
O
O
O
-
N
N
N
N
N
H
2
O
O
H
O
H
H
H
H
C
H
2
H
O
P
O
C
O
O
-
R
O
S
C
R
O
C
o
A
R
C
O
O
-
P
P
i
C
o
A
S
H
A
M
P
2
P
i
N
N
N
N
N
H
2
O
O
H
O
H
H
H
H
C
H
2
H
O
P
O
P
O
P
-
O
O
O
-
O
-
O
O
O
-
N
N
N
N
N
H
2
O
O
H
O
H
H
H
H
C
H
2
H
O
P
O
C
O
O
-
R
O
S
C
R
O
C
o
A
R
C
O
O
-
P
P
i
C
o
A
S
H
A
M
P
2
P
i
Fatty acid activationEMBED ChemDraw.Document.4.5
fatty acid
ATP
acyl-
adenylate
acyl-CoA
_1104392193.cdx -
Summary of fatty aid activation:
fatty acid + ATP acyladenylate + PPi
PPi 2 Pi
acyladenylate + HS-CoA acyl-CoA + AMP
Overall:
fatty acid + ATP + HS-CoA acyl-CoA + AMP + 2 Pi
-
For most steps of the pathway there are multiple enzymes specific for particular fatty acid chain lengths.
b-Oxidation pathway:
Fatty acids are degraded in the mitochondrial matrix via the b-Oxidation Pathway.
Fatty acyl-CoA formed in cytosol by enzymes of outer mitochondrial membrane & ER
inner
membrane
-Oxidation pathway in matrix
Mitochondrion
-
Fatty acyl-CoA formed outside can pass through the outer mitochondrial membrane (which has large VDAC channels), but cannot penetrate the inner membrane.
Many of the constituent enzymes are soluble proteins located in the mitochondrial matrix.
But enzymes specific for very long chain fatty acids are associated with the inner membrane, facing the matrix.
Fatty acyl-CoA formed in cytosol by enzymes of outer mitochondrial membrane & ER
inner
membrane
-Oxidation pathway in matrix
Mitochondrion
-
Carnitine Palmitoyl Transferases catalyzes transfer of a fatty acid between the thiol of Coenzyme A and the hydroxyl on carnitine.
Transfer of the fatty acid moiety across the mitochondrial inner membrane involves carnitine.
H
3
C
N
C
H
2
C
H
C
H
2
C
H
3
C
H
3
O
H
C
O
O
-
+
R
C
S
C
o
A
O
+
H
3
C
N
C
H
2
C
H
C
H
2
C
H
3
C
H
3
O
C
O
O
-
+
C
R
O
+
H
S
C
o
A
H
3
C
N
C
H
2
C
H
C
H
2
C
H
3
C
H
3
O
H
C
O
O
-
+
R
C
S
C
o
A
O
+
H
3
C
N
C
H
2
C
H
C
H
2
C
H
3
C
H
3
O
C
O
O
-
+
C
R
O
+
H
S
C
o
A
EMBED ChemDraw.Document.4.5
carnitine
fatty acyl carnitine
Carnitine Palmitoyl Transferase
_1008786327.cdx -
Carnitine-mediated transfer of the fatty acyl moiety into the mitochondrial matrix is a 3-step process:
1. Carnitine Palmitoyl Transferase I, an enzyme on the cytosolic surface of the outer mitochondrial membrane, transfers a fatty acid from CoA to the OH on carnitine.
2. An antiporter in the inner mitochondrial membrane mediates exchange of carnitine for acylcarnitine.
cytosol mitochondrial matrix
O O
R-C-SCoA HO-carnitine HO-carnitine R-C-SCoA
HSCoA R-C-O-carnitine R-C-O-carnitine HSCoA
O O
1
2
3
-
3. Carnitine Palmitoyl Transferase II, an enzyme within the matrix, transfers the fatty acid from carnitine to CoA. (Carnitine exits the matrix in step 2.)
The fatty acid is now esterified to CoA in the matrix.
cytosol mitochondrial matrix
O O
R-C-SCoA HO-carnitine HO-carnitine R-C-SCoA
HSCoA R-C-O-carnitine R-C-O-carnitine HSCoA
O O
1
2
3
-
Control of fatty acid oxidation is exerted mainly at the step of fatty acid entry into mitochondria.
Malonyl-CoA (which is also a precursor for fatty acid synthesis) inhibits Carnitine Palmitoyl Transferase I.
Malonyl-CoA is produced from acetyl-CoA by the enzyme Acetyl-CoA Carboxylase.
H
3
C
C
S
C
o
A
O
C
H
2
C
S
C
o
A
O
-
O
O
C
H
3
C
C
S
C
o
A
O
C
H
2
C
S
C
o
A
O
-
O
O
C
EMBED ChemDraw.Document.4.5
acetyl-CoA
malonyl-CoA
_1033147502.cdx -
Activated Kinase, leading to decreased malonyl-CoA.
The decrease in malonyl-CoA concentration leads to increased activity of Carnitine Palmitoyl Transferase I.
Increased fatty acid oxidation then generates acetyl-CoA, for entry into Krebs cycle with associated ATP production.
AMP-Activated Kinase, a sensor of cellular energy levels, is allosterically activated by AMP, which is high in concentration when [ATP] is low.
Acetyl-CoA Carboxylase is inhibited when phosphorylated by AMP-
H
3
C
C
S
C
o
A
O
C
H
2
C
S
C
o
A
O
-
O
O
C
H
3
C
C
S
C
o
A
O
C
H
2
C
S
C
o
A
O
-
O
O
C
EMBED ChemDraw.Document.4.5
acetyl-CoA
malonyl-CoA
ATP + HCO3
ADP + Pi
Acetyl-CoA Carboxylase
(inhibited by AMP-Activated Kinase)
_1155582881.cdx -
AMP-Activated Kinase functions under a variety of conditions that lead to depletion of cellular ATP (reflected as increased AMP), including:
glucose deprivation, exercise, hypoxia & ischaemia.
AMP-Activated Kinase regulates various metabolic pathways to:
promote catabolism leading to ATP synthesis
(e.g., stimulation of fatty acid oxidation)inhibit energy-utilizing anabolic pathways
(e.g., fatty acid synthesis).AMP-Activated Kinase in the hypothalamus of the brain is involved also in regulation of food intake.
-
bond between carbon atoms 2 & 3.
There are different Acyl-CoA Dehydrogenases for short (4-6 C), medium (6-10 C),long and very long (12-18 C) chain fatty acids.
Very Long Chain Acyl-CoA Dehydrogenase is bound to the inner mitochondrial membrane. The others are soluble enzymes located in the mitochondrial matrix.
b-Oxidation Pathway:
Step 1. Acyl-CoA Dehydrogenase catalyzes oxidation of the fatty acid moiety of acyl-CoA to produce a double
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
H
H
O
1
2
3
a
b
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
O
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
H
O
H
2
O
F
A
D
H
2
F
A
D
H
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
H
+
+
N
A
D
H
N
A
D
+
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
H
H
O
1
2
3
a
b
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
O
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
H
O
H
2
O
F
A
D
H
2
F
A
D
H
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
H
+
+
N
A
D
H
N
A
D
+
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
EMBED ChemDraw.Document.4.5
fatty acyl-CoA
trans-2-enoyl-CoA
fatty acyl-CoA acetyl-CoA
(2 C shorter)
Acyl-CoA Dehydrogenase
_1008570455.cdx_1008572580.cdx_1008574690.cdx_1008571291.cdx_1008570153.cdx -
FAD is the prosthetic group that functions as e- acceptor for Acyl-CoA Dehydrogenase. Proposed mechanism:
A Glu side-chain carboxyl extracts a proton from the a-carbon of the substrate, facilitating transfer of 2 e- with H+ (a hydride) from the b position to FAD.
The reduced FAD accepts a 2nd H+, yielding FADH2.
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
H
H
O
1
2
3
a
b
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
O
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
H
O
H
2
O
F
A
D
H
2
F
A
D
H
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
H
+
+
N
A
D
H
N
A
D
+
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
H
H
O
1
2
3
a
b
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
O
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
H
O
H
2
O
F
A
D
H
2
F
A
D
H
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
H
+
+
N
A
D
H
N
A
D
+
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
EMBED ChemDraw.Document.4.5
fatty acyl-CoA
trans-2-enoyl-CoA
fatty acyl-CoA acetyl-CoA
(2 C shorter)
Acyl-CoA Dehydrogenase
_1008570455.cdx_1008572580.cdx_1008574690.cdx_1008571291.cdx_1008570153.cdxH
3
N
+
C
C
O
O
-
C
H
2
C
H
2
C
H
O
-
O
H
3
N
+
C
C
O
O
-
C
H
2
C
H
2
C
H
O
-
O
EMBED ChemDraw.Document.6.0
glutamate
_1126617240.unknown -
The carbonyl O of the thioester substrate is hydrogen bonded to the 2'-OH of the ribityl moiety of FAD, giving this part of FAD a role in positioning the substrate and increasing acidity of the substrate a-proton.
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
H
H
O
1
2
3
a
b
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
O
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
H
O
H
2
O
F
A
D
H
2
F
A
D
H
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
H
+
+
N
A
D
H
N
A
D
+
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
H
H
O
1
2
3
a
b
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
O
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
H
O
H
2
O
F
A
D
H
2
F
A
D
H
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
H
+
+
N
A
D
H
N
A
D
+
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
EMBED ChemDraw.Document.4.5
fatty acyl-CoA
trans-2-enoyl-CoA
fatty acyl-CoA acetyl-CoA
(2 C shorter)
Acyl-CoA Dehydrogenase
_1008570455.cdx_1008572580.cdx_1008574690.cdx_1008571291.cdx_1008570153.cdx -
The carbonyl O of the thioester substrate is hydrogen bonded to the 2'-OH of the ribitol moiety of FAD, giving the sugar alcohol a role in positioning the substrate and increasing acidity of the substrate a-proton.
C
C
C
H
C
C
H
C
N
C
C
N
N
C
N
H
C
H
3
C
H
3
C
O
O
C
H
2
H
C
H
C
H
C
H
2
C
O
H
O
P
O
P
O
O
O
-
O
O
-
R
i
b
o
s
e
O
H
O
H
A
d
e
n
i
n
e
C
C
C
H
C
C
H
C
N
C
C
H
N
N
H
C
N
H
C
H
3
C
H
3
C
O
O
C
H
2
H
C
H
C
H
C
H
2
C
O
H
O
P
O
P
O
O
O
-
O
O
-
R
i
b
o
s
e
O
H
O
H
A
d
e
n
i
n
e
C
C
C
H
C
C
H
C
N
C
C
N
N
C
N
H
C
H
3
C
H
3
C
O
O
C
H
2
H
C
H
C
H
C
H
2
C
O
H
O
P
O
P
O
O
O
-
O
O
-
R
i
b
o
s
e
O
H
O
H
A
d
e
n
i
n
e
C
C
C
H
C
C
H
C
N
C
C
H
N
N
H
C
N
H
C
H
3
C
H
3
C
O
O
C
H
2
H
C
H
C
H
C
H
2
C
O
H
O
P
O
P
O
O
O
-
O
O
-
R
i
b
o
s
e
O
H
O
H
A
d
e
n
i
n
e
EMBED ChemDraw.Document.4.5
EMBED ChemDraw.Document.4.5
FAD
FADH2
2 e + 2 H+
dimethylisoalloxazine
_1001749585.cdx_1066634605.cdx_1066634638.cdx_1001749759.cdx_1001749458.cdx -
The reactive Glu and FAD are on opposite sides of the substrate at the active site.
Thus the reaction is stereospecific, yielding a trans double bond in enoyl-CoA.
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
H
H
O
1
2
3
a
b
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
O
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
H
O
H
2
O
F
A
D
H
2
F
A
D
H
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
H
+
+
N
A
D
H
N
A
D
+
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
H
H
O
1
2
3
a
b
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
O
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
H
O
H
2
O
F
A
D
H
2
F
A
D
H
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
H
+
+
N
A
D
H
N
A
D
+
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
EMBED ChemDraw.Document.4.5
fatty acyl-CoA
trans-2-enoyl-CoA
fatty acyl-CoA acetyl-CoA
(2 C shorter)
Acyl-CoA Dehydrogenase
_1008570455.cdx_1008572580.cdx_1008574690.cdx_1008571291.cdx_1008570153.cdx -
FADH2 is reoxidized by transfer of 2 electrons to an electron transfer flavoprotein (ETF), which in turn passes the electrons to coenzyme Q of the respiratory chain.
Matrix
H+ + NADH NAD+ + 2H+ 2H+ + O2 H2O
2 e
I Q III IV+ +
4H+ 4H+ 2H+
Intermembrane Space
cyt c
-
Step 2.
Enoyl-CoA Hydratase catalyzes stereospecific hydration of the trans double bond produced in the 1st step, yielding L-hydroxyacyl-Coenzyme A.
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
H
H
O
1
2
3
a
b
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
O
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
H
O
H
2
O
F
A
D
H
2
F
A
D
H
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
H
+
+
N
A
D
H
N
A
D
+
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
H
H
O
1
2
3
a
b
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
O
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
H
O
H
2
O
F
A
D
H
2
F
A
D
H
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
H
+
+
N
A
D
H
N
A
D
+
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
EMBED ChemDraw.Document.4.5
fatty acyl-CoA
trans-2-enoyl-CoA
3-L-hydroxyacyl-CoA
fatty acyl-CoA acetyl-CoA
(2 C shorter)
Acyl-CoA Dehydrogenase
Enoyl-CoA Hydratase
_1008570455.cdx_1008572580.cdx_1008574690.cdx_1008571291.cdx_1008570153.cdx -
Step 3.
Hydroxyacyl-CoA Dehydrogenase catalyzes oxidation of the hydroxyl in the b position (C3) to a ketone.
NAD+ is the electron acceptor.
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
H
H
O
1
2
3
a
b
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
O
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
H
O
H
2
O
F
A
D
H
2
F
A
D
H
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
H
+
+
N
A
D
H
N
A
D
+
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
H
H
O
1
2
3
a
b
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
O
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
H
O
H
2
O
F
A
D
H
2
F
A
D
H
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
H
+
+
N
A
D
H
N
A
D
+
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
EMBED ChemDraw.Document.4.5
fatty acyl-CoA
trans-2-enoyl-CoA
3-L-hydroxyacyl-CoA
-ketoacyl-CoA
fatty acyl-CoA acetyl-CoA
(2 C shorter)
Acyl-CoA Dehydrogenase
Enoyl-CoA Hydratase
Hydroxyacyl-CoA
Dehydrogenase
-Ketothiolase
_1008570455.cdx_1008571291.cdx_1008572580.cdx_1008570153.cdx -
A cysteine S attacks the b-keto C.
Acetyl-CoA is released, leaving the fatty acyl moiety in thioester linkage to the cysteine thiol.
The thiol of HSCoA displaces the cysteine thiol, yielding fatty acyl-CoA (2 C less).
Step 4.
b-Ketothiolase catalyzes thiolytic cleavage.
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
EMBED ChemDraw.Document.4.5
-ketoacyl-CoA
fatty acyl-CoA acetyl-CoA
(2 C shorter)
Acyl-CoA Dehydrogenase
Enoyl-CoA Hydratase
-Ketothiolase
_1127239342.cdxH
3
N
+
C
C
O
O
-
C
H
2
S
H
H
H
3
N
+
C
C
O
O
-
C
H
2
S
H
H
EMBED ChemDraw.Document.6.0
cysteine
_1123783145.unknown -
A membrane-bound trifunctional protein complex with two subunit types expresses the enzyme activities for steps 2-4 of the b-oxidation pathway for long chain fatty acids.
Equivalent enzymes for shorter chain fatty acids are soluble proteins of the mitochondrial matrix.
-
Summary of one round of the b-oxidation pathway:
fatty acyl-CoA + FAD + NAD+ + HS-CoA
fatty acyl-CoA (2 C less) + FADH2 + NADH + H+
+ acetyl-CoA
The b-oxidation pathway is cyclic.
The product, 2 carbons shorter, is the input to another round of the pathway.
If, as is usually the case, the fatty acid contains an even number of C atoms, in the final reaction cycle butyryl-CoA is converted to 2 copies of acetyl-CoA.
-
NADH produced during fatty acid oxidation is reoxidized by transfer of 2e- to respiratory chain complex I.
Transfer of 2e- from complex I to oxygen causes sufficient proton ejection to yield approximately 2.5 ATP.
Recall that 4H+ enter the matrix per ATP synthesized, taking into account transmembrane flux of ADP, ATP & Pi.
Matrix
H+ + NADH NAD+ + 2H+ 2H+ + O2 H2O
2 e
I Q III IV+ +
4H+ 4H+ 2H+
Intermembrane Space
ADP + Pi ATP
3H+
F1
Fo
cyt c
-
FADH2 of Acyl-CoA Dehydrogenase is reoxidized by transfer of 2e- via ETF to CoQ of the respiratory chain.
H+ ejection from the matrix that accompanies transfer of 2e- from coenzyme Q to oxygen, leads to production of approximately 1.5 ATP.
Matrix
H+ + NADH NAD+ + 2H+ 2H+ + O2 H2O
2 e
I Q III IV+ +
4H+ 4H+ 2H+
Intermembrane Space
ADP + Pi ATP
3H+
F1
Fo
cyt c
-
Acetyl-CoA can enter Krebs cycle, yielding additional NADH, FADH2, and ATP.
Fatty acid oxidation is a major source of cell ATP.
-
Catabolism of two 6-C glucose through Glycolysis, Krebs, & ox phos yields about 60 ~P bonds of ATP (30/glucose).
Compare energy yield oxidizing a 12-C fatty acid. Assume:
1.5 ATP produced per FADH2 reoxidized in the respiratory chain (via coenzyme Q).
2.5 ATP produced per NADH reoxidized in the respiratory chain.
Problem
(See web handout, tutorial)
Matrix
H+ + NADH NAD+ + 2H+ 2H+ + O2 H2O
2 e
I Q III IV+ +
4H+ 4H+ 2H+
Intermembrane Space
ADP + Pi ATP
3H+
F1
Fo
cyt c
-
How many "high energy" (~) bonds are utilized in activating the fatty acid, by esterifying it to coenzyme A? (-)________
How many times is the b-oxidation pathway repeated during oxidation of a 12-C fatty acid? _________
How many each of NADH______, FADH2______, and Acetyl CoA______ are produced, per 12-carbon fatty acid, in the b-oxidation pathway?
Oxidation of each acetyl CoA in Krebs cycle yields 3 NADH and one FADH2 (from succinate), resulting in additional production of _______NADH and _______FADH2.
Thus the yield is a total of _______NADH and _______FADH2.
In the respiratory chain, approx. 2.5 ~ bonds of ATP are produced per NADH and 1.5 ~ bonds of ATP per FADH2 (electrons entering the respiratory chain via coenzyme Q). Thus from reoxidation of NADH and FADH2 a total of _______ ~ bonds of ATP are produced per 12-C fatty acid.
Add to this the ~P bonds of GTP produced in Krebs Cycle (one GTP per acetyl-CoA) for a total of _______ ~P bonds produced.
Summing input and output yields a total of _______ ~P bonds per 12-C fatty acid oxidized. Does fat yield more energy than carbohydrate? _______
2
5
5
5
6
18
6
23
11
74
80
78
YES
-
Human genetic diseases have been identified that involve mutations in:
the plasma membrane fatty acid transporter CD36
Carnitine Palmitoyltransferases I & II (required for transfer of fatty acids into mitochondria)
Acyl-CoA Dehydrogenases for various chain lengths of fatty acids
Hydroxyacyl-CoA Dehydrogenases for medium & short chain length fatty acids
Medium Chain b-Ketothiolase
the trifunctional protein complex
Electron Transfer Flavoprotein (ETF).
-
Human genetic diseases:
Symptoms vary depending on the specific genetic defect but may include:
hypoglycemia and failure to increase ketone body production during fasting
fatty degeneration of the liver
heart and/or skeletal muscle defects
maternal complications of pregnancy
sudden infant death (SIDS).
Hereditary deficiency of Medium Chain Acyl-CoA Dehydrogenase (MCAD), the most common genetic disease relating to fatty acid catabolism, has been linked to SIDS.
-
The reactions presented accomplish catabolism of a fatty acid with an even number of C atoms & no double bonds.
Additional enzymes deal with catabolism of fatty acids with an odd number of C atoms or with double bonds.
The final round of b-oxidation of a fatty acid with an odd number of C atoms yields acetyl-CoA & propionyl-CoA.
Propionyl-CoA is converted to the Krebs cycle intermediate succinyl-CoA, by a pathway involving vitamin B12 (to be presented later).
-
Most double bonds of naturally occurring fatty acids have the cis configuration.
As C atoms are removed two at a time, a double bond may end up in the wrong position or wrong configuration to be the correct substrate for Enoyl-CoA Hydratase.
The reactions that allow unsaturated fatty acids to be fully catabolized by the b-oxidation pathway are summarized in the textbook.
-
b-Oxidation of very long-chain fatty acids also occurs within peroxisomes.
FAD is e- acceptor for peroxisomal Acyl-CoA Oxidase, which catalyzes the 1st oxidative step of the pathway.
Single membrane
Enzymes, some of which produce H2O2 , &
always including Catalase, that degrades H2O2.
Crystalline inclusion often present
Peroxisome
-
Within the peroxisome, FADH2 generated by fatty acid oxidation is reoxidized producing hydrogen peroxide:
FADH2 + O2 FAD + H2O2
The peroxisomal enzyme Catalase degrades H2O2:
2 H2O2 2 H2O + O2
These reactions produce no ATP.
Once fatty acids are reduced in length within the peroxisomes they may shift to the mitochondria to be catabolized all the way to CO2.
Carnitine is involved in transfer of fatty acids into and out of peroxisomes.
-
Serious genetic diseases are associated with defects in or deficiency of enzymes of the peroxisomal b-oxidation system.
Peroxisomes also contain enzymes for an essential a-oxidation pathway that degrades fatty acids having methyl branches, such as phytanic acid, a breakdown product of chlorophyll.
-
This impedes entry of acetyl-CoA into Krebs cycle.
Acetyl-CoA in liver mitochondria is converted then to ketone bodies, acetoacetate & b-hydroxybutyrate.
During fasting or carbohydrate starvation, oxaloacetate is depleted in liver due to gluconeogenesis.
Glucose-6-phosphataseglucose-6-P glucose
Gluconeogenesis Glycolysispyruvate
fatty acids
acetyl CoA ketone bodies
cholesterol
oxaloacetate citrate
Krebs Cycle -
Ketone body synthesis:
b-Ketothiolase. The final step of the b-oxidation pathway runs backward.
HMG-CoA Synthase catalyzes condensation with a 3rd acetate moiety (from acetyl-CoA).
HMG-CoA Lyase cleaves HMG-CoA to yield acetoacetate & acetyl-CoA.
H
3
C
C
H
2
C
C
O
O
S
C
o
A
H
3
C
C
O
S
C
o
A
H
S
C
o
A
H
2
C
C
H
2
C
C
O
H
O
S
C
o
A
C
H
3
C
O
-
O
H
3
C
C
O
S
C
o
A
+
H
3
C
C
O
S
C
o
A
H
S
C
o
A
-
O
C
H
2
C
C
O
O
C
H
3
H
3
C
C
O
S
C
o
A
+
H
3
C
C
H
2
C
C
O
O
S
C
o
A
H
3
C
C
O
S
C
o
A
H
S
C
o
A
H
2
C
C
H
2
C
C
O
H
O
S
C
o
A
C
H
3
C
O
-
O
H
3
C
C
O
S
C
o
A
+
H
3
C
C
O
S
C
o
A
H
S
C
o
A
-
O
C
H
2
C
C
O
O
C
H
3
H
3
C
C
O
S
C
o
A
+
EMBED ChemDraw.Document.4.5
acetyl-CoA
acetyl-CoA
acetoacetyl-CoA
acetyl-CoA
HMG-CoA
acetoacetate acetyl-CoA
Thiolase
HMG-CoA Synthase
HMG-CoA Lyase
_1008616771.cdx_1008617041.cdx_1008616263.cdx -
Ketone bodies are transported in the blood to other cells, where they are converted back to acetyl-CoA for catabolism in Krebs cycle, to generate ATP.
While ketone bodies thus function as an alternative fuel, amino acids must be degraded to supply input to gluconeogenesis when hypoglycemia occurs, since acetate cannot be converted to glucose.
b-Hydroxybutyrate Dehydrogenase catalyzes reversible interconversion of the ketone bodies acetoacetate & b-hydroxybutyrate.
C
H
3
C
C
H
2
C
O
O
-
O
C
H
3
C
H
C
H
2
C
O
O
-
H
O
C
H
3
C
C
H
2
C
O
O
-
O
C
H
3
C
H
C
H
2
C
O
O
-
H
O
acetoacetate D--hydroxybutyrate
EMBED ChemDraw.Document.4.5
H+
NADH NAD+
-Hydroxybutyrate Dehydrogenase
_977223612.cdx_977224386.cdxC
O
O
-
1
2
3
4
a
b
g
fatty acid with a cis
-
D
9
double bond
glycerol fatty acid triacylglycerol
H
2
C
H
C
H
2
C
O
H
O
H
O
H
H
2
C
H
C
H
2
C
O
O
O
C
R
O
C
C
R
O
R
O
H
O
C
R
O
glycerol glycerol-3-P
dihydroxyacetone-P
C
H
2
C
H
C
H
2
O
H
H
O
O
P
O
3
-
C
H
2
C
H
C
H
2
O
H
H
O
O
H
C
H
2
C
C
H
2
O
H
O
P
O
3
-
O
ATP ADP
H
+
+
NAD
+
NADH
1
2
Fatty acid activation
N
N
N
N
N
H
2
O
O
H
O
H
H
H
H
C
H
2
H
O
P
O
P
O
P
-
O
O
O
-
O
-
O
O
O
-
N
N
N
N
N
H
2
O
O
H
O
H
H
H
H
C
H
2
H
O
P
O
C
O
O
-
R
O
S
C
R
O
C
o
A
R
C
O
O
-
P
P
i
C
o
A
S
H
A
M
P
2
P
i
fatty acid
ATP
acyl
-
adenylate
acyl
-
CoA
b
-Oxidation
pathway in
matrix
Fatty
acyl-CoA formed in cytosol by enzymes
of outer mitochondrial membrane & ER
Mitochondrion
H
3
C
N
C
H
2
C
H
C
H
2
C
H
3
C
H
3
O
H
C
O
O
-
+
R
C
S
C
o
A
O
+
H
3
C
N
C
H
2
C
H
C
H
2
C
H
3
C
H
3
O
C
O
O
-
+
C
R
O
+
H
S
C
o
A
carnitine
fatty acyl carnitine
Carnitine Palmitoyl
Transferase
cytosol
mitochondrial matrix
O
O
R
-
C
-
SCoA HO
-
carnitine HO
-
carnitine R
-
C
-
SCoA
HSC
oA R
-
C
-
O
-
carnitine
R
-
C
-
O
-
carnitine HSCoA
O
O
1
2
3
H
3
C
C
S
C
o
A
O
C
H
2
C
S
C
o
A
O
-
O
O
C
acetyl
-
CoA
malonyl
-
CoA
H
3
C
C
S
C
o
A
O
C
H
2
C
S
C
o
A
O
-
O
O
C
acetyl
-
CoA
malonyl
-
CoA
ATP +
HCO
3
-
ADP + P
i
Acetyl
-
CoA
Carboxylase
(inhibited by
AMP
-
Activated
Kinase)
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
H
H
O
1
2
3
a
b
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
O
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
H
O
H
2
O
F
A
D
H
2
F
A
D
H
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
H
+
+
N
A
D
H
N
A
D
+
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
fatty acyl
-
CoA
trans
-
D
2
-
enoyl
-
CoA
Acyl
-
CoA Dehydrogenase
H
3
N
+
CCOO
CH
2
CH
2
C
H
O
O
glutamate
C
C
C
H
C
C
H
C
N
C
C
N
N
C
N
H
C
H
3
C
H
3
C
O
O
C
H
2
H
C
H
C
H
C
H
2
C
O
H
O
P
O
P
O
O
O
-
O
O
-
R
i
b
o
s
e
O
H
O
H
A
d
e
n
i
n
e
C
C
C
H
C
C
H
C
N
C
C
H
N
N
H
C
N
H
C
H
3
C
H
3
C
O
O
C
H
2
H
C
H
C
H
C
H
2
C
O
H
O
P
O
P
O
O
O
-
O
O
-
R
i
b
o
s
e
O
H
O
H
A
d
e
n
i
n
e
FAD
FADH
2
2 e
-
+ 2 H
+
dimethylisoalloxazine
Matrix
H
+
+
NADH
NAD
+
+
2H
+
2H
+
+
O
2
H
2
O
2
e
-
I
Q
III
IV
+ +
4H
+
4H
+
2H
+
Intermembrane Space
cyt
c
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
H
H
O
1
2
3
a
b
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
O
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
H
O
H
2
O
F
A
D
H
2
F
A
D
H
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
H
+
+
N
A
D
H
N
A
D
+
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
fatty acyl
-
CoA
trans
-
D
2
-
enoyl
-
CoA
3
-
L
-
hydroxyacyl
-
CoA
Acyl
-
CoA Dehydrogenase
Enoyl
-
CoA Hydratase
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
H
H
O
1
2
3
a
b
H
3
C
(
C
H
2
)
n
C
C
C
S
C
o
A
H
H
O
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
H
O
H
2
O
F
A
D
H
2
F
A
D
H
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
H
+
+
N
A
D
H
N
A
D
+
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
3
-
L
-
hydroxyacyl
-
CoA
b
-
ketoacyl
-
CoA
fatty acyl
-
CoA acetyl
-
CoA
(2 C shorter)
Hydroxyacyl
-
CoA
Dehydrogena
se
b
-
Ketothiolase
H
3
C
(
C
H
2
)
n
C
C
H
2
C
S
C
o
A
O
O
C
H
3
C
S
C
o
A
O
H
3
C
(
C
H
2
)
n
C
S
C
o
A
+
O
H
S
C
o
A
b
-
ketoacyl
-
CoA
fatty acyl
-
CoA acetyl
-
CoA
(2 C shorter)
b
-
Ketothiolase
H
3
N
+
CCOO
CH
2
SH
H
cysteine
Matrix
H
+
+
NADH
NAD
+
+
2H
+
2H
+
+
O
2
H
2
O
2
e
-
I
Q
III
IV
+ +
4H
+
4H
+
2H
+
Intermembrane Space
cyt
c
3H
+
F
1
F
o
ADP
+
P
i
ATP
Single membrane
Enzymes, some of which produce H
2
O
2
, &
always including
Catalase, that degrades H
2
O
2
.
Crystalline inclusion
often present
Peroxisome
Glucose
-
6
-
phosphatase
glucose
-
6
-
P glucose
Gluconeogenesis
Glycolysis
pyruvate
fatty acids
acetyl CoA
ketone bodies
cholesterol
oxaloacetate citrate
Krebs Cycle
H
3
C
C
H
2
C
C
O
O
S
C
o
A
H
3
C
C
O
S
C
o
A
H
S
C
o
A
H
2
C
C
H
2
C
C
O
H
O
S
C
o
A
C
H
3
C
O
-
O
H
3
C
C
O
S
C
o
A
+
H
3
C
C
O
S
C
o
A
H
S
C
o
A
-
O
C
H
2
C
C
O
O
C
H
3
H
3
C
C
O
S
C
o
A
+
acetyl
-
CoA
acetyl
-
CoA
acetoacetyl
-
CoA
acetyl
-
CoA
HMG
-
CoA
acetoacetate
acetyl
-
CoA
Thiolase
HMG
-
CoA Synthase
HMG
-
CoA Lyase
b
-
Hydroxybutyrate Dehydrogenase
C
H
3
C
C
H
2
C
O
O
-
O
C
H
3
C
H
C
H
2
C
O
O
-
H
O
acetoacetate
D
-
b
-
hydroxybutyrate
H
+
+
NADH NAD
+