Essential Cell Biology 5th edition

440 CHAPTER 13 How Cells Obtain Energy from Food
Figure 13–12 The citric acid cycle
catalyzes the complete oxidation of
acetyl groups supplied by acetyl CoA.
The cycle begins with the reaction of
acetyl CoA (derived from pyruvate as
shown in Figure 13–10) with oxaloacetate
to produce citric acid (citrate). The number
of carbon atoms in each intermediate is
shaded in yellow. (See also Panel 13–2,
pp. 442–443.) The steps of the citric acid
cycle are reviewed in Movie 13.2.
NADH
O
H 3 C C S–CoA
acetyl CoA
2C
HS–CoA
oxaloacetate
6C
citrate
4C
STEP 1
STEP 2
6C
STEP 8
NADH
4C
STEP 7
4C
STEP 6
CITRIC ACID
CYCLE
4C
STEP 5
STEP 3
C O 2
5C
STEP 4
4C
NADH
C O 2
FADH 2
GTP
NET RESULT: ONE TURN OF THE CYCLE PRODUCES THREE NADH, ONE GTP, AND
ONE FADH 2 , AND RELEASES TWO MOLECULES OF CO 2
acid cycle are supplied by atmospheric O 2 . In fact, these oxygen atoms
come from water (H 2 O). As illustrated at the top of Panel 13–2, three
molecules of H 2 O are split as they enter each turn of the cycle, and their
oxygen atoms are ultimately used to make CO 2. As we see shortly, the
O 2 that we breathe is actually reduced to H 2 O by the electron-transport
chain; it does not form the CO 2 that we exhale.
Thus far, we have discussed only one of the three types of activated carriers
that are produced by the citric acid cycle—NADH. In addition to three
molecules of NADH, each turn of the cycle also produces one molecule of
FADH 2 (reduced flavin adenine ECB5 e13.12-13.12
dinucleotide) from FAD and one molecule
of the ribonucleoside triphosphate GTP (guanosine triphosphate)
from GDP (see Figure 13–12). The structures of these two activated carriers
are illustrated in Figure 13–13. GTP is a close relative of ATP, and
(A)
– O
O
P
O –
O
O
P
O –
O
O
P
O –
O
O
guanine
N
C
C NH
HC
C C
N N NH
CH 2
2 O
ribose
OH OH
GDP
(B)
H 3 C
H 3 C
C
C
H
C
C
H
2 H + 2 e –
FAD FADH 2
N
O
C
H
N
C C NH
C
C C C
C
N N O
CH 2
N
H
GTP
Figure 13–13 Each turn of the citric acid cycle produces one molecule of GTP
and one molecule of FADH 2 . (A) GTP and GDP are close relatives of ATP and ADP,
respectively, the only difference being the substitution of the base guanine for adenine.
(B) Despite its very different structure, FADH 2 , like NADH and NADPH (see Figure 3–34),
is a carrier of hydrogen atoms and high-energy electrons. It is shown here in its oxidized
form (FAD), with the part of the structure involved in accepting and donating hydrogens
highlighted in yellow. FAD can accept two hydrogen atoms, along with their electrons,
to form the reduced FADH 2 . The atoms involved are shown in their reduced form in the
excerpt to the right.
H
H
H
C
C
C
H 2 C
OH
OH
OH
O P
FAD
P
O
CH 2
RIBOSE
ADENINE