Introduction to Glycolysis for basic biochemistry
- 1. Glycolysis
- 2. Lesson Learning Outcome Upon completion of this lecture, should be able to: • understand the glycolytic pathway • fates of pyruvate students
- 4. Cellular Respiration • Is a set of metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste product. • The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy in the process. • Cellular respiration is considered an exothermic redox reaction which releases heat.
- 7. The Glycolytic Pathway Glucose is converted to two pyruvate
- 8. Glycolysis
- 10. Glycolysis • In cytoplasm, an anaerobic process which generates ATP, NAPH and pyruvate • Glycolysis: a series of 10 enzyme-catalyzed reactions by which glucose is oxidized to two molecules of pyruvate – there is net conversion of 2ADP to 2ATP C6 H1 2 O6 Glucose glycolysis O 2 CH3 CCOO- Pyruvate + 2 H+ + 2 ADP + 2 Pi + 2 ATP C6 H1 2 O6 Glucose O 2 CH3 CCOO- Pyruvate
- 11. Fates of Pyruvate • Pyruvate is most commonly metabolized in one of three ways, depending on the type of organism and the presence or absence of O2 O CH3 CCOO- Pyruvate OH CH3 CHCOO- Lactate CH3 CH2 OH + CO2 Ethanol 3 CO2 + 2 H2 O aerobic conditions plants and animals anaerobic conditions contracting muscle anaerobic conditions fermentation in yeast
- 12. Reactions of glycolysis • Reaction 1: phosphorylation of -D-glucose to give glucose-6-phosphate OH OH HO HO CH2 OH O + -O- P-O- P-O- AM P O- O- ATP -D-Glucose hexokinase Mg 2 + OH HO HO CH2 OPO3 2 - O OH -D-Glucose-6-phosphate + O -O- P-O- AM P O- ADP O O
- 13. – this reaction is driven by the free energy of hydrolysis of ATP – These two reaction are coupled, so the overall reaction is the sum of the two and is exergonic – The enzyme that catalyzes this reaction is hexokinase – Glucose-6-phosphate inhibits hexokinase – feedback inhibition
- 14. • Reaction 2: isomerization of glucose-6-phosphate to fructose-6-phosphate -D-Glu • The enzyme that catalyzes this reaction is glucosephosphate isomerase • The aldehyde group at C1 is reduced to hydroxyl, and the C2-hydroxyl is oxidized to give the ketone group of fructose- 6-phosphate 2 3 6 CH OPO 2 - OH HO HO CH2 OPO3 O 2 - phosphogluco- isomerase 1 2 6
- 16. • Reaction 3: phosphorylation of fructose-6-phosphate 1 CH2 OH OH H O H HO -D-Fructose-6-phosphate 6 CH2 OPO3 2 - HO H + ATP phospho- fructokinase Mg 2 + 2 3 1 CH2 OPO3 2 - O OH H H HO 6 CH OPO HO H -D-Fructose-1,6-bisphosphate • The phosphorylation of fructose-6-phosphate is highly exergonic and irreversible – enzyme responsible is phosphofructokinase 2 - + ADP
- 17. • Reaction 4: cleavage of fructose-1,6-bisphosphate to two triose phosphates by enzyme aldolase 2 C=O CH OPO3 HO H H OH H OH CH2 OP Fruct 2- aldolase CH2 OPO3 2 - C=O CH2OH Dihydr
- 18. • Reaction 5: isomerization of triose phosphates – catalyzed by triosephosphate isomerase – reaction involves two successive keto-enol tautomerizations – only the D enantiomer of glyceraldehyde 3- phosphate is formed CH2 OH C= O CH2 OPO3 2 - Dihydroxyacetone phosphate CHO H C OH CH2 OPO3 2 - D-Glyceraldehyde 3-phosphate CHOH C-OH CH2 OPO3 2 - An enediol intermediate
- 19. • Reaction 6: oxidation of the -CHO group of D- glyceraldehyde-3-phosphate – the -CHO group is oxidized to a carboxyl group – the oxidizing agent, NAD+, is reduced to NADH G - C - H + A t w o - e l e c t r o n o x i d a t i o n O H 2 O G - C - O H O 2 H + 2 e - H + 2 e - N A D H A t w o - e l e c t r o n r e d u c t i o n + + + N A D + + O G - C - H + H 2 O + N A D + O G - C - O H H + +
- 20. ◾the overall reaction involves an exergonic oxidation and an endergonic phosphorylation ◾the overall reaction is slightly endergonic Go' = +49.3 kJ•mol -1 Go' = -43.1 kJ•mol -1 oxidation: phosphorylation: O C-O- O O C-H to C-O- O O O C-H to to C-O- P-O- O- O O C-O- P-O- O- Go' = +6.2 kJ•mol -1
- 21. • Reaction 7: transfer of a phosphate group from 1,3- bisphosphoglycerate to ADP – this reaction is called substrate-level phosphorylation + 1,3-Bisphospho- glycerate COO- H C OH CH2 OPO3 2 - 3-Phosphoglycerate CH OPO 2 3 2 - O C-OPO3 2 - H C OH + O -O- P-O-AMP O- ADP phospho- glycerate kinase Mg2+ -O- P-O- P-O-AMP O O O- O- ATP
- 22. the sum of the endergonic – this reaction is phosphorylation of ADP and the exergonic hydrolysis of the mixed phosphate anhydride phosphorylation: Go' = -49.3 kJ•mol -1 Go' = +0.5 kJ•mol -1 O C-O- O O C-O-P-O- O- ADP + Pi hydrolysis: Go' = -18.8 kJ•mol -1 + Pi ATP + H2 O O O C-O- P-O- O- + ADP + Pi O C-O- + ATP + H2 O
- 23. • Reaction 8: isomerization of 3-phosphoglycerate to 2-phosphoglycerate COO- H C OH CH2 OPO3 2 - 3-Phosphoglycerate COO- H C OPO3 2 - CH2 OH 2-Phosphoglycerate phosphoglycerate mutase
- 24. • Reaction 9: dehydration of 2-phosphoglycerate 3 COO- H C OP O CH2 OH 2-Phosp 2- COO- C enolase Mg2+
- 25. • Reaction 10: phosphate transfer to ADP stage 1: transfer of the phosphate group COO- C OPO CH2 3 2 - Phosphoenol- pyruvate + -O- P-O- P-O-AMP O- O- ATP COO- C-OH CH2 Enol of pyruvate O -O- P-O-AMP O- ADP + pyruvate kinase Mg2+ O O
- 26. Stage 2: enolization to pyruvate COO- C-OH CH2 Enol of pyruvate COO- C= O CH3 Pyruvate
- 28. Glycolysis • Summing these 10 reactions gives the net equation for glycolysis C6H12O6 Glucose + 2NAD+ + 2HPO4 2- + 2ADP glycolysis O 2CH3CCOO- Pyruvate + 2NADH+ 2A TP + 2H2O+ 2H+
- 29. Energetics of Glycolysis • Three reactions exhibit particularly large decreases in free energy; the enzymes that catalyze these reactions are sites of allosteric control – hexokinase – phosphofructokinase – pyruvate kinase
- 30. • Fructose is phosphorylated by fructokinase (liver) or hexokinase (adipose) on the 1 or 6 positions resp. • Fructose-6-phosphate is an intermediate of glycolysis. • Fructose-1-phosphate is acted upon by an aldolase- like enzyme that gives DHAP (dihydroxyacetone phosphate) and glyceraldehyde. • DHAP is a glycolysis intermediate and glyceraldehyde can be phosphorylated to glyceraldehyde-3-P. • Glycerol is phosphorylated to G-3-P which is then converted to glyceraldehyde 3 phosphate.
- 31. • Galactose has a slightly complicated multi-step pathway for conversion to glucose-1-phosphate. • gal gal-1-P UDP-gal UDP-glc glc-1-P. • If this pathway is disrupted because of defect in one or more enzymes involved in the conversion of gal to glc-1-P, then galactose accumulates in the blood and the subject suffers from galactosemia which is a genetic disorder, an inborn error of metabolism.
- 33. Reactions of Pyruvate • Pyruvate does not accumulate in cells, but rather undergoes one of three enzyme- catalyzed reactions, depending of the type of cell and its state of oxygenation – reduction to lactate – reduction to ethanol – oxidation and decarboxylation to acetyl-CoA
- 34. Lactate Fermentation ⦿In vertebrates under anaerobic conditions, the most important pathway for the regeneration of NAD+ is reduction of pyruvate to lactate ◾lactate dehydrogenase (LDH) is a tetrameric isoenzyme consisting of H and M subunits; H4 predominates in heart muscle, and M4 in skeletal muscle O CH CCOO- + NADH + H+ 3 Pyruvate OH CH3 CHCOO- + NAD+ Lactate lactate dehydrogenase
- 35. Pyruvate to Lactate while lactate fermentation allows glycolysis to continue, it increases the concentration of lactate and also of H+ in muscle tissue when blood lactate reaches about 0.4 mg/100 mL, muscle tissue becomes almost completely exhausted C6H12O6 Gluco OH lactate fermentation
- 36. Pyruvate to Ethanol • Yeasts and several other organisms regenerate NAD+ by this two-step pathway; decarboxylation of pyruvate to acetaldehyde 3 P y r u v a t e O p y r u v a t e d e c a r b o x y l a s e O C H C C O O - + H + 3 O C H C H + N A D H + reduction of acetaldehyde to ethanol alcohol H + A ce ta lde hy de de hy dro g e na s e C H 3 C H 2 O H + E t h a n o l N A D +
- 37. Pyruvate to Acetyl-CoA • Under aerobic conditions, pyruvate undergoes oxidative decarboxylation – the carboxylate group is converted to CO2 – the remaining two carbons are converted to the acetyl group of acetyl-CoA 3 P y ru v a t e O C H 3 C S C o A + A c e t y l - C o A C O 2 + N A D H o x i d a t i v e d e c a r b o x y l a t i o n O C H C C O O - + N A D + + C o A S H
- 39. End of lecture