80
Chapter 3
The primary molecule entering at the beginning of the
Krebs cycle is
acetyl coenzyme A (acetyl CoA):
O
B
CH
3
—C—S—CoA
Coenzyme A (CoA) is derived from the B vitamin pantothenic
acid and functions primarily to transfer acetyl groups, which
contain two carbons, from one molecule to another. These
acetyl groups come either from pyruvate—the end product of
aerobic glycolysis—or from the breakdown of fatty acids and
some amino acids.
CH
2
OH
O
HO
OH
H
H
2
C
P
O
O
O
O
CH
2
ATP
ADP
O
H
OH
H
HO
OH
H
H
H
P
i
OH
H
P
O
O
O
O
CH
2
OH
O
OH
H
OH
H
HO
OH
H
H
H
Glucose
Glucose 6-phosphate
Dihydroxyacetone
phosphate
HO
H
H
H
2
C
P
O
O
O
O
O
OH
H
H
Fructose 6-phosphate
CH
2
P
O
O
O
O
OH
CH
2
P
O
C
O
OH
CH
2
O
P
O
O
O
CH
O
CO
CH
2
OH
H
H
P
O
O
O
O
CH
2
O
C
CH
P
O
O
O
O
ATP
ADP
O
H
2
O
ATP
ADP
P
O
COO
CH
3
OO
O
Fructose 1,6-bisphosphate
2-Phosphoglycerate
Phosphoenolpyruvate
3-Phosphoglyceraldehyde
COOH
P
O
OH
CH
2
O
CH
O
O
O
ATP
ADP
P
O
COO
OH
CH
2
O
CH
O
O
C
O
C
CH
2
COO
NADH + H
+
NAD
+
OH
CH
COO
CH
3
Pyruvate
To Krebs cycle
(anaerobic)
(aerobic)
3-Phosphoglycerate
1,3-Bisphosphoglycerate
Lactate
NAD
+
OH
NADH + H
+
3
2
1
4
5
6
7
8
9
10
Figure 3–41
Glycolytic pathway. Under anaerobic conditions, every molecule of glucose that enters the pathway produces a net synthesis of two molecules
of ATP. Note that at the pH existing in the body, the products produced by the various glycolytic steps exist in the ionized, anionic form
(pyruvate, for example). They are actually produced as acids (pyruvic acid, for example) that then ionize.
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