The Metabolism of Fats

The Science Behind Health
With Doctor Bones (Don R. Mueller, Ph.D.)
The Funny Man of Health
Educator
Entertainer
J
U
G
G
L
E
R
Scientist

Exploring the Metabolism of Fats
Fats and oils are the body's most concentrated form of energy,
providing 9 kilocalories (kcal) of energy per gram of fat (oil). This is
more than twice the energy per gram provided by carbohydrates
(4 kcal/gram) and proteins (4 kcal/gram).
The molecular structure of fat is
relatively simple. Fat molecules are
constructed from one molecule of
glycerol and three fatty acid molecules.
This is why Fats are also called Triacylglycerols. Oils have the same
chemical structure as fats, but are liquids rather than solids at
room temperature. Quite useful for cooking purposes.

Fatty acids have a long hydrocarbon chain, spanning from 4 carbon
atoms to as many as 30 carbon atoms (although 12 to 24 carbons is
more common) with a carboxyl (-COOH) group at one end.
Fatty Acid Molecule

Fatty acids having one or more double bonds between carbon
atoms in the hydrocarbon chain are called unsaturated fatty acids.
Saturated fatty acids have no such double bonds between carbon
atoms.
Mono-unsaturated fats (oils) have only
one carbon-carbon double bond.
Those fats (oils) with two or more such
bonds are called polyunsaturated.
Fats with only carbon-carbon single
bonds are called saturated fats.
Butter and coconut oil are examples
of saturated fats (oils).

Fatty Acids + Glycerol = Fat Molecule
From the figure, we can envision glycerol as the "backbone" of a
fat molecule to which three fatty acid molecules are bonded.
Glycerol is an alcohol
with three hydroxyl (-OH)
groups. It is a thick,
sweet tasting liquid, that
dissolves easily in water,
because the -OH groups
can form hydrogen bonds
with water (H2O).

How Fats are metabolized
The metabolism of fats involves both the creation and the
breakdown of fat molecules. Metabolism is usually divided
into two basic types: Anabolism and Catabolism.
1) Anabolism is concerned with chemical reactions that
synthesize or construct new (generally more complex)
compounds from simpler compounds at the expense of an
energy input.
2) Catabolism, involves chemical reactions, which
breakdown more complex compounds (molecules) into
simpler compounds (structures) often with the release of
energy.

Before entering cells, fats (also called lipids) are broken down to
fatty acids and glycerol by fat-specific enzymes known as lipases
(triglyceridases). Lipases attack the bonds between glycerol and
the respective fatty acids in the fat molecule in a process called
hydrolysis, which adds H2O across these bonds.
Glycerol, released by this
reaction, can be converted to
glyceraldehyde 3-phosphate
for entry into glycolysis and
then on to the Kreb's Cycle
for further production of ATP.

1. Dehydrogenation (loss of hydrogen to form a C=C double bond)
2. Hydration (the addition of water across the C=C double bond)
3. Oxidation (loss of electrons and the formation of a ketone C=O)
4. Thiolysis (the splitting of coenzyme-A activated fatty acids)
Inside the highly-folded
structure of the inner
mitochondrial membrane,
chemical energy contained
in the free fatty acids is
released via a four-step
process known as the
Beta-oxidation cycle:

Fatty acids on route for beta-oxidation are “activated” for
degradation by coenzyme-A (HS-CoA), which forms a chemical
bond with the fatty acid (R-CH2CH2COOH).
Activating fatty acids, in this manner, requires energy in the form
of ATP, which is needed only once per fatty acid molecule
degraded. The result is referred to as a fatty acyl-CoA thioester or
fatty acyl-CoA for short. [Thio means, "containing sulfur"].
R-CH2CH2COOH + HS-CoA + ATP
R-CH2CH2CO-S-CoA + AMP + PPi
Fatty Acid Coenzyme-A
Fatty Acyl-CoA

Beta-oxidation is so named, because in this process, the beta-
carbon of the activated fatty acid is oxidized from CH2 to a ketone
C=O. The numbering system is explained as follows:
The first carbon in the figure is
called the carboxyl carbon (C=O).
The fatty acid numbering system
begins here. The carbon atom
adjacent to the carboxyl carbon is
called the alpha-carbon (α). The
beta-carbon (β) follows the alpha-
carbon and so it goes.
The last carbon atom in the chain is designated the omega-carbon
(ω), which is fitting, as ω is the last letter in the Greek alphabet.
α β ω
1 2 3

Each cycle through, releases a
single acetyl-CoA molecule,
reducing the initial fatty acid by
two-carbon units.
Beta-oxidation can convert
fatty acids having an even
number of carbon atoms in
their acyl chain, completely to
acetyl-CoA.
For example, a fatty acid with
16 carbons in its acyl chain is
converted to 8 acetyl-CoA
molecules via beta-oxidation.
The Four-Step Beta-Oxidation Cycle of Fatty Acids
Dehydrogenation
Hydration
Oxidation
Thiolysis
FAD+
H2O
NAD+
SCoA

The acetyl-CoA resulting from
beta-oxidation of fatty acids,
enters the Kreb's Cycle (Citric
Acid Cycle), where it is further
oxidized to CO2, along with
generating more of the
electron carriers, NADH and
FADH2. They in turn, transfer
their energy to the electron
transport chain, thus driving
the production of more ATP.
This can be represented as a
three-stage process as is
shown in the diagram:
Entry into the Kreb’s Cycle

Interestingly, fatty acids possessing an odd number of carbons in
their acyl chain, are also metabolized via beta-oxidation. However,
in this case, the cycle ends with the three-carbon propionyl-CoA,
which cannot enter a further round of beta-oxidation.
The propionyl-CoA from beta-oxidation is subsequently converted
to succinyl-CoA whereupon it may enter the Kreb's cycle.

The Metabolism of Fats

More Related Content

The Metabolism of Fats