Cellular Structure, Proteins, and Metabolism
85
Carbohydrate Metabolism
Carbohydrate Catabolism
In the previous sections, we described the major pathways of
carbohydrate catabolism: the breakdown of glucose to pyruvate
or lactate by way of the glycolytic pathway, and the metabolism
of pyruvate to carbon dioxide and water by way of the Krebs
cycle and oxidative phosphorylation.
The amount of energy released during the catabolism of
glucose to carbon dioxide and water is 686 kcal/mol of glucose:
C
6
H
12
O
6
+ 6 O
2
⎯→
6 H
2
O + 6 CO
2
+ 686 kcal/mol
About 40 percent of this energy is transferred to ATP.
Figure
3–46
summarizes the points at which ATP forms during glu-
cose catabolism. A net gain of two ATP molecules occurs by
substrate-level phosphorylation during glycolysis, and two
more are formed during the Krebs cycle from GTP, one from
each of the two molecules of pyruvate entering the cycle. The
majority of ATP molecules glucose catabolism produces—34
ATP per molecule—form during oxidative phosphorylation
from the hydrogens generated at various steps during glucose
breakdown.
Because, in the absence of oxygen, only two molecules
of ATP can form from the breakdown of glucose to lactate,
Table 3–10
Characteristics of Oxidative Phosphorylation
Entering substrates
Hydrogen atoms obtained from NADH + H
+
and FADH
2
formed (1) during glycolysis, (2) by the Krebs cycle
during the breakdown of pyruvate and amino acids, and (3) during the breakdown of fatty acids
Molecular oxygen
Enzyme location
Inner mitochondrial membrane
ATP production
3 ATP formed from each NADH + H
+
2 ATP formed from each FADH
2
Final products
H
2
O—one molecule for each pair of hydrogens entering pathway
Net reaction
1
2
O
2
+ NADH + H
+
+ 3 ADP + 3 P
i
⎯→
H
2
O + NAD
+
+ 3 ATP
Glycolysis
Glucose
2 Pyruvate
2 ATP
Krebs cycle
(mitochondria)
2 Acetyl coenzyme A
4 H
2
O
2 H
2
O
4 CO
2
2 ATP
Oxidative phosphorylation
(mitochondria)
2 FADH
2
2 (NADH + H
+
)
ATP
ATP
ATP
6 O
2
Cytochromes
34 ATP
6 (NADH + H
+
)
2 CO
2
12 H
2
O
2 (NADH + H
+
)
C
6
H
12
O
6
+
6 O
2
+
38 ADP
+
38 P
i
6 CO
2
+
6 H
2
O
+
38 ATP
(cytosol)
Figure 3–46
Pathways of glycolysis and aerobic glucose catabolism and their linkage to ATP formation.
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