GlycolysisChapter 16 – Voet and Voet 2nd EditionWed. 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.
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
Step 2 – Phosphoglucose Isomerase (PGI) [Phosphohexose isomerase] Phosphoglucose Isomerase catalyzes the conversion of G6P to F6P, 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 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.
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 hexokinase- deficiency results in increased hemoglobin oxygen affinity. [BPG] [BPG]
(I) Step 9 - Enolase: Second “High-Energy” Intermediate Formation. (Dehydration reaction)
Pyruvate Kinase Tautomerization of enolpyruvate to pyruvate.