A-6
Appendix D
Two Important Metabolic Pathways
Finally, glycolysis produces ATP. The phosphate group, with
its high-energy bond, is transferred from 1,3-bisphosphoglyceric
acid to ADP. For each glucose molecule that began glycolysis,
step 7 produces two molecules of ATP, because every product
after the sugar-splitting step (step 4) is doubled. Of course, two
ATPs were invested to get sugar ready for splitting. The ATP
ledger now stands at zero. By the end of step 7, glucose has
been converted to two molecules of 3-phosphoglyceric acid. This
compound is not a sugar. The sugar was oxidized to an organic
acid back in step 6, and now the energy made available by that
oxidation has been used to make ATP.
Next, an enzyme relocates the remaining phosphate group
of 3-phosphoglyceric acid to form 2-phosphoglyceric acid. This
prepares the substrate for the next reaction.
An enzyme forms a double bond in the substrate by
removing a water molecule from 2-phosphoglyceric acid to form
phosphoenolpyruvic acid, or PEP. This results in the electrons of
the substrate being rearranged in such a way that the remaining
phosphate bond becomes very unstable; it has been upgraded
to high-energy status.
The last reaction of glycolysis produces another molecule of
ATP by transferring the phosphate group from PEP to ADP.
Because this step occurs twice for each glucose molecule, the
ATP ledger now shows a net gain of two ATPs. Steps 7 and 10
each produce two ATPs for a total credit of four, but a debt of
two ATPs was incurred from steps 1 and 3. Glycolysis has repaid
the ATP investment with 100% interest. In the meantime,
glucose has been broken down and oxidized to two molecules
of pyruvic acid, the compound produced from PEP in step 10.
(1,3-Bisphosphoglyceric acid)
ADP
Phosphoglycerokinase
3-Phosphoglyceric
acid (2 molecules)
Phosphoglyceromutase
2-Phosphoglyceric
acid (2 molecules)
Enolase
OH
C O
CHOH
CH
2
O
OH
C
O
C
H
CH
2
OH
H
2
O
O
OH
C O
C
CH
2
ADP
O
~
2-Phosphoenolpyruvic
acid (PEP)
(2 molecules)
Pyruvate kinase
OH
C O
Pyruvic acid
(2 molecules)
Lactic acid
(2 molecules)
CH
3
OH
Oxygen
present
Oxygen
deficit
Phase 1 Sugar activates by phosphorylation
Phase 2 Sugar cleavage
Glucose (6C)
Fructose-1,6-bisphosphate (6C)
2 ADP
C
O
C
H
OH
CH
3
Glyceraldehyde
3-phosphate (3C)
Phase 3
Sugar oxidation
and formation
of ATP
2
P
i
4 ADP
Oxygen
deficit
Dihydroxyacetone
phosphate (3C)
2 Pyruvic acid (3C)
Oxygen
present
Aerobic
pathway
2 Lactic acid
C O
Lactate
dehydro-
genase
P
P
P
ATP
ATP
To Krebs
cycle
(aerobic
pathway)
2
2 NAD
+
2 NAD
+
NADH+H
+
2 NADH+H
+
NAD
+
NADH+H
+
7
8
9
10
4 ATP
2 ATP
7
8
9
10
Summary
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