Oxidation of Fatty acid

1. FATTY ACID OXIDATION

2. Main Functions of Lipids
• Energy stores
• Fuel molecules
• Components of membranes
• Signalling molecules

4. Utilization of Fatty Acids as Fuel Requires 3 Stages of
Processing.
1. The lipids have to be mobilized. In this process,
triacylglycerols (which are the main storage forms
of dietary lipids) are degraded to fatty acids and
glycerol, which are released from the adipose
tissue and transported to energy-requiring tissues.
2. In these tissues, the fatty acids is activated and
transported into mitochondria for degradation.
3. The fatty acids are broken down in a step-by step
process into acetyl CoA

5. • Each step in FA oxidation involves acyl coA derivatives, is catalysed by
separate enzymes, utilizes NAD+ and FAD as coenzymes , and
generates ATP.

6. Utilization of fatty acids for energy
production varies from tissue to tissue
and with the metabolic status of the
individual. While they are a major source
of energy in cardiac and skeletal muscle,
the brain utilizes them poorly due to
limited transport across the blood-brain
barrier..

7. When hormones such as epinephrine or glucagon are
secreted in response to low levels of glucose, it
triggers an intracellular second messenger cascade
that phosphorylates hormone sensitive lipase to
break triglycerides into glycerol and free fatty acids.
The free fatty acids move into the blood stream
where they are bound by serum albumin and
transported to the tissue in which fatty acid oxidation
is to take place. They are then released by albumin
and they move into the cytosol.
Fatty acids that are to be oxidized for energy are first
activated in the cytosol, then shuttled into the
mitochondria for oxidation

9. • The pyrophosphate is subsequently hydrolyzed to 2Pi.Therefore, the
activation of a fatty acid consumes two high energy phosphate bonds.
• PPi + H2O 2Pi + 2H+

10. The enzymes of fatty acid oxidation are located
in the mitochondrial matrix. Therefore, fatty
acyl CoAs generated in the cytosol must be
transported into the mitochondrial matrix. Long
chain acyl-CoA and FFA cannot penetrate the
inner mitochondrial membrane so fatty acyl
CoA enters the mitochondria via a special
mechanism known as the carnitine fatty acyl
carrier system.
Carnitine (β-hydroxy-γtrimethylammonium
butyrate ) is widely distributed and very
abundant in muscle.

11. Carnitine fatty acyl carrier system
• Acyl CoA is conjugated to carnitine by carnitine
acyltransferase I (palmitoyltransferase I) located on
the outer mitochondrial membrane
• Acyl carnitine is shuttled inside the mitochondria by a
translocase
• Acyl carnitine (such as palmitoylcarnitine) is
converted to acyl CoA by carnitine acyltransferase
(palmitoyltransferase) II located on the inner
mitochondrial membrane.
• The liberated carnitine returns to the outer
mitochondrial membrane

12. The carnitine carrier system depends on the presence of
coA on both sides of the inner mitochondrial membrane.
It functions primarily in the transport of fatty acyl CoAs
with 12-18 carbon units. Entry of shorter chain fatty acids
is independent of the carrier system. They cross the inner
mitochondia membrane and they become activated to
their CoA derivative in the matrix.

13. https://youtu.be/ueQqdrqBGiU
Watch this short video on the Carnitine shuttle system

14. β-Oxidation of Fatty Acids
In the Mitochondrial, matrix, fatty acyl CoAs are
oxidized to acetyl CoA by a recurring reaction
sequence that cleaves successive two carbon units off
the fatty acid chain. This process is known as β-
oxidation. The reactions of β-oxidation are as follows
1. Oxidation: The fatty acylCoA is oxidized by the
appropriate acyl coA dehydrogenase . FAD is
reduced in the process.

16. The mitochondrion contains at least 4 dehydrogenases
specific for fatty acyl CoAs of different chain lengths.
They are very long chain, long chain, medium chain
and short chain acyl-CoA dehydrogenases( VLCAD,
LCAD, MCAD and SCAD).
• VL CAD – oxidizes straight chain acyl-CoA from C 12 –
C 24.
• M CAD has broad chain length specificity but is most
active with C6 and C8 substrates.
• S CAD order of preferred C4 > C6 > C8
• LCAD is involved in initiating the oxidation of
branched chain FA .

17. 2. Hydration. The unsaturated fatty acyl CoA is
hydrated by an enoyl CoA hydratase to yield the β-
hydroxyacyl derivative. The hydratases also show
chain length specificity

19. 3. Oxidation
The β- hydroxy derivative is oxidized by β-
hydroxyacylCoA dehydrogenase to the
corresponding β- ketoacyl CoA , with the
reduction of an NAD+

21. 4. Thiolysis
This involves the thiolytic cleavage of the bond in the β-
keto derivative by an incoming CoA to yield acetyl CoA
and the shortened fatty acyl CoA. The reaction is
catalyzed by thiolase.
The shortened fatty acid chain is now ready for the next
cycle of β- oxidation

23. β-oxidation pathway

24. Net ATP Yield from Palmitate Oxidation
• Each cycle of β- oxidation produces one FADH2,
one NADH and one Acetyl CoA. During the last β- oxidation cycle,
two acetyl CoA s are formed. Thus, the products of complete
oxidation of palmitate are
8 Acetyl CoA,7 FADH and 7NADH. Oxidation of FADH2 and NADH
by electron transport and oxidative phosphorylation yield
respectively 1.5 and 2.5 ATPs, and oxidation of acetyl CoA by
the TCA cycle coupled to electron transport and oxidative
phosphorylation yields 10 ATPs.,
Therefore, the total yield of oxidation of palmitate to CO2 and
H2O is 108. However, two high energy phosphate bonds,
equivalent of 2 ATPs are consumed in the activation step. Thus,
the net yield of palmitate oxidation is 106ATPs

25. • Fatty acids that contain an odd number of carbon
atoms, certain unsaturated fatty acids, and
methylated fatty acids require modification of the β-
oxidation sequence.
Oxidation of unsaturated Fatty acids
• The double bond that is generated between the α and
β carbons in the first step of β oxidation is in the trans
configuration. Hydration of this bond by the hydratase
yields L-hydroxyacyl coA, which is the required
substrate of the next enzyme, β-hydroxyacylcoA DH.
OTHER MECHANISMS OF FATTY ACID OXIDATION

26. • The double bonds in most unsaturated Fas are
however in the Cis form, and yields the D-
stereoisomer when hydrated.
• These isomers are converted to the L-isomers by a
racemase, and β oxidation then proceeds normally
• If the double bond in an unsaturated FA is located bw
β and γ Cs, an isomerase moves the bond to the
correct position for β oxidation to proceed.

28. Cis 3 enoyl coA
trans- 2 enoyl coA
trans-2, cis-4 dienoyl coA
trans -3 enoyl coA trans- 2 enoyl CoA

29. Cis -3 enoyl -coA
A trans- 2 enoyl
coA
trans- 2 -enoyl coA
Beta Oxidation
Cis- 4-enoyl coA

30. Oxidation of Fatty acids containing an odd number of Carbons
Fatty acids that contain an odd number of carbons are oxidized
by β oxidation until a final three carbon propionyl CoA is
obtained . Propionyl CoA is utilized as follows
Propionyl CoA is carboxylated to D-methyl malonyl CoA by
propionyl CoA carboxylase
Methylmalonyl CoA racemase then converts D-methyl malonyl
CoA to L-methyl malonylCoA.
L-methyl malonyl CoA undergoes rearrangement by methyl
malonyl CoA mutase to yield succinyl CoA. Succinyl CoA then
enters the TCA cycle.

31. Metabolism of Propionyl CoA

32. α and ω- Oxidation of fatty acids
Animal tissues contain minor pathways that involve
oxidation of fatty acids at the α- and ω- carbons. The
products of α and ω oxidation can enter β- oxidation.
α oxidation
This involves hydroxylation on C-2. The resulting α –
hydroxy derivatives may be further oxidized to yield
CO2 and fatty acids consisting of one less carbon atom,
which may then be metabolized by β-oxidation.It
occurs in the endoplasmic reticulum and peroxisomes
and is very important in the oxidation of methylated
fatty acids. It is a method of generating odd-chain fatty
acids.

33. Phytanic acid is hydroxylated at the α C .
C1 is released as CO2 and the product,
pristanic acid undergoes β-oxidation

34. ω-Oxidation
• Occurs in the endoplasmic reticulum of liver or kidney
cells.
• The terminal carbon is progressively oxidized first to
an alcohol and then to a carboxylic acid, creating a
molecule with a carboxylic acid on both ends. The
first step in the pathway is catalyzed by a cytochrome
P450 mixed function oxidase and requires both
oxygen and NADPH. Oxidation of the alcohol is
catalyzed by an alcohol dehydrogenase while an
aldehyde dehydrogenase catalyzes the formation of
the dicarboxylic acid.

35. ω-Oxidation
If the initial substrate was a long chain fatty acid, then the resulting
dicarboxylic acid can enter the beta-oxidation pathway to be
shortened at both ends of the molecule at the same time. When
beta-oxidation is complete, the product is either succinate or
adipate.

36. ω-oxidation pathway

37. • Very long chain fatty acids undergo preliminary
beta oxidation in peroxisomes. The shortened FA
diffuses to the Mitochondria for further oxidation.
• The initial dehydrogenation in peroxisomes is
catalyzed by an FAD containing acyl coA DH.
• The FADH2 produced is oxidized by molecular oxygen
which is reduced to H2O2 . Thus , no ATP is generated
by this step. The H2O2 is reduced to water by
catalase.
• Other metabolic roles of peroxisomes include chain
shortening of dicarboxylic acids and conversion of
cholesterol into bile acids.

38. Regulation of Fatty acid oxidation
The rate of fatty acid oxidation is controlled by regulating the entry of
substrate into the mitochondria.The key enzyme is CAT1. This enzyme
is inhibited by malonyl CoA, whose formation is the committed step
of fatty acid synthesis. When fatty acid synthesis is occurring in the
cell(at times of high energy charge), the concentration of malonyl CoA
is increased, leading to inhibition of CATI.

39. When the energy charge of the cell is low, there is an
accompanying decrease in the concentrations of malonyl CoA,
resulting in the activation of FA oxidation. Thus, CATI is inhibited
in the fed state, while it is very active in the fasted state.
FA oxidation in muscle is also regulated by malonyl CoA, even
though this tissue does not synthesize Fas. Muscle contains an
isozyme of acetyl coA carboxylase which produces malonyl CoA
solely for the purpose of regulating CAT I.

40. • Acetyl coA carboxylase is activated by citrate and inhibited by
phosphorylation. It is phosphorylated by Protein kinase A and AMP-
dependent kinase. Phosphorylation by PKA allows the enzyme to be
regulated by dietary status while phosphorylation by AMP-dependent
kinase links the rate of FA oxidation to the energy status of the cell.