82
Chapter 3
occurs during reaction 5 in which inorganic phosphate is trans-
ferred to guanosine diphosphate (GDP) to form guanosine tri-
phosphate (GTP). The hydrolysis of GTP, like that of ATP, can
provide energy for some energy-requiring reactions. In addition,
the energy in GTP can be transferred to ATP by the reaction
GTP + ADP
34
GDP + ATP
The formation of ATP from GTP is the only mecha-
nism by which ATP is formed within the Krebs cycle. Why,
then, is the Krebs cycle so important? Because the hydrogen
atoms transferred to coenzymes during the cycle (plus the free
hydrogen ions generated) are used in the next pathway, oxida-
tive phosphorylation, to form large amounts of ATP.
The net result of the catabolism of one acetyl group from
acetyl CoA by way of the Krebs cycle can be written:
Acetyl CoA + 3 NAD
+
+ FAD + GDP + P
i
+ 2 H
2
O
⎯→
2 CO
2
+ CoA + 3 NADH + 3 H
+
+ FADH
2
+ GTP
Table 3–9
summarizes the characteristics of the Krebs
cycle reactions.
Oxidative Phosphorylation
Oxidative phosphorylation
provides the third, and quanti-
tatively most important, mechanism by which energy derived
from fuel molecules can be transferred to ATP. The basic prin-
ciple behind this pathway is simple: The energy transferred to
H
CH
3
CoA
SH
S
O
C
CH
2
Oxidative
phosphorylation
Malate
C
H
CH
2
CoA
HO
COO
COO
COO
Acetyl coenzyme A
Oxaloacetate
α
-Ketoglutarate
Citrate
CO
2
O
CH
2
COO
C
COO
OH
CH
2
COO
H
COO
OH
C
COO
CH
2
COO
C
C
O
C
COO
COO
CH
2
CH
2
NADH + H
+
H
2
O
NADH + H
+
COO
NADH + H
+
CO
2
Isocitrate
O
C
ATP
GDP
Fumarate
CoA
FADH
2
COO
P
i
COO
COO
CH
2
CH
2
CH
Succinyl coenzyme A
CH
Succinate
ADP
GTP
H
2
O
CoA
CoA
COO
COO
CH
2
CH
2
S
3
1
2
4
5
6
7
8
Figure 3–44
The Krebs cycle pathway. Note that the carbon atoms in the two molecules of CO
2
produced by a turn of the cycle are not the same two
carbon atoms that entered the cycle as an acetyl group (identifi ed by the dashed boxes in this fi
gure).
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