2. Importance of fatty acid
Fatty acids are building blocks of phospholipids and
glycolipids
Many proteins are modified by the covalent attachment
of fatty acids, which targets them to membrane locations
Fatty acids are fuel molecules : They are stored as
TAGs (uncharged esters of fatty acids with glycerol)
Fatty acids mobilized from TAGs are oxidized to
meet the energy needs of a cell or organism
Fourth, fatty acid derivatives serve as hormones and
intracellular messengers
3. Importance of fatty acid as fuel
TAGs are highly concentrated stores of metabolic
energy because they are reduced and anhydrous
The yield from the complete oxidation of fatty acids is
about 9 kcal g-1 (38 kJ g-1), in contrast with about 4
kcal g-1 (17 kJ g-1) for carbohydrates and proteins
Consider a typical 70-kg man, who has fuel reserves of
100,000 kcal in TAGs, 25,000 kcal in protein, 600 kcal
in glycogen, and 40 kcal in glucose
TAGs constitute about 11 kg of his total body weight
5. β-Oxidation of fatty acids
Fatty acid in body mostly oxidised by β-oxidation
Oxidation of fatty acid on the β carbon
Two-carbon fragments are successively removed
from the carboxyl end of the fatty acyl CoA,
producing acetyl CoA, NADH, and FADH2
Tissue location for oxidation : Most of the tissue in
the body
6. Steps of -oxidation
Activation of fatty acid occuring in cytosol
Transport of Fatty acids into mitochondria
-Oxidation proper in mitochondrial matrix.
7. Activation of fatty acid
Eugene Kennedy and Albert Lehninger showed in
1949 that fatty acids are oxidized in mitochondria
Subsequent work demonstrated that they are activated
before they enter the mitochondrial matrix
ATP drives formation of a thioester linkage between the
carboxyl group of FAs and the sulfhydryl group of CoA
Activation reaction : outer mitochondrial membrane,
catalyzed by acyl CoA synthetase
9. Transport of long-chain fatty acids (LCFA) into
the mitochondria
Special transport mechanism : Carnitine shuttle
Activated LCFA are transported across the membrane
by conjugating them to carnitine, a zwitterionic alcohol
The acyl group is transferred from the sulfur atom of
CoA to the hydroxyl group of carnitine to form acyl
carnitine (carnitine acyltransferase I)
Second, the acylcarnitine is transported into the
mitochondrial matrix in exchange for free carnitine by
carnitine–acylcarnitine translocase (CPT-II, or CAT-II)
11. Acyl Carnitine Translocase
The entry of acyl
carnitine into the
mitochondrial matrix is
mediated by a translocase
Carnitine returns to the
cytosolic side of the
inner mitochondrial
membrane in exchange
for acyl carnitine
12. Inhibitor of the Carnitine shuttle
Malonyl CoA inhibits CPT-I, thus preventing the entry of
long-chain acyl groups into the mitochondrial matrix
Therefore, newly made palmitate cannot be transferred
into the mitochondria and degraded
The phosphorylation and inhibition of acetyl CoA
carboxylase decreases malonyl CoA production,
removing the break on fatty acid oxidation
Fatty acid oxidation is also regulated by the acetyl CoA
to CoA ratio: As the ratio increases, the thiolase reaction
decreases
13. Sources of Carnitine
Diet, found primarily in meat products
Also synthesized from the amino acids lysine and
methionine
This enzymatic pathway found in the liver and kidney
but not in skeletal or heart muscle
Skeletal muscle : 97% of all carnitine in the body
14. Carnitine deficiencies
Decreased ability of tissues to use LCFA as a metabolic
fuel
Results in the accumulation of toxic amounts of free
fatty acids and branched-chain acyl groups in cells.
Causes of Secondary carnitine deficiency
1. Patients with liver disease
2. Malnutrition or those on strictly vegetarian diets
3. Increased requirement for carnitine (for e.g,
pregnancy, severe infections, burns, or trauma)
4. Patients undergoing hemodialysis
15. Congenital deficiencies of Carnitine palmitoyltransferase
(CPT) system, cause primary carnitine deficiency
Genetic CPT-I deficiency affects the liver
CPT-II deficiency occurs primarily in cardiac and
skeletal muscle
Symptoms of carnitine deficiency range from
cardiomyopathy to muscle weakness with
myoglobinemia following prolonged exercise
Primary carnitine deficiency
16. Entry of short- and medium-chain fatty acids
into the mitochondria
Fatty acids shorter than 12-C can cross the inner mit.
membrane without the aid of carnitine or the CPT system
Once inside the mitochondria, they are activated to their
CoA derivatives by matrix enzymes, and are oxidized.
17. β-Oxidation proper
A saturated acyl CoA is degraded by a recurring sequence
of four reactions:
Oxidation by FAD, hydration, oxidation by NAD+, and
thiolysis by CoA
The fatty acyl chain is shortened by 2-C atoms as a result
of these reactions
FADH2
, NADH, and acetyl CoA are generated
Because oxidation is on the β-carbon, this series of
reactions is called the β-oxidation pathway
18. Enzymes involved in the β-oxidation of Fatty acyl
Coa
Oxidation: the acyl CoA
undergoes dehydrogenation by
acyl CoA dehydrogenase.
Hydration: the enoyl CoA
hydratase brings about the
hydration of double bond to
form β-hydroxy acyl CoA.
Oxidation: β-hydroxy acyl CoA
dehydrogenase catalyses the
oxidation to form β-keto acyl
CoA.
Thiolytic cleavage: the thiolase
cleaves acetyl CoA from acyl CoA
19. Figure 3. Processing
and
-oxidation of palmitoyl
CoA
matrix side
inner mitochondrial
membrane
2 ATP
3 ATP
respiratory chain
recycle
6 times
Carnitine
translocase
Palmitoylcarnitine
Palmitoylcarnitine
Palmitoyl-CoA
+ Acetyl CoACH3-(CH)12-C-S-CoA
O
oxidation
FAD
FADH2
hydration H2O
thiolase CoA
oxidation
NAD+
NADH
Citric
acid
cycle 2 CO2
S
U
M
M
A
R
Y
20. Energy yield from fatty acid oxidation
For example, the oxidation of a molecule of palmitoyl
CoA to CO2 and H2O produces 8 acetyl CoA, 7 NADH,
and 7 FADH2
The Complete Oxidation of Palmitate Yields 129
Molecules of ATP
22. Medium-Chain Fatty Acyl CoA Dehydrogenase
(MCAD) deficiency
In mitochondria, there are 4 fatty acyl CoA dehydrogenase
species (SCFA, MCFA, LCFA, VLCFA)
MCAD deficiency : autosomal recessive disorder, is one of
the most common inborn errors of metabolism
Incidence - 1:12,000 births in the West, and 1:40,000
worldwide
It causes a decrease in fatty acid oxidation and severe
hypoglycemia
Treatment includes a carbohydrate-rich diet
23. Oxidation of fatty acids with an odd
number of carbons
Reactions proceeds by the same
steps as that of fatty acids with
an even number, until the final
three carbons are reached
This compound, propionyl CoA,
is metabolized by a three-step
pathway
Only example of a glucogenic
precursor generated from fatty
acid oxidation
24. Oxidation of unsaturated fatty acids
Provides less energy than that of saturated fatty acids
Oxidation of monounsaturated fatty acids, such as
18:1(9) (oleic acid) requires one additional enzyme, 3,2-
enoyl CoA isomerase
Oxidation of PUFAs, such as 18:2(9,12) (linoleic acid),
requires an NADPH-dependent 2,4-dienoyl CoA
reductase in addition to the isomerase
So, an Isomerase and a Reductase Are Required for
the Oxidation of Unsaturated Fatty Acids
25. β-oxidation in the peroxisome
VLCFAs, or those 20 carbons long or longer, undergo a
preliminary β-oxidation in peroxisomes.
The shortened fatty acid is then transferred to a
mitochondrion for further oxidation.
In contrast to mitochondrial β-oxidation, the initial
dehydrogenation in peroxisomes is catalyzed by an FAD-
containing acyl CoA oxidase.
26. Zellweger syndrome
X-linked adrenoleukodystrophy : Genetic defects in the ability
to transport VLCFA across the peroxisomal membrane
A peroxisomal biogenesis disorder in
all tissues resulting from the absence of
functional peroxisomes
Characterized by liver, kidney, and
muscle abnormalities and usually
results in death by age six
The syndrome is caused by a defect in
the import of enzymes into the
peroxisomes
27. α-Oxidation of Fatty acids
Branched-chain fatty acid, phytanic acid: not a substrate
for acyl CoA dehydrogenase because of the methyl
group on its third (β) carbon
Instead, it is hydroxylated at the α-carbon by fatty acid
α-hydroxylase
The product is decarboxylated and then activated to its
CoA derivative, which is a substrate for the enzymes of
β-oxidation.
28. Refsum’s disease
Rare, autosomal recessive disorder caused by a
deficiency of α-hydroxylase
This results in the accumulation of phytanic acid in
the plasma and tissues
Symptoms are primarily neurologic
Treatment involves dietary restriction to halt
disease progression
29. ω-Oxidation
ω-Oxidation (at the methyl terminus) also is known,
and generates dicarboxylic acids.
Normally a minor pathway of the ER
Its up-regulation is seen with conditions such as MCAD
deficiency that limit fatty acid β-oxidation
30.
31. REFERENCES
Biochemistry 5th edition by Jeremy M. Berg,
JL Tymoczko, Lubert Stryer
Lippincots Illustrated Biochemistry 3rd
edition
Harpers Illustrated Biochemistry 28th edition
Lehningers principles of Biochemistry, 5th
edition
Editor's Notes
A small band of cytoplasm surrounds the large deposit of triacylglycerols
These amphipathic molecules are important components of biological membranes
..The basis of this large difference in caloric yield is that fatty acids are much more reduced. Furthermore, triacylglycerols are nonpolar, and so they are stored in a nearly anhydrous form, whereas much more polar proteins and carbohydrates are more highly hydrated. In fact, 1 g of dry glycogen binds about 2 g of water. Consequently, a gram of nearly anhydrous fat stores more than six times as much energy as a gram of hydrated glycogen, which is likely the reason that triacylglycerols rather than glycogen were selected in evolution as the major energy reservoir
... If this amount of energy were stored in glycogen, his total body weight would be 55 kg greater
.....
The utility of triacylglycerols as an energy source is dramatically illustrated by the abilities of migratory birds, which can fly great distances without eating. Examples are the American golden plover and the ruby-throated sparrow. The golden plover flies from Alaska to the southern tip of South America; a large segment of the flight (3800 km, or 2400 miles) is over open ocean, where the birds cannot feed. The ruby-throated hummingbird can fly nonstop across the Gulf of Mexico. Fatty acids provide the energy source for both these prodigious feats.
Paul Berg showed that the activation of a fatty acid is accomplished in two steps. First, the fatty acid reacts with ATP to form an acyl adenylate. In this mixed anhydride, the carboxyl group of a fatty acid is bonded to the phosphoryl group of AMP. The other two phosphoryl groups of the ATP substrate are released as pyrophosphate. The sulfhydryl group of CoA then attacks the acyl adenylate, which is tightly bound to the enzyme, to form acyl CoA and AMP
[Note: CPT-I is also known as CAT-I for carnitine acyltransferase I.]
..Carnitine palmitoyltransferase II (CPT-II, or CAT-II)—an enzyme of the inner mitochondrial membrane—catalyzes the transfer of the acyl group from carnitine to CoA in the mitochondrial matrix, thus regenerating free carnitine.
Therefore, these tissues are totally dependent on carnitine provided by endogenous synthesis or the diet, and distributed by the blood.
(such as those produced during the catabolism of the branched-chain amino acids)
Genetic CPT-I deficiency affects the liver, where an inability to use LCFA for fuel greatly impairs that tissue's ability to synthesize glucose during a fast
This is an example of how the impaired flow of a metabolite from one cell compartment to another results in pathology. Treatment includes avoidance of prolonged fasts, adopting a diet high in carbohydrate and low in LCFA, but supplemented with medium-chain fatty acid and, in cases of carnitine deficiency, carnitine
[Note: medium-chain fatty acids are plentiful in human milk. Because their oxidation is not dependent on CPT-I, it is not subject to inhibition by malonyl CoA.]
each of which has a specificity for either short-, medium-, long-, or very-long-chain fatty acids
(because the tissues cannot obtain full energetic benefit from fatty acids and, therefore, must now rely on glucose).
[Note: Infants are particularly affected by MCAD deficiency, because they rely for their nourishment on milk, which contains primarily medium-chain fatty acids.
MCAD deficiency has been identified as the cause of some cases originally reported as sudden infant death syndrome (SIDS) or Reye syndrome
[Note: Propionyl CoA is also produced during the metabolism of certain amino acids
The enzyme, methylmalonyl CoA mutase, requires a coenzyme form of vitamin B12 (deoxyadenosylcobalamin) for its action. The mutase reaction is one of only two reactions in the body that require vitamin B12 (see p. 375). [Note: In patients with vitamin B12 deficiency, both propionate and methylmalonate are excreted in the urine. Two types of inheritable methylmalonic acidemia and aciduria have been described: one in which the mutase is missing or deficient(or has reduced affinity for the coenzyme), and one in which the patient is unable to convert vitamin B12 into its coenzyme form. Either type results in metabolic acidosis, with developmental retardation seen in some patients.]
.provides less energy than that of saturated fatty acids
..because unsaturated fatty acids are less highly reduced and, therefore, fewer reducing equivalents can be produced from these structures
...3,2-enoyl CoA isomerase : which converts the 3-trans derivative obtained after three rounds of β-oxidation to the 2-trans derivative that can serve as a substrate for the hydratase
The FADH2 produced is oxidized by molecular oxygen, which is reduced to H2O2. The H2O2 is reduced to H2O by catalase
Here we see a pathological condition resulting from an inappropriate cellular distribution of enzymes
lead to accumulation of VLCFA in the blood and tissues
ω-Oxidation (at the methyl terminus) also is known. Normally a minor pathway, its up-regulation is seen with MCAD deficiency
