Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

HOW CELLS OBTAIN ENERGY FROM FOOD
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tricarboxylic acid cycle or the Krebs cycle. The citric acid cycle accounts for about
two-thirds of the total oxidation of carbon compounds in most cells, and its major
end products are CO 2 and high-energy electrons in the form of NADH. The CO 2
is released as a waste product, while the high-energy electrons from NADH are
passed to a membrane-bound electron-transport chain (discussed in Chapter
14), eventually combining with O 2 to produce H 2 O. The citric acid cycle itself does
not use gaseous O 2 (it uses oxygen atoms from H 2 O). But the cycle does require O 2
in subsequent reactions to keep it going. This is because there is no other efficient
way for the NADH to get rid of its electrons and thus regenerate the NAD + that is
needed.
The citric acid cycle takes place inside mitochondria in eukaryotic cells. It
results in the complete oxidation of the carbon atoms of the acetyl groups in acetyl
CoA, converting them into CO 2 . But the acetyl group is not oxidized directly.
Instead, this group is transferred from acetyl CoA to a larger, four-carbon molecule,
oxaloacetate, to form the six-carbon tricarboxylic acid, citric acid, for which
the subsequent cycle of reactions is named. The citric acid molecule is then gradually
oxidized, allowing the energy of this oxidation to be harnessed to produce
energy-rich activated carrier molecules. The chain of eight reactions forms a cycle
because at the end the oxaloacetate is regenerated and enters a new turn of the
cycle, as shown in outline in Figure 2–57.
We have thus far discussed only one of the three types of activated carrier
molecules that are produced by the citric acid cycle; NADH, the reduced form of
the NAD + /NADH electron carrier system (see Figure 2–36). In addition to three
molecules of NADH, each turn of the cycle also produces one molecule of FADH 2
(reduced flavin adenine dinucleotide) from FAD (see Figure 2–39), and one molecule
of the ribonucleoside triphosphate GTP from GDP. The structure of GTP is
illustrated in Figure 2–58. GTP is a close relative of ATP, and the transfer of its
terminal phosphate group to ADP produces one ATP molecule in each cycle. As
we discuss shortly, the energy that is stored in the readily transferred electrons of
NADH and FADH 2 will be utilized subsequently for ATP production through the
Figure 2–56 The oxidation of fatty acids
to acetyl CoA. (A) Electron micrograph
of a lipid droplet in the cytoplasm. (B) The
structure of fats. Fats are triacylglycerols.
The glycerol portion, to which three fatty
acids are linked through ester bonds,
is shown in blue. Fats are insoluble in
water and form large lipid droplets in the
specialized fat cells (adipocytes) in which
they are stored. (C) The fatty acid oxidation
cycle. The cycle is catalyzed by a series of
four enzymes in mitochondria. Each turn of
the cycle shortens the fatty acid chain by
two carbons (shown in red) and generates
one molecule of acetyl CoA and one
molecule each of NADH and FADH 2 .
(A, courtesy of Daniel S. Friend.)
(A)
(C)
R
CH 2
fatty acyl CoA
CH 2
CH 2
C
O
activated fatty acid
enters cycle
rest of
hydrocarbon tail
S–CoA
fat droplet
fatty acyl CoA
shortened by
two carbons
R
CH 2
C
O
S–CoA
cycle repeats
until fatty acid
is completely
degraded
CH 2
CH
O
O
O
C
O
C
O
1 µm
hydrocarbon tail
hydrocarbon tail
HS–CoA
R
CH 2
O
C
O
CH 3 C
S–CoA
acetyl CoA
O
CH 2 C
S–CoA
R
CH 2 CH CH
OH H
R CH 2 C C
H H
FAD
FADH 2
O
C
S–CoA
H 2 O
O
C
S–CoA
(B)
CH 2 O C hydrocarbon tail
ester bond
triacylglycerol
NADH
+ H +
NAD +