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Summary of events that must occur before
triacyglycerols (TAGs) can reach the bloodstream
through the digestive process.
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 triacylglycerol's
rather than glycogen were selected
in evolution as the major energy
The glycogen and glucose stores
provide enough energy to
sustain biological function for
about 24 hours, whereas the
Triacylglycerol stores allow
survival for several weeks.
TRIGLYCERIDES V/S GLYCOGEN
Lipolysis - hydrolysis of
triacylglycerol's by lipases.
A hormone-sensitive lipase converts
TGs to free fatty acids and
Monoacylglycerol is hydrolyzed to fatty
acid and glycerol or by a hormone-
sensitive lipase or by more specific and
more active monoacylglycerol lipase
Hydrolysis of stored triacylglycerols in adipose tissue is
triggered by hormones that stimulate cAMP production
within adipose cells.
phosphate is an
• Glycolytic pathway
Oxidation of Glycerol
Glycerol – glycerol 3-phosphate - 1 ATP
Glycerol 3-phosphate - dihydroxyaceton phosphate
2.5ATP (1 NADH)
Glyceraldehyde 3-phosphate – pyruvate
4,5 ATP (1NADH + 2 ATP)
Pyruvate – acetyl CoA 2.5 ATP (1 NADH)
Acetyl CoA in Krebs cycle 10 ATP (3NADH + 1 FADH2 +
Total 19,5-1 = 18,5 ATP
ATP Generation from
TYPES OF FATTY ACID OXIDATION
Fatty acids can be oxidized by:
Overview of beta oxidation:
A saturated acyl Co A is degraded by a
recurring sequence of four reactions:
1) Oxidation 2) Hydration
3) Oxidation 4) Thiolysis
• Acyl CoA undergoes dehydrogenation
by an FAD-dependent flavoenzyme,
acyl CoA Dehydrogenase.
• A double bond is formed between α
and β carbons (i.e., 2 and 3 carbons)
Enoyl CoA hydrates brings.
About the hydration of the
double bond to form β -
β-Hydroxyacyl CoA dehydrogenase
catalyzes the second oxidation and
The product formed is β-ketoacyl CoA.
The final reaction in β -oxidation is the liberation
of a 2 carbon fragment, acetyl CoA from acyl CoA.
This occurs by a thiolytic cleavage catalysed by β-
ketoacyl CoA thiolase (or thiolase).
The new acyl CoA, containing two carbons less
than the original, reenters the β-oxidation cycle.
The process continues till the fatty acid is
completely oxidized. 12
1. Activation of fatty
acids in the cytosol
2. Transport of
3. Beta oxidation
proper in the
Fatty acids to be oxidized must be entered
the following steps:
1) Activation of FA:
This proceeds by FA thiokinase (acyl COA
synthetize) present in endoplasmic
reticulum and in the outer mitochondrial
membrane. Thiokinase requires ATP,
COA SH, Mg++. The product of this
reaction is acyl COA and water.
Fatty acids must first be converted to an
active intermediate before they can be
catabolized. This is the only step in the
complete degradation of a fatty acid that
requires energy from ATP. The activation
of a fatty acid is accomplished in
The beta oxidation of fatty
acids involve three stages:
ions and most
The mitochondrion contained the enzymes responsible for
electron transport and oxidative phosphorylation.
• ATP is converted to
AMP + P~P, the energy
released is utilized for
formation of high
energy bond (thioester
bond) in acyl COA (RCO
~ S COA).
• The high energy of P~P
is lost by
two high energy
phosphates are lost
2-Transport of fatty acyl CoA
from cytosol into mitochondria:
Long chain acyl CoA cannot readily
traverse the inner mitochondria
membrane and so a special
transport mechanism called
carnitine shuttle is needed.
It is synthesized in liver and kidney
It is essential for oxidation of long
chain fatty acids.
Carnitine is not required for the
permeation of medium chain acyl
CoA into the mitochondrial matrix.
ammonium butyrate) is a carrier.
Acyl groups from acyl COA is
transferred to hydroxyl group of
carnitine to form acyl carnitine,
catalyzed by carnitine
acyltransferase I, located in the
outer mitochondrial membrane.
Acylcarnitine is then shuttled across the inner mitochondrial membrane by
a translocase enzyme.
The acyl group is transferred back to CoA on the inner border of the matrix
side of the inner mitochondrial membrane by carnitine acyl transferase II.
Finally, carnitine is returned to the cytosolic side by translocase, in
exchange for an incoming acyl carnitine.
СН3 С О
STEPS OF BETA OXIDATION
Electrons from the FADH2 prosthetic group of the reduced acyl CoA
dehydrogenase are transferred to electron-
transferring flavoprotein (ETF).
ETF donates electrons to ETF: ubiquinone reductase, an iron-sulfur
Ubiquinone is thereby reduced to ubiquinol, which delivers its high-
potential electrons to the second proton-pumping site of the respiratory
The first step is the
removal of two
hydrogen atoms from
the 2(α)- and 3(β)-
catalyzed by acyl-CoA
requiring FAD. This
results in the
formation of Δ2-trans-
enoyl-CoA and FADH2.
Water is added
to saturate the
and form 3-
by Δ 2-enoyl-
dehydrogenation on the
3-carbon catalyzed by
CoA dehydrogenase to
form the corresponding
compound. In this case,
NAD+ is the coenzyme
split at the 2,3-
by thiolase (3-
acetyl-CoA and a
new acyl-CoA two
than the original
STEPS OF BETA OXIDATION
formed in the
at reaction 2.
can be oxidized to
CO2 and water via
the citric acid cycle
oxidation of fatty
acids is achieved. 21
BETA OXIDATION- ENERGY YIELD
Total ATP per turn of
the fatty acid spiral is:
Step 1 - FAD into e.t.c.
= 2 ATP
Step 3 - NAD+ into
e.t.c. = 3 ATP
Total ATP per turn of
spiral = 5 ATP
NET ATP from Fatty Acid
Spiral = 35 - 1 = 34 ATP
One turn of the fatty acid spiral
produces ATP from the interaction
of the coenzymes FAD (step 1) and
NAD+ (step 3) with the electron
Example with Palmitic Acid = 16
carbons = 8 acetyl groups.
Number of turns of fatty acid spiral
= 8-1 = 7 turns
ATP from fatty acid spiral = 7 turns
and 5 per turn = 35 ATP.
Cycles of Beta Oxidation
Beta-oxidation is the
process by which fatty
acids, in the form of
Acyl-CoA molecules, are
broken down in
mitochondria and/or in
the entry molecule for
the Citric Acid cycle
β- OXIDATION OF ODD CHAIN FATTY ACIDS
The propionyl residue
from an odd-chain fatty
acid is the only part of a
fatty acid that is
CoA cannot be converted
into pyruvate or
Oxaloacetate in animals.
Fatty acids with an odd number
of carbon atoms are oxidized by
the pathway of β-oxidation,
producing acetyl-CoA, until a
residue remains. This
compound is converted to
Succinyl-CoA, a constituent of
the citric acid cycle
BETA OXIDATION OF UNSATURATED FATTY
In the oxidation of unsaturated fatty acids, most of
the reactions are the same as those for saturated
fatty acids, only two additional enzymes an
isomerase and a reductase are needed to degrade
a wide range of unsaturated fatty acids.
Energy yield is less by the oxidation of unsaturated
fatty acids since they are less reduced.
Per double bonds 2 ATP are less formed, since the
first step of dehydrogenation to introduce double
bond is not required, as the double already exists.
BETA OXIDATION OF UNSATURATED FATTY
Palmitoleoyl Co A
undergoes three cycles of
degradation, which are carried
out by the same enzymes as in
the oxidation of saturated fatty
The cis- Δ 3-enoyl CoA formed
in the third round is not a
substrate for acyl CoA
An isomerase converts this
double bond into a trans- Δ 2
The subsequent reactions are
those of the saturated fatty
acid oxidation pathway, in
which the trans- Δ 2-enoyl CoA
is a regular substrate .
BETA OXIDATION OF POLY UNSATURATED FATTY ACIDS
A different set of
enzymes is required
for the oxidation of
Linoleic acid, a C18
acid with cis-Δ 9 and
cis-Δ12 double bonds.
The cis- Δ 3 double bond
formed after three rounds
of β oxidation is converted
into a trans- Δ 2 double
bond by isomerase.
The acyl CoA produced by another
round of β oxidation contains a cis- Δ 4
double bond. Dehydrogenation of this
species by acyl CoA dehydrogenase
yields a 2,4-dienoyl intermediate,
which is not a substrate for the next
enzyme in the β -oxidation pathway.
This impasse is circumvented
by 2,4-dienoyl CoA
reductase, an enzyme that
uses NADPH to reduce the
to trans-D 3-enoyl CoA. 29
MINOR PATHWAYS OF FATTY ACID OXIDATION
3) Peroxisomal fatty
Occurs for the chain
shortening of very
long chain fatty acids.
1) α- Oxidation- Oxidation
occurs at C-2 instead of C-3
, as in β oxidation
2) ω- Oxidation –
Oxidation occurs at
the methyl end of
the fatty acid
Unsaturated fatty acids:
- mono, ∆9 (odd) - poly, ∆9, ∆12 (odd, even)
-> isomerization, reduction
Odd chain length fatty acids:
-> propionyl-CoA in the last cycle
Very long-chain fatty acids (> C22 atoms):
-> first β-oxidation in peroxisomes
Branched chain fatty acids:
- chlorophyll’s phytanic acid
-> α-oxidation, formyl-CoA + propionyl-CoA
Special cases of β-oxidation
BIOLOGICAL SIGNIFICANCE OF ALPHA OXIDATION
1) α- Oxidation is most
suited for the
oxidation of phytanic
acid, produced from
dietary phytol, a
chlorophyll of plants.
2) The hydroxy fatty acids
produced as intermediates
of this pathway like
Cerebronic acid can be used
for the synthesis of
3) Odd chain fatty acids
decarboxylation in this
pathway, can be used for the
synthesis of sphingolipids
and can also undergo beta
oxidation to form propionyl
co A and Acetyl co A. The
number of acetyl co A depend
upon the chain length.
Propionyl co A is converted to
Succinyl co A to gain entry in
to TCA cycle for further
Phytanic acid is a significant constituent of
milk lipids and animal fats.
Normally it is metabolized by an initial α-
hydroxylation followed by
dehydrogenation and decarboxylation.
Beta oxidation can not occur initially because
of the presence of 3- methyl groups, but it can
proceed after decarboxylation.
The whole reaction produces three
molecules of propionyl co A, three
molecules of Acetyl co A, and one molecule
of iso butyryl co A . 33
Phytanic acid is
Phytanic acid α
yield CO2 and
odd chain fatty
acid that can be
OMEGA OXIDATION OF FATTY ACIDS
Involves hydroxylation and
occurs in the endoplasmic
reticulum of many tissues.
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 P450,
O2 and NADPH, as well as the
Hydroxy fatty acids can be
further oxidized to a
dicarboxylic acid via
sequential reactions of
Alcohol dehydrogenase and
The process occurs primarily
with medium chain fatty
Dicarboxylic acids so formed can
undergo beta oxidation to produce
shorter chain dicarboxylic acids such as
Adipic acids(C6) and succinic acid (C4).
PEROXISOMAL OXIDATION OF VERY LONG CHAIN FATTY ACIDS
In peroxisomes, a flavoprotein dehydrogenase transfers electrons to O2
to yield H2O2 instead of capturing the high-energy electrons as FADH2, as
occurs in mitochondrial beta oxidation.
Catalase is needed to convert the hydrogen peroxide produced in the
initial reaction into water and oxygen.
Subsequent steps are identical with their mitochondrial counterparts,
They are carried out by different isoform of the enzymes.
The specificity of the peroxisomal enzymes is for longer chain
fatty acids. Thus peroxisomal enzymes function to shorten the
chain length of relatively long chain fatty acids to a point at which
beta oxidation can be completed in mitochondria.
Fatty Acid Synthesis
Occurs mainly in liver and
adipocytes, in mammary
glands during lactation
Occurs in cytoplasm
FA synthesis and
degradation occur by two
Three stages of fatty acid synthesis:
A. Transport of acetyl CoA into cytosol
Acetyl CoA from catabolism of carbohydrates
and amino acids is exported from
mitochondria via the citrate transport
Cytosolic NADH also converted to NADPH
Two molecules of ATP are expended for each
round of this cyclic pathway
B. Carboxylation of acetyl CoA.
C. Assembly of fatty acid chain
NADH + H+
NADPH + H+
Transfer of acetyl CoA from mitochondria to cytosol
of Acetyl CoA
Enzyme: acetyl CoA carboxylase
Prosthetic group - biotin
A carboxybiotin intermediate is
ATP is hydrolyzed.
The CO2 group in carboxybiotin
is transferred to acetyl CoA to
form malonyl CoA.
Acetyl CoA carboxylase is the
Five separate stages:
1. Loading of precursors via thioester derivatives.
2. Condensation of the precursors.
C. The Reactions
of Fatty Acid
The elongation phase of fatty acid synthesis starts with
the formation of acetyl ACP and malonyl ACP.
Acetyl transacylase and malonyl transacylase catalyze
Acetyl CoA + ACP acetyl ACP + CoA
Malonyl CoA + ACP malonyl ACP + CoA
Acetyl ACP and
malonyl ACP react
to form acetoacetyl
is reduced to D-3-
NADPH is the
ACP is dehydrated
to form crotonyl
The final step in the cycle
reduces crotonyl ACP to
NADPH is reductant.
Enzyme - enoyl ACP
This is the end of first
elongation cycle (first round).
In the second round butyryl
ACP condenses with malonyl
ACP to form a C6--ketoacyl
Reduction, dehydration, and
a second reduction convert
the C6--ketoacyl ACP into a
C6-acyl ACP, which is ready
for a third round of
Rounds of synthesis
continue until a
C16 palmitoyl group is
hydrolyzed by a
Final reaction of FA
1. Phosphorilation of
glycerol through the action
of glycerol kinase:
2. Reduction of
which is the product of the
aldolase reaction of
phosphate is reduced to
glycerol 3-phosphate by the
phosphate dehydrogenase of
ATP + glycerol glycerol
3-phosphate + ADP
phosphate + NADH + H+
glycerol 3-phosphate +
The ways of formation of active form of glycerol.
There are two ways of formation of active form of glycerol.
Biosynthesis of Triacylglycerol
The first stage in triacylglycerol formation is the acylation
of the free hydroxyl groups of glycerol phosphate by two
molecules of fatty acyl-CoA to yield first a
lysophosphotidic acid and then a phosphatidic acid:
H2C O P
R1 - COSKoA KoA - SH H2C
H2C O P
H2C O P
R2 - COSKoA KoA - SH
H2C O P
KOLEVA, L. (2017) – The Notes or Lectures of Subject of Plant Chemistry in
Biochemistry: Biochemistry. Faculty of Agronomy. Plovdiv: Agricultural
University of Plovdiv; Bulgaria.
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