88
Chapter 3
22 carbons, 16 and 18 being most common. The catabolism of
one 18-carbon saturated fatty acid yields 146 ATP molecules.
In contrast, as we have seen, the catabolism of one glucose mol-
ecule yields a maximum of 38 ATP molecules. Thus, taking
into account the difference in molecular weight of the fatty acid
and glucose, the amount of ATP formed from the catabolism of
a gram of fat is about 2
1
2
times greater than the amount of ATP
produced by catabolizing 1 gram of carbohydrate. If an average
person stored most of his or her energy as carbohydrate rather
than fat, body weight would have to be approximately 30 per-
cent greater in order to store the same amount of usable energy,
and the person would consume more energy moving this extra
weight around. Thus, a major step in fuel economy occurred
when animals evolved the ability to store fuel as fat.
Fat Synthesis
The synthesis of fatty acids occurs by reactions that are almost
the reverse of those that degrade them. However, the enzymes
in the synthetic pathway are in the cytosol, whereas (as we
have just seen) the enzymes catalyzing fatty acid breakdown
are in the mitochondria. Fatty acid synthesis begins with cyto-
plasmic acetyl coenzyme A, which transfers its acetyl group to
another molecule of acetyl coenzyme A to form a four-carbon
chain. By repetition of this process, long-chain fatty acids are
built up two carbons at a time. This accounts for the fact that
all the fatty acids synthesized in the body contain an even
number of carbon atoms.
Once the fatty acids are formed, triglycerides can be syn-
thesized by linking fatty acids to each of the three hydroxyl
groups in glycerol, more specifi cally, to a phosphorylated form
of glycerol called
α
-glycerol phosphate.
The synthesis of tri-
glyceride is carried out by enzymes associated with the mem-
branes of the smooth endoplasmic reticulum.
Compare the molecules produced by glucose catabo-
lism with those required for synthesis of both fatty acids and
α
-glycerol phosphate. First, acetyl coenzyme A, the starting
material for fatty acid synthesis, can be formed from pyruvate,
the end product of glycolysis. Second, the other ingredients
C
18
Fatty acid
O
AMP+2P
i
CH
2
CoA
SH
(CH
2
)
14
CoA
S
H
2
O
CH
3
CH
2
COOH
(CH
2
)
14
CH
3
C
+
H
2
O
O
CH
2
(CH
2
)
14
CoA
S
CH
3
C
CH
2
CoA
SH
CH
3
(CH
2
)
14
S
CoA
C
O
C
O
CoA
S
C
+
CH
3
Acetyl CoA
9 ATP
H
2
O
O
2
CO
2
CH
2
FADH
2
ATP
FAD
NAD
+
NADH
H
+
O
139 ATP
Coenzyme—2H
cycle
Krebs
Oxidative
phosphorylation
Figure 3–49
Pathway of fatty acid catabolism, which takes place in the mitochondria. The energy equivalent of two ATP is consumed at the start of the
pathway.
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