Glycolysis
Chapter 16 – Voet and Voet 2nd Edition
Wed. September 25, 2002
1. The Glycolytic Pathway
2. The Reactions of Glycolysis
3. Fermentation: The Anaerobic Fate of
Pyruvate
4. The Glycolytic Flux
5. Metabolism of Hexoses Other Than
Glucose
Glycolysis
Glucose is converted
to pyruvate while
generating two ATPs.
2 molecules of NAD+
are converted to
2 molecules of NADH.
The oxidizing power of
NAD+ must be recycled.
B. Pathway overview
Pathway Overview
• There are 10 enzyme-catalyzed reactions
considered to occur in two stages
– Stage I (reactions 1-5): Preparatory stage where
glucose is phosphorylated and cleaved to yield 2
molecules of glyceraldehyde-3-phosphate (GAP).
Stage I uses 2 ATPs.
– Stage II (reactions 6-10) Payoff stage where 2
GAPs converted to pyruvate and generation of 4
ATPs.
Glycolytic Pathway
Stage 1
Stage 2
2. The Reactions of Glycolysis
Stage I (Preparatory Stage)
1. Hexokinase (first ATP utilization)
2. Phosphoglucose Isomerase (PGI)
3. Phosphofructokinase -1 (PFK-1)
(second ATP utilization)
4. Aldolase
5. Triose Phosphate Isomerase (TIM)
THE PREPARATORY PHASE
Step 1 –
Hexokinase
Step 2 –
Phosphoglucose Isomerase
(PGI)
[Phosphohexose isomerase]
Phosphoglucose Isomerase
catalyzes the conversion of
G6P to F6P, the isomerization
of an aldose to a ketose.
The isomerization of an aldose to a ketose
(C) Step 3 - Phosphofructokinase 1:
Second ATP utilization
D. Step 4 - Aldolase
• Aldolase catalyzes cleavage of fructose1,6-bisphosphate (FBP) in reaction 4 of
glycolysis.
• This forms two trioses
– Glyceraldehyde-3-phosphate (GAP)
– Dihydroxyacetone phosphate (DHAP).
Step 4 - Aldolase. Aldol cleavage of FBP to form two
Trioses (GAP and DHAP)
Note that the atom
numbering system
changes. Atoms 1, 2,
and 3 of glucose become
atoms 3,2, and 1 of
DHAP. Atoms 4, 5, and
6 become atoms
1, 2, and 3 of GAP.
(E) Step 5 - Triose Phosphate Isomerase
(TIM)
Only GAP continues
along the glycolytic
pathway.
Fate of the carbon atoms of glucose in the
formation of glyceraldehyde-3-phosphate.
Stage II - payoff phase
• 6. Glyceraldehyde-3-Phosphate Dehydrogenase
(GAPDH) first "High-energy" intermediate
formation.
• 7. Phosphoglycerate Kinase (PGK): First ATP
Generation.
• 8. Phosphoglycerate Mutase (PGM).
• 9. Enolase: second "High-energy" intermediate
formation.
• 10. Pyruvate Kinase (PK): Second ATP generation.
(F) Step 6 - Glyceraldehyde-3-Phosphate Dehydrogenase
(GAPDH): First “High-Energy” Intermediate Formation.
Glyceraldehyde-3-phosphate dehydrogenase
reaction
(G). Step 7 - Phosphoglycerate Kinase (PGK):
First ATP Generation.
Mechanism of the PGK reaction.
The energetics of the overall GAPDH-PGK reaction pair.
(H). Step 8 - Phosphoglycerate Mutase (PGM).
Reaction Mechanism of PGM
(1) Catalytic amounts of 2,3-Bisphosphoglycerate
Are required for enzymatic activity.
(2) Incubation of the enzyme with catalytic amounts of
32P-labeled 2,3-BPG yields a 32P-labeled enzyme.
Glycolysis influences oxygen transport
2,3-BPG binds to deoxyhemoglobin and alters the oxygen
affinity of hemoglobin. Erythrocytes synthesize and degrade
2,3-BPG by a detour from the glycolytic pathway.
Lower [BPG]
in erythrocytes
resulting from
hexokinasedeficiency
results in
increased
hemoglobin
oxygen
affinity.
[BPG]
[BPG]
(I) Step 9 - Enolase: Second “HighEnergy” Intermediate Formation.
(Dehydration reaction)
(J) Step 10 - Pyruvate Kinase (PK) : Second
ATP Generation.
Pyruvate Kinase
Tautomerization of
enolpyruvate to pyruvate.