Alpha oxidation, beta Oxidation, omega oxidation.

1. Submitted by-
Vivek kumar
M.Sc Microbiology 1st sem
Bangalore university

2. Introduction
 Fatty acids are aliphatic carboxylic acids containing a long
hydrocarbon chain.
 The hydrocarbon chain may be saturated(with no double
bond) or unsaturated ( containing double bond).
 Triacylglycerols(fats)are the most abundant source of
energy and provide energy twice as much as carbohydrates
and protein.
 Fatty acid can be obtained from diet, adipolysis and de
novo synthesis.

3.  Mitochondrial oxidation of fatty acids takes place in 3
stages-
In 1st stage β oxidation of fatty acid undergo oxidative
removal of successive 2-C units in form of acetyl Co-A.
In 2nd stage of fatty acid oxidation, the acetyl group of acetyl
co-A are oxidized to co2 in the citric acid cycle which also
takes place in the mitochondrial matrix.
The first 2 stages of fatty acid oxidation produce the reduce
electron carriers NADH and FADH2 which in 3rd stage
donate electron to mitochondrial respiratory chain, through
which the electron pass to oxygen with the concomitant
phosphorylation of ADP to ATP.

5. Types of Fatty Acid
Oxidation-
 Alpha oxidation- predominantly takes place in
brain and liver, one carbon is lost in the form of co2
per cycle
 Beta oxidation- major mechanism, occurs in the
mitochondria matrix. 2-C units are released as acetyl
CoA per cycle.
 Omega oxidation- minor mechanism, but becomes
important in condition of impaired beta oxidation.

6. Beta oxidation
overview-
 A saturated acyl Co-A is degraded by recurring sequence
of 4 reaction.
 4 enzyme catalyzed reaction make up the 1st stage of fatty
acid oxidation.
 The fatty acid chain is shortened by 2 carbon atoms as a
result of these reaction FADH2, NADH and acetyl Co-A
are generated.
 Oxidation is on the β-carbon and the chain is broken
between the α (2) and β (3) carbon atom, hence known as
β oxidation.

8. Activation of Fatty Acids-
 Fatty acids must first be converted to an active
intermediate before they can be catabolized.
 This is the only step in the in complete degradation of
fatty acid that requires energy from ATP.
 Fatty acids are activated to fatty acyl CoA in a reaction
catalyzed by acyl CoA synthase.

9. Transport of Fatty Acids into mitochondria
 The inner mitochondrial membrane is not permeable to
long chain fatty acyl CoA.
 The fatty acyl CoA group is transferred to carnitine which
function as a carrier, catalyzed by carnitine acyltransferase
Ι present on the outer surface of mitochondrial
membrane.

10.  In the mitochondrial
matrix, the acyl group from
acyl carnitine is transferred
to CoA by the enzyme
carnitine acyltransferase ΙΙ.

11. Steps of β oxidation
Step 1-α,β
dehydrogenation of acyl
CoA
• The first step is the
removal of two hydrogen
atoms from the 2(α) and
3(β) carbon atoms,
catalyzed by acyl CoA
dehydrogenase and
requiring FAD. This result
in the formation of trans-
∆² enoyl CoA and FADH2

12.  The electron removal from the fatty acyl CoA are
transferred to FAD and the reduced form of the
dehydrogenase immediately donates its electron to an
electron carrier of the mitochondrial respiratory chain, the
electron transferring flavoprotein (ETF).
 The oxidation catalyzed by acyl CoA dehydrogenase is
analogous to succinate dehydrogenation in the citric acid
cycle. In both the reaction, the enzyme is bound to the
inner membrane, a double bond is introduced into a
carboxylic acid between the α and β carbon. FAD is the
electron acceptor, and electron from the reaction
ultimately enter the respiratory chain and pass to O2 with
concomitant synthesis of about 1.5 ATP molecules.

13. Step 2 hydration of
α,β unsaturated acyl
CoA
• In this step, a molecule
of water is added to the
double bond of the
trans ∆² enoyl CoA to
form the L
stereoisomer of β
hydroxyacyl-CoA. And
reaction is catalyzed by
enoyl-CoA hydratase.

14. Step 3 Oxidation of β-
hydroxyacyl-CoA
• The 3-hydroxy derivative
undergoes further
dehydrogenation on the
3-carbon catalyzed by
L(+)-3-hydroxyacyl-CoA
dehydrogenase to form
the corresponding 3-
ketoacyl-CoA compound.
In this case, NAD+ is the
coenzyme involved.

15. Step 4 Thiolysis
 3-ketoacyl-CoA is spilt at
the 2,3- position by
thiolase (3-ketoacyl-CoA-
thiolase), forming acetyl-
CoA two carbons shorter
than the original acyl-
CoA molecule.

16.  The acyl-CoA formed in the
cleavage reaction reenters
the oxidative pathway at
reaction 2.
 Since acetyl-CoA can be
oxidized to CO2 and water
via the citric acid cycle the
complete oxidation of fatty
acids is achieved.

17. Stoichiometry of β-oxidation
 The yields of NADH, FADH2 and ATP in the successive
steps of palmitocyl-CoA oxidation. Because of the
activation of palmitate to palmitoyl-CoA breaks both
phosphoanhydride bonds in ATP, the energetic cost of
activating a fatty acid is equivalent to 2ATP, and the net
gain per molecule of palmitate is 106 ATP.
 The standard free-energy change for the oxidation of
palmitate to CO2 and H2O is about 9800 kJ/mol. Under
standard condition the energy recovered as the phosphate
bond energy of ATP is 106×30.5 kJ/mol=3230 kJ/mol, about
33% of the theoretical maximum.

18. Energy yield
 Energy yield by the complete oxidation of one mol of
Palmitic acid-
The degradation of palmitoyl CoA (C 16-acyl CoA) requires
seven reaction cycles. In the seventh cycle, the C4-ketoacyl
CoA is thiolyzed to two molecules of acetyl CoA.
106 ATP are produced by the complete oxidation of one mol
of Palmitic acid.

19. Oxidation of Unsaturated Fatty Acid
 Most of the fatty acids in the triacylglycerol and
phospholipids of animal and plants are unsaturated having
one or more double bonds. These bonds are in the cis
configuration and cannot be acted upon by enoyl-CoA
hydratase, the enzyme catalyzing the addition of H2O to
the trans double bond of the ∆²-enoyl-CoA generated
during β oxidation. Two auxillarly enzymes are needed for
β-oxidation of the common unsaturated fatty acids: an
isomerase and a reductase.

20. Oxidation of Monounsaturated Fatty Acid
 Oleate is an abundant 18-carbon monounsaturated fatty
acid with a cis double bond between C-9 and C-10.
 In the first step of oxidation, oleate is converted into
oleoyl-CoA and like the saturated fatty acids, enter the
mitochondrial matrix via carnitine shuttle.
 Oleoyl-CoA then undergoes 3 passes through the fatty acid
oxidation cycle to yield 3 molecule of acetyl CoA and co-
enzyme A, ester of ∆³, 12-Carbon unsaturated fatty acid,
cis-∆³ dodecenoyl-CoA. This product cannot serve as a
substrate for enoyl-CoA hydratase, which acts only on
trans double bonds.

21.  The auxiliary enzyme ∆³,∆²-enoyl CoA isomerase
isomerize the cis ∆³ enoyl-CoA to trans ∆²-enoyl-CoA,
which is converted by enoyl-CoA hydratase into the
corresponding L-β- hydroxyacyl CoA.
 This intermediate is now acted upon by the remaining
enzymes of β-oxidation to yield acetyl-CoA and the co-
enzyme A ester of a 10-carbon saturated fatty acids
decanoyl CoA.
 The latter undergoes four more passes through the
pathway to yield five more molecules of acetyl-CoA.
 Altogether, 9 acetyl-CoA are produced from one molecules
of the 18-Carbon oleate.

23. Oxidation of a Polyunsaturated Fatty Acid
 The other auxiliary enzyme (a reductase) is required for
oxidation of polyunsaturated fatty acids- for example the
18-carbon linoleate, which has a cis-∆⁹,cis-∆¹²
configuration.
 Linoleoyl-CoA undergoes three passes through the β-
oxidation sequences to yield three molecules of acetyl-CoA
and the coenzyme A ester of a 12-carbon unsaturated fatty
acid with a cis-∆³, cis ∆⁶ configuration.
 This intermediate cannot be used by enzymes of β-
oxidation pathway, its double bonds are in the wrong
position and have the wrong configuration (cis, not trans).

24.  However, the combined
action of enoyl-CoA
isomerase and 2,4-dienoyl-
CoA reductase allows re-
entry of the intermediate
into the β oxidation
pathway and its
degradation to 6 acetyl-
CoA. The overall result is
conversion of linoleate to 9
molecules of acetyl-CoA.

25. Alpha oxidation
 α-oxidation is a process in which fatty acid are shortened
by one carbon atom.
 α-oxidation was 1st observed in seeds and leaf tissues of
plant. The original substrate have been demonstrated in
the microsomes of brain and liver and tissues also.
 Involves decarboxylation process for the removal of single
carbon atom at one time with the resultant production of
an odd chain fatty acids, that can be subsequently oxidized
by β-oxidation for energy production. It is strictly an
aerobic process.
 The process involves hydroxylation of the α carbon with a
specific α-hydroxylase that requires fe++ and vitamin
C/FH4 as co-factor.

26. • Hydroxy fatty acids can be converted to the α-keto acid,
followed by oxidative decarboxylation resulting in the
formation of long chain fatty chain with an odd number of
carbon atoms.
 The initial hydroxylation reaction is catalyzed by a
mitochondrial enzyme, monoxygenase that requires O2,
Mg²⁺, NADPH and heat stable co-factor.
 Normally metabolized by an initial α-hydroxylation
followed by dehydrogenation and decarboxylation.
 Whole reaction produces 3 molecule of propionyl CoA, 3
molecules of acetyl CoA, and 1 molecule of iso-butyryl
CoA.

27. Biological significance
 α-oxidation is most suited for
oxidation of phytanic acid,
produced from dietary phytol.
 Phytanic acid is a significant
constituent of milk lipids and
animal fats.
 Enzymatic deficiency in α-
oxidation leads to Refsum’s
disease.

28. Omega oxidation
 The biological oxidation of fatty acid at the ω carbon atom
was 1st reported by Verkade, who isolated from the urine,
dicarboxylic acids of same length chain those were fed in
form of triglycerides.
 He proposed that certain acids were first oxidized at the ω
carbon atom and then further metabolized by β oxidation
proceeding from both ends of the dicarboxylic acids.
 Minor pathway for the fatty acid oxidation also involves
hydroxylation and occur in the endoplasmic reticulum of
many tissues.

29.  Hydroxylation takes place on the methyl carbon at the
other end of the molecule from the carboxyl group or on
the carbon next to the methyl end.
 It uses the mixed function oxidase, type of reaction
requiring cytochrome P45o, O2 and NADPH, as well as
necessary enzymes.
 Hydroxy fatty acid can be further oxidized to a dicarboxylic
acid via sequential reaction of alcohol dehydrogenase and
aldehyde dehydrogenase. The process occurs primarily
with medium chain fatty acids.

30.  The dicarboxylic acids so
formed can be activated at
either end of molecule to
form a CoA ester, which can
undergo β-oxidation to
produce shorter chain
dicarboxylic acids such as
adipic acids and succinic
acid.

31. Biological significance
 The microsomal (endoplasmic reticulum) pathway of
fatty acid ω-oxidation represents a minor pathway of
overall fatty acid oxidation.
 In certain pathophysiological states, such as diabetes,
chronic alcohol consumption and starvation, the ω-
oxidation pathway may provide an effective means for
the elimination of toxic levels of free fatty acids.

32. Reference
 David L.Nelson, Michael M.Cox. 2008. Lenninger
“Principle of Biochemistry”. W.H Freeman and
Company. Chapter-7 “Fatty Acid Catabolism”. Page
649-664.