Download Fatty Acid Catabolism

Fatty Acid Catabolism
March 21, 2003
Bryant Miles
Fatty acids are lipids. Fatty acids are also one of the major forms of storage of metabolic energy. There
are two distinct advantages in storing metabolic energy as fatty acids. (1) Fatty acids are mainly
composed of –CH2- groups which are fully reduced. Therefore, the oxidation of these reduced carbons
will yield more energy than oxidized forms of carbon. (2) Because fatty acids are lipids, they are
hydrophobic. They do not need to be solvated in contrast to carbohydrates such as glycogen. Dehydrated
glycogen will absorb twice its dry weight of water when it is rehydrated. The long greasy tails of fatty
acids pack tightly in storage tissues, allowing for concentrated storage.
Adipose Cell shown below.
Modern diets are high in fats.
Fatty acids provide 30 to 60% of the calories in the average
American’s diet.
Our evolutionary ancestors ate lean diets.
Diary products were not part of their diet, and the meat they
consumed was from fast moving animals which are low in fat.
The domesticated animals such as cows and pigs we eat today
were bred to have high fat content because high fat content
correlates to a better tasting animal.
The consequences of a high fat diet are evident, increased
obesity, diabetes, heart disease, ect.
Fatty acids are stored as triacylglycerols.
Triacylglycerols are our principal reserve of stored energy.
The potential energy stored in triacylglycerols in the average
person greatly exceeds the energy stored in forms of protein and
carbohydrate.
Stored Metabolic Fuels in your average 70 kg person
Energy
Weight
Stored Metabolite
(kJ/g)
(g)
Triacylglycerols (adipose tissue)
37
15,000
Protein (muscle)
17
6,000
Glycogen (liver)
16
120
Glycogen (muscle)
16
70
Glucose (blood)
16
20
Total
Stored Potential Energy
(kJ)
555,000
102.000
1,920
1,120
320
660,360
I. Releasing Fatty Acids From Adipose Tissue.
The fatty acids stored in the adipose tissue are mobilized in response to hormone messengers such as
epinephrine, norepinephrine, glucagon and adrenocorticotropic hormone. These signal molecules bind to
specific receptors in the plasma membrane. In adipose tissue the receptors are the 7TM receptors that
activate adenylate cyclase which turns on cAMP production which in turn activates protein kinase A,
which phosphorylates triacylglycerol lipase. The phosphorylation of this lipase activates it. The activated
triacylglycerol lipase hydroyzes the ester bond of the C-1 or C-3 triacylglycerols. The subsequent action
of diacylglycerol lipase and monoacylglycerol lipase yields fatty acids and glycerol. The adipose cell
then releases the fatty acids and the glycerol into the blood where they are carried in complexes with
serum albumin to sites of utilization.
The glycerol formed from lipolysis is absorbed by the liver where it is phosphorylated by glycerol kinase
to glycerol-3-phosphate which is then reduced by glycerol phosphate dehydrogenase into
dihydroxyacetone phosphate which can be converted into glyceraldehyde 3-phosphate by triose phosphate
isomerase.
The fate of the triose phosphate formed from glycerol can be used in both glycolytic and gluconeogenic
pathways depending on the needs of the organism.
II. Absorption of Dietary Fatty Acids
Dietary triacylglycerols digestion begins in the low pH environment of the stomach by some acid hardy
lipases. Most triacylglycerides pass untouched into the duodenum where the triacylglycerols are
emulsified by bile salts. Alkaline pancreatic juice raises the pH allowing the hydrolysis of
triacylglycerols by pancreatic lipases, and esterases. The pancreatic lipases cleave the C-1 and C-3
positions of fatty acids. The esterases cleave at the C-2 position. The fatty acids and 2monoacylglycerols liberated are absorbed by the villi of the intestinal mucosa.
The intestinal mucosal cells take the fatty acids and reconvert them back to triacylglycerols.
Triacylglycerols are insoluble in water. It they were directly released into the blood they would aggregate
and impede blood flow. Intestinal cells take the triacylglycerols and package them into lipoprotein
transport particles called chylomicrons. These chylomicrons are mainly composed of triacylglycerols
surrounded by phospholipids and which are enclosed by proteins. The protein components are called
apolipoproteins. Chylomicrons carry triacylglycerols, cholesterol and fat soluble vitamins in the blood to
the tissues that need them. The chylomicrons are released by exocytosis into the lymph system which in
turn releases them into the blood.
Chylomicrons
Peripheral Protein
Chylomicrons contain phospholipids and proteins on the
surface so that the hydrophilic surfaces are in contact
with water. The hydrophobic molecules are enclosed in
the interior. The lone hydroxyl group of cholesterol
molecules is oriented towards the outer surface shown
here as black dots.
Chylomicrons bind to membrane bound lipoprotein lipases located on adipose and muscle tissues where
the triacylglycerols are once again hydrolyzed into fatty acids. The fatty acids are transported into the
adipose or muscle cells where they are once again resynthesized into triacylglycerols and stored. In the
muscle they can be oxidized to provide energy. As the tissues absorb the fatty acids and
monoacylglycerols, the chylomicrons progressively shrink until they are reduced down to cholesterol
enriched remnants. The remnants are absorbed by the liver releasing the dietary cholesterol.
III. Activation and Transport of Fatty Acids Into the Mitochondrial Matrix.
Fatty acids are oxidized in the matix of the mitochondrian. Short chain fatty acids are transported into the
mitochondrial membrane as free fatty acids. But long chain fatty acids are activated before transportation
into the mitochondrial matrix. The long fatty acids are activated by acyl CoA synthetase which is
located on the outer mitochondrial membrane. The activation occurs in two steps. First the fatty acid
reacts with ATP to form an acyl-adenylate and pyrophosphate. The sulfhyryl group of CoA then attacks
the acyl-adenylate intermediate to form acyl-CoA and AMP.
The free energy of this reaction is near zero making it readily reversible.
Fatty acid + ATP + CoA acyl-CoA + AMP + PPi ∆Go’ = −0.8 kJ/mol
If we couple the activity of the enzyme inorganic pyrophosphatase, the reaction becomes:
Fatty acid + ATP + CoA acyl-CoA + AMP + 2Pi ∆Go’ = −34.3 kJ/mol
The coupling of this enzyme makes the reaction irreversible. This is yet another example of that
reoccurring theme of biosynthetic reactions that are made irreversible by the activity of inorganic
pyrophosphatase.
NH2
N
Fatty acyl-CoA synthetase
O
O
O P O P O
-
O
-
O
O
H
H
O
-
O
P
O-
N
O
-
P O
O-
N
H
O-
H
C
O
OH
N
H
H
OH
CH2(CH2)13CH3
O
O
O
P O-
NH2
O-
N
N
pyrophosphatase
O
CH3(CH2)14
2
O
O P O H
O-
O
C O P O
O-
S
N
H
N
O
H
H
H
OH OH
CoA
NH2
N
O
CH3(CH2)14 C S CoA +
O
O P O
O-
N
H
N
N
O
H
H
H
OH OH
The long chain fatty acids activated on the outer mitochondrial membrane need to be transported into the
mitochondrial matrix to be oxidized. There is a special transport mechanism to carry long chain acylCoA molecules across the inner mitochondrial membrane. The long chain fatty acid molecules are
transferred from Coenzyme A to carnitine. Carnitine is a zwitterionic alcohol. The acyl group is
transferred from CoA to the hydroxyl group of carnitine to form acyl-carnitine by the enzyme carnitine
acyltransferase I which is bound to the outer mitochondrial membrane.
The acyl carnitine is then shuttled across the inner mitochondrial membrane by a specific translocase.
Once the acyl carnitine is on the matrix side of the
membrane then it is transferred back to CoA by the
enzyme carnitine acyltransferase II which is the exact
reverse of the reaction that took place in the cytosol.
Finally the same translocase returns carnitine to the
intermembrane space.
This translocase antiports carnitine out of the matrix
while simultaneously transporting acyl-carnitine in.
IV. β-Oxidation of Fatty Acids
O
H
H2
C
H
C
Cβ
R
S
CoA
Short chain fatty acids are transported into the
mitochondrial matrix as free acids. In the matrix they are
activated by acyl-CoA synthetase into acyl-CoA. Long
chain fatty acids are carried across the inner
mitochondrial membrane as acyl-carnitine. In the matrix
the acyl groups are transferred to CoA to form Acyl-CoA
as described above.
Cα
C
H2
H
H
Acyl-CoA
FAD
Acyl-CoA Dehydrogenase
FADH2
H
H2
C
R
O
C
Cβ
S
CoA
Cα
C
H2
Saturated fatty acyl-CoA molecules are degraded in the
matrix by a reoccurring sequence of four reactions in a
process called β-oxidation. The overall strategy is to
create a carbonyl at the β-carbon by oxidizing the Cα-Cβ
bond to form an olefin, with subsequent hydration and
oxidation analogous to the reaction sequence of succinate
dehydrogenase, fumarase and malate dehydrogenase in
the citric acid cycle. The last reaction is the reverse of a
Claisen condensation producing acetyl CoA and a fatty
acid chain that is 2 carbon atoms shorter.
The process of β-oxidation is shown to the left.
H
trans-∆2-enoyl CoA
H 2O
Enoyl-CoA Hydratase
R
O
H
HO
H2
C
C
Cβ
C
H2
S
CoA
Cα
H
H
L-3-Hydroxyacyl CoA
NAD+
L-3-Hydroxyacyl CoA Dehydrogenase
O
NADH + H+
R
C
Cβ
C
H2
CoASH
O
O
H2
C
H2
C
S
R
CoA
Cα
Cβ
C
H2
Thiolase
H
H
H
3-Ketoacyl CoA
H
Cα
H
O
C
S
CoA
S
CoA
The first reaction is catalyzed by acyl-CoA
dehydrogenase. There are actually three of these water
soluble enzymes. They differ only in their substrate
specificity for either long chain, medium chain or short
chain acyl-CoAs. All three of these acyl-CoA
Mechanism of Acyl-CoA Dehydrogenase
FAD
O
H
H2
C
H
Cβ
C
H2
R
S
C
CoA
Cα
H
H
Acyl-CoA
:B
H
O
Cβ
C
ENZ
dehydrogenases contain a tighly bound FAD prosthetic group which
becomes reduced by the hydride transferred during the oxidation of the
fatty acid. The FADH2 produced transfers its electrons to an electron
transfer flavoprotein (ETF). The reduced ETF is reoxidized by
specific oxidoreductase which is an iron sulfer protein which transfers
the electron to an electron transport chain which transfers the electrons
to CoQ to form CoQH2. CoQH2 carries the electrons to Complex III,
cytochrome c reductase which transfers the electrons to cytochrome c
via the Q cycle generating a proton gradient. We have already studied
this mitochondrial respiratory chain which results in 1.5 ATP
molecules per FADH2.
O
H2
C
C
H2
R
S
CoA
H
H2
C
Cα
H
C
Cβ
R
S
CoA
Cα
C
H2
H
H
H
Acyl-CoA
-
FADH
H
B
ENZ
FAD
ETFOX
CoQ
2Cyt cOX
FADH2
ETFRED
CoQH2
2Cyt cRED
Acyl-CoA
Dehydrogenase
H
FADH2
:B
+
H2
C
ENZ
R
O
C
Cβ
S
1/2O2
H2O
CoA
Cα
C
H2
H
The next step of β-oxidation is the addition of water across the double bond in a stereospecific manner.
This reaction is catalyzed by enoyl-CoA hydratase also called crotonase. This enzyme converts transenoyl CoA into L-β-hydroxyacyl-CoA.
The next step is the oxidation of the hydroxyl group into the ketone. This second oxidation reaction is
catalyzed by L-hydroxyacyl-CoA dehydrogenase which uses NAD+ as the oxidant to produce NADH and
the β-ketoacyl-CoA. This NADH is produced in the matrix of the mitochondria so it can readily transfer
the electrons to Complex I of the electron transport chain to produce 2.5 ATP molecules.
O
O
H2
C
C
H2
C
S
S
Cβ
CoA
C
H2
R
Cβ
R
H2
C
O
CoA
S
Cα
H
H
S
H
H
B:
ENZ
ENZ
B:
-
ENZ
O
H2
C
ENZ
C
H2
R
Cβ
S
H
O
R
H
-
O
H2
C
C
C
H2
Cβ
S
ENZ
Cα
O
C
S
H
CoA
B
ENZ
CoA
H
O
H2
C
R
H
H
ENZ
Cα
H
S
CoA
S
B
ENZ
Mechanism of Thiolase
C
H2
Cβ
H
S
B:
ENZ
ENZ
S
C
o
A
The final step is catalyzed by thiolase which
involves the attack of a cysteine residue of the
enzyme on the β-keto carbon atom. This is
followed by bond cleavage to produce the
enolate of acetyl CoA and a thioester
intermediate. Subsequent attack of a second
CoA molecule yields a new, 2 carbon shorter
acyl-CoA.
The repetition of the β-oxidation cycle yields
successive acetyl CoA units.
V. Complete Oxidation of Palmitate.
Beginning with palmitoyl CoA
O
S
CoA
+ 7 FAD + 7NAD + 7 H2O + 7CoA
7 Cycles of β-oxidation
O
8
+ 7 FADH2 + 7NADH
C
H3C
S
CoA
Citric acid cycle
8 2CO2 + 3NADH + GTP + FADH2
7FADH2 + 8 FADH2 = 15 FADH2 X 1.5 ATP/FADH2 = 22.5 ATP
7NADH + 24NADH = 31 NADH X 2.5 ATP/NADH = 77.5 ATP
8GTP = 8ATP
108 ATP
If we begin with palmitate, it takes two ATP equivalents (one in the palmitoyl-adenylate formation and
one in the pyrophosphatase reaction) to convert palmitate into palmitoyl CoA
O
O-
ATP + CoASH
AMP + 2 Pi
O
S
The net ATP production beginning with palmitate is 108 – 2 = 106 ATP.
The net reaction:
CH3(CH2)14CO2- + 106Pi + 106ADP + 23O2 106 ATP + 16CO2 + 130 H2O
CoA