|Figure 9 Oxidation of fatty acids. Fats
and oils are hydrolyzed to
form glycerol and
fatty acids. CoA derivatives of the fatty acids
oxidized in mitochondria by NAD+ and
FAD to β-oxo-derivatives.
CoA cleaves these
derivatives to yield acetyl CoA and a fatty acid
CoA molecule that is two carbons shorter.
The process continues
until the fatty acid
has been completely converted to acetyl
The acetyl moiety is oxidized in the
citric acid cycle to CO2 and
complete oxidation of a fatty acid of about
molecular weight of glucose yields
four times more ATP than that
Fats and oils are ubiquitous biological molecules that are
major energy reserves in animals and developing plants.
Fats and oils are esters of glycerol, a three-carbon compound
with hydroxyl groups on all three carbons, and carboxylic
acids with long hydrocarbon chains. The most
common fats and oils contain fatty acids with straight
chains with an even number of carbon atoms. Most often,
the total number of carbons in a fatty acid in a triglyceride
ranges from 14 to 18. The difference between a fat and an
oil is simply melting temperature. Oils are liquid at room
temperature, whereas fats are solid. Familiar examples are
olive oil and butter.
The most significant reason for this difference in melting
temperatures between fats and oils is the degree of
unsaturation (double bonds) of the fatty acids they contain.
The introduction of double bonds into a hydrocarbon
chain causes perturbations in the structure of the chain
that decrease its ability to pack the chains closely into
a solid structure. Olive oil contains far more unsaturated
fatty acids than butter does and is thus a liquid at room
temperature and even in the cold.
Regardless of the physical properties of triglycerides,
they are the long-term energy reserves of higher organisms.
Consider the fact that the complete oxidation of
triglycerides to CO2
and water yields 9 kcal/g, whereas
that of the carbohydrate storage polymers, starch and
glycogen, yields just 4 kcal/g. When it is also remembered
that fats and oils shunwater, but glycogen and starch
are more hydrophilic, triglycerides have an additional advantage
over the glucose polymers as deposits of potential
free energy. As hydrophobic moieties, fats and oils require
less intracellular space than that required by the glucose
Thefirst stepinthebreakdownof triglycerides (Fig. 9) is
their conversion by hydrolysis to their components, glycerol
and fatty acids. Glycerol is a close relative of the threecarbon
compounds involved in the catabolism of glucose
and may be completely oxidized to CO2
and water by
glycolysis and the tricarboxylic acid cycle.
The fatty acids are first converted to CoA derivatives at
the expense of the hydrolysis of ATP and then transported
into mitochondria where they are broken down sequentially,
two carbon units at a time, by a pathway known as
β-oxidation (see Fig. 9). The fatty acyl CoA derivatives
undergo oxidation at the carbon that is β to the carboxyl
carbon from that of a saturated carbon–carbon bond to that
of an oxo-saturated carbon bond. Enzymes that contain
FADor use NAD+
as the electron acceptors catalyze these
reactions. As is the case in the oxidation of carbohydrates,
the NADH and FADH2
generated by the β-oxidation of
fatty acids are converted to their oxidized forms by the
mitochondrial electron transport chain, which results in
the formation of ATP by oxidative phosphorylation.
Once β-oxidation is complete, the terminal two carbons
of the fatty acid chain are then released as acetyl CoA.
Oxidation and cleavage of the fatty acid continue until
it is entirely converted to acetyl CoA. The conversion of
a saturated fatty acid with 18 carbon atoms to 9 acetyl
CoA produces 8 NADH and 8 FADH2
. The acetyl CoA is
burned by the citric acid cycle to generate more ATP. The
high caloric content of fats pays off to cells in the yield of