GlycolysisChapter 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

GlycolysisGlucose is convertedto pyruvate whilegenerating 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 GlycolysisStage 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

Phosphoglucose Isomerasecatalyzes the conversion ofG6P to F6P, the isomerizationof an aldose to a ketose.

Step 2 – Phosphoglucose Isomerase (PGI)

[Phosphohexose isomerase]

The isomerization of an aldose to a ketose

(C) Step 3 - Phosphofructokinase 1:

Second ATP utilization

D. Step 4 - Aldolase

• Aldolase catalyzes cleavage of fructose-1,6-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 twoTrioses (GAP and DHAP)

Note that the atomnumbering system changes. Atoms 1, 2, and 3 of glucose becomeatoms 3,2, and 1 ofDHAP. Atoms 4, 5, and6 become atoms 1, 2, and 3 of GAP.

(E) Step 5 - Triose Phosphate Isomerase (TIM)

Only GAP continuesalong the glycolyticpathway.

Fate of the carbon atoms of glucose in theformation 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 dehydrogenasereaction

(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-BisphosphoglycerateAre required for enzymatic activity.

(2) Incubation of the enzyme with catalytic amounts of32P-labeled 2,3-BPG yields a 32P-labeled enzyme.

Glycolysis influences oxygen transport

2,3-BPG binds to deoxyhemoglobin and alters the oxygenaffinity of hemoglobin. Erythrocytes synthesize and degrade 2,3-BPG by a detour from the glycolytic pathway.

Lower [BPG]in erythrocytesresulting fromhexokinase-deficiencyresults inincreasedhemoglobinoxygenaffinity.

[BPG]

[BPG]

(I) Step 9 - Enolase: Second “High-Energy” Intermediate Formation.

(Dehydration reaction)

(J) Step 10 - Pyruvate Kinase (PK) : Second ATP Generation.

Pyruvate Kinase

Tautomerization ofenolpyruvate to pyruvate.