926
UNIT 4
Maintenance of the Body
24
Lipid Metabolism
Describe the process by which fatty acids are oxidized for
energy.
Define ketone bodies, and indicate the stimulus for their
formation.
Fats are the body’s most concentrated source of energy. Tey
contain very little water, and the energy yield from fat catabo-
lism is approximately twice that from either glucose or protein
catabolism—9 kcal per gram of fat versus 4 kcal per gram of
carbohydrate or protein. Most products of fat digestion are
transported in lymph in the form of fatty-protein droplets called
chylomicrons
(see Chapter 23). Eventually, enzymes on capillary
endothelium hydrolyze the lipids in the chylomicrons, and the
Krebs
cycle
H
2
O
Glycerol
Fatty acids
Coenzyme A
Lipase
β
Oxidation
in the mito-
chondria
Cleavage
enzyme
snips off
2C fragments
NAD
+
NADH + H
+
Glycolysis
Glyceraldehyde
3-phosphate
(a glycolysis intermediate)
Pyruvic acid
Lipids
Acetyl CoA
FAD
FADH
2
ATP
Figure 24.14
Initial phase of lipid oxidation.
Glycerol is
converted to a glycolysis intermediate, and completes the glycolytic
pathway through pyruvic acid to acetyl CoA. The fatty acids undergo
beta (
b
) oxidation in which they are activated by a coupled reaction
with ATP, combined with coenzyme A, and then oxidized twice
(reducing NAD
1
and FAD). The acetyl CoA created in
b
oxidation is
cleaved off and the process begins again.
resulting fatty acids and glycerol are taken up by body cells and
processed in various ways.
Oxidation of Glycerol and Fatty Acids
Of the various lipids, only triglycerides are routinely oxidized
for energy. Teir catabolism involves the separate oxidation of
their two different building blocks: glycerol and fatty acid chains
(Figure 24.14)
.
Most body cells easily convert glycerol to glyceraldehyde
3-phosphate, a glycolysis intermediate that eventually enters the
Krebs cycle. Glyceraldehyde is equal to half a glucose molecule,
and A±P energy harvest from its complete oxidation is approxi-
mately half that of glucose (15 A±P/glycerol).
Beta oxidation
, the initial phase of fatty acid oxidation, oc-
curs in the mitochondria. Te net result is that the fatty acid
chains are broken apart into two-carbon
acetic acid
fragments,
and coenzymes (FAD and NAD
1
) are reduced (Figure 24.14,
right side). Each acetic acid molecule is fused to coenzyme A,
forming acetyl CoA. Te term “beta oxidation” reflects the fact
that the carbon in the beta (third) position is oxidized during
the process and in each case the fatty acid is cleaved between the
alpha and beta carbons. Oxaloacetic acid then picks up acetyl
CoA, which enters the aerobic pathways to be oxidized to CO
2
and H
2
O.
Notice that unlike glycerol, which enters the glycolytic path-
way, acetyl CoA resulting from fatty acid breakdown
cannot
be
used for gluconeogenesis. Why? Tis is because the metabolic
pathway is irreversible past pyruvic acid.
Lipogenesis
Tere is a continuous turnover of triglycerides in adipose tissue.
New fats are “put in the larder” for later use, while stored fats
are broken down and released to the blood. Tat bulge of fatty
tissue you see today does
not
contain the same fat molecules it
did a month ago.
Glycerol and fatty acids from dietary fats not immediately
needed for energy are recombined into triglycerides and stored.
About 50% ends up in subcutaneous tissue, and the balance is
stockpiled in other fat depots of the body.
±riglyceride synthesis, or
lipogenesis
, occurs when cellular
A±P and glucose levels are high (
Figure 24.15
, magenta ar-
rows). Excess A±P also leads to an accumulation of acetyl CoA
and glyceraldehyde 3-PO
4
, two intermediates of glucose metab-
olism that would otherwise feed into the Krebs cycle. But when
these two metabolites are present in excess, they are channeled
into triglyceride synthesis pathways.
Acetyl CoA molecules are condensed together, forming
fatty acid chains that grow two carbons at a time. (Tis ac-
counts for the fact that almost all fatty acids in the body con-
tain an even number of carbon atoms.) Because acetyl CoA,
an intermediate in glucose catabolism, is also the
starting
point
for fatty acid synthesis, glucose is easily converted to fat.
Glyceraldehyde 3-PO
4
is converted to glycerol, which is con-
densed with fatty acids to form triglycerides. Consequently,
even with a low-fat diet, carbohydrate intake can provide
all
the raw materials
needed to make triglycerides. When blood
previous page 960 Human Anatomy and Physiology (9th ed ) 2012 read online next page 962 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off