Page 10 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

P1: FYD Revised Pages
Encyclopedia of Physical Science and Technology EN002H-54 May 17, 2001 20:22

106 Bioenergetics

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 melt-
ing temperatures between fats and oils is the degree of
unsaturation (double bonds) of the fatty acids they con-
tain. 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 organ-
isms. Consider the fact that the complete oxidation of
triglycerides to CO 2 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 remem-
bered that fats and oils shun water, but glycogen and starch
are more hydrophilic, triglycerides have an additional ad-
vantage 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
polymers.
Thefirststepinthebreakdownoftriglycerides(Fig.9)is
their conversion by hydrolysis to their components, glyc-
erol and fatty acids. Glycerol is a close relative of the three-
carbon compounds involved in the catabolism of glucose
and may be completely oxidized to CO 2 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 sequen-
tially, 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
FIGURE 9 Oxidation of fatty acids. Fats and oils are hydrolyzed to
+
FAD or use NAD as the electron acceptors catalyze these form glycerol and fatty acids. CoA derivatives of the fatty acids are
reactions. As is the case in the oxidation of carbohydrates, oxidized in mitochondria by NAD and FAD to β-oxo-derivatives.
+
the NADH and FADH 2 generated by the β-oxidation of CoA cleaves these derivatives to yield acetyl CoA and a fatty acid
fatty acids are converted to their oxidized forms by the CoA molecule that is two carbons shorter. The process continues
until the fatty acid has been completely converted to acetyl CoA.
mitochondrial electron transport chain, which results in
The acetyl moiety is oxidized in the citric acid cycle to CO 2 and
the formation of ATP by oxidative phosphorylation. water. The complete oxidation of a fatty acid of about the same
Once β-oxidation is complete, the terminal two carbons molecular weight of glucose yields four times more ATP than that
of the fatty acid chain are then released as acetyl CoA. of glucose.
Oxidation and cleavage of the fatty acid continue until
it is entirely converted to acetyl CoA. The conversion of
D. Catabolism of Proteins and Amino Acids
a saturated fatty acid with 18 carbon atoms to 9 acetyl
CoA produces 8 NADH and 8 FADH 2 . The acetyl CoA is In addition to containing carbohydrates and fats, diets may
burned by the citric acid cycle to generate more ATP. The be rich inproteins.The catabolism of proteins results in the
high caloric content of fats pays off to cells in the yield of generation of their component parts, amino acids. When
ATP. the dietary amino acid requirements of an individual are