1. Glycolysis and Gluconeogenesis
Alice Skoumalová

3. 1. Glycolysis

4. Glucose:
the universal fuel for human cells
Sources:
diet (the major sugar in our diet)
internal glycogen stores
blood (glucose homeostasis)
Glucose oxidation:
after a meal: almost all tissues
during fasting: brain, erythrocytes

5. oxidation and cleavage of glucose
Glycolysis:
oxidation and cleavage of glucose
ATP generation (with and without oxygen)
all cells
in the cytosol (the reducing equivalents are transferred to the electron-transport chain by the shuttle)
ATP is generated:
1. via substrate-level phosphorylation
2. from NADH
3. from oxidation of pyruvate
Regulation of glycolysis:
1. Hexokinase
2. Phosphofructokinase
3. Pyruvate Kinase
Generation of precursors for biosynthesis:
fatty acids
amino acids
ribosis-5-P

6. Anaerobic glycolysis
a limited supply of O2
no mitochondria
increased demands for ATP
Lactic acidemia
in hypoxia

7. Phosphorylation of glucose:
irreversible
Glucose 6-P:
cannot be transported back across the plasma membrane
a precursor for many pathways that uses glucose
Hexokinases
Glucokinase (liver, β-cell of the pancreas)
high Km

8. Michaelis-Menten kinetics

9. 1. Conversion of glucose 6-P to the triose phosphates
2. Oxidation and substrate-level phosphorylation

10. essential for the subsequent cleavage
1. Conversion of glucose 6-P to the triose phosphates
essential for the subsequent cleavage
irreversible
regulation

11. Substrate-level phophorylation Substrate-level phophorylation
2. Oxidation and substrate-level phosphorylation
Substrate-level phophorylation
Substrate-level phophorylation

12. Glucosis + 2 NAD+ + 2 Pi + 2 ADP
Summary of the glycolytic pathway:
Glucosis + 2 NAD+ + 2 Pi + 2 ADP
2 pyruvate + 2 NADH + 4 H+ + 2 ATP + 2 H2O
∆G0´ = - 22 kcal (it cannot be reversed without the expenditure of energy!)

13. Aerobic glycolysis:
involving shuttles that transfer reducing equivalents across the mitochondrial membrane

14. Glycerol 3-phosphate shuttle:

15. Malate-aspartate shuttle:

16. Anaerobic glycolysis:
dissociation and formation of H+
Energy yield 2 mol of ATP

17. Major tissues of lactate production:
(in a resting state)
Daily lactate production
115 (g/d)
Erythrocytes 29
Skin 20
Brain 17
Sceletal muscle 16
Renal medulla 15
Intestinal mucosa 8
Other tissues 10

18. Cori cycle:
Lactate can be further metabolized by:
heart, sceletal muscle
Lactate dehydrogenase: a tetramer (subunits M and H)

19. Pyruvate + NADH + H+ lactate + NAD+
Lactate dehydrogenase LD
Pyruvate + NADH + H+ lactate + NAD+
5 isoenzymes:
Heart (lactate)
Muscle (pyruvate)

20. Biosynthetic functions of glycolysis:

21. Regulation

22. tissue-specific isoenzymes (low Km, a high afinity)
glucokinase (high Km)
the rate-limiting, allosteric enzyme
tissue-specific isoenzymes
Fructose 2,6-bis-phosphate:
is not an intermediate of glycolysis!
Phosphofructokinase-2: inhibited through phosphorylation - cAMP-dependent protein kinase (inhibition of glycolysis during fasting-glucagon)

23. the liver isoenzyme - inhibition by cAMP-dependent protein kinase (inhibition of glycolysis during fasting)
Lactic acidemia:
increased NADH/NAD+ ratio inhibition of pyruvate dehydrogenase

24. 2. Gluconeogenesis

25. Pyruvate → Phosphoenolpyruvate Fructose-1,6-P → Fructose-6-P
Gluconeogenesis:
synthesis of glucose from noncarbohydrate precursors → to maintain blood glucose levels during fasting
liver, kidney
fasting, prolonged exercise, a high-protein diet, stress
Specific pathways:
Pyruvate → Phosphoenolpyruvate
Fructose-1,6-P → Fructose-6-P
Glucose-6-P → Glucose

26. Precursors for gluconeogenesis
lactate (anaerobic glycolysis)
amino acids (muscle proteins)
glycerol (adipose tissue)

27. Conversion of pyruvate to phosphoenolpyruvate
1. Pyruvate → Oxaloacetate
Pyruvate carboxylase
2. Oxaloacetate → PEP
Phosphoenolpyruvate-carboxykinase

28. Conversion of phosphoenolpyruvate to glucose
3. Fructose-1,6-P → Fructose-6-P
Fructose 1,6-bisphosphatase (cytosol)
4. Glucose-6-P → Glucose
Glucose 6-phosphatase (ER)

29. Regulation of gluconeogenesis:
concomitant inactivation of the glycolytic enzymes and activation of the enzymes of gluconeogenesis
1. Pyruvate → PEP
Phosphoenolpyruvate carboxykinase - induced by glucagon, epinephrine, and cortisol
2. Fructose 1,6-P → Fructose 6-P
Fructose 1,6-bisphosphatase - inhibited by fructose 2,6-P
3. Glucose 6-P → Glucose
Glucose 6-phosphatase - induced during fasting

31. Summary
Glycolysis
Generation of ATP (with or without oxygen)
The role of glycolysis in different tissues
Lactate production
Regulation
Gluconeogenesis
Activation during fasting, prolonged exercise, after a high-protein diet
Precursors: lactate, glycerol, amino acids
3 key reactions: Pyruvate → PEP Fructose-1,6-P→ Fructose-6-P Glucose-6-P → Glucose

32. Pictures used in the presentation:
Marks´ Basic Medical Biochemistry, A Clinical Approach, third edition, 2009 (M. Lieberman, A.D. Marks)