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Metabolism ii-chp-17-bioc-361-version-dec-2012 - Glycolysis


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This is the glycolysis component of Bioc (chem) 361 at UAE University. Some from Campbell 6th ed and the rest from General, Organic, and Biochemistry, 5th edition (2007), by K.J.Denniston, J.J.Topping, and R.L.Caret.

Metabolism ii-chp-17-bioc-361-version-dec-2012 - Glycolysis

  1. 1. Mary K. Campbell Shawn O. Farrell Chapter 17 GlycolysisPaul D. Adams • University of Arkansas
  2. 2. The Overall Pathway of Glycolysis• Glycolysis is the first stage of glucose metabolism• One molecule of glucose is converted to fructose- 1,6-bisphosphate, which gives rise to two molecules of pyruvate• It plays a key role in the way organisms extract energy from nutrients• Once pyruvate is formed, it has one of several fates
  3. 3. Glycolysis Overview • The anaerobic oxidation of glucose to give two molecules of pyruvate • Glucose + 2 ADP + 2 Pi + 2 NAD+ 2 pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O • Pyruvate used in follow-up reactions to sustain glycolysis • NADH must be reoxidized so that glycolysis can continue
  4. 4. Fates of Pyruvate From Glycolysis
  5. 5. The Reactions of Glycolysis• Phosphorylation of glucose to give glucose-6- phosphate• Isomerization of glucose-6-phosphate to give fructose-6-phosphate• Phosphorylation of fructose-6-phosphate to yield fructose-1,6-bisphosphate• Cleavage of fructose-1,6,-bisphosphate to give glyceraldehyde-3-phosphate and dihyroxyacetone phosphate• Isomerization of dihyroxyacetone phosphate to give glyceraldehyde-3-phosphate
  6. 6. The Reactions of Glycolysis (Cont’d)• Oxidation of glyceraldehyde-3-phosphate to give 1,3-bisphosphoglycerate• Transfer of a phosphate group from 1,3- bisphosphoglycerate to ADP to give 3- phosphoglycerate• Isomerization of 3-phosphoglycerate to give 2- phosphoglycerate• Dehydration of 2-phosphoglycerate to give phosphoenolpyruvate• Transfer of a phosphate group from phosphoenolpyruvate to ADP to give pyruvate
  7. 7. Conversion of Glucose to Glyceraldehyde-3-Phosphate (Cont’d)• Fructose-1,6-bisphosphate is split into two 3-carbon fragments• Reaction catalyzed by aldolase• Side chains of an essential Lys and Cys play key roles in catalysis
  8. 8. Conversion of Glucose to Glyceraldehyde-3-Phosphate (Cont’d)• In step 5, dihydroxyacetone phosphate (DHAP) is converted to glyceraldehyde-3-phosphate• These compounds are trioses• This reaction has small +∆G (0.58kcal/mol-1)• Remember that glycolysis has several reactions that have very negative ∆G values, and drive other reactions to completion, so that the overall process is negative
  9. 9. Glycolysis Reactions 1 and 2Reaction 1 (INVESTMENT 1)• Substrate glucose is phosphorylated by hexokinase• Product is glucose-6-phosphate • Source of the phosphoryl group is ATP • Expenditure of ATP early in the pathway works as energy “debt” necessary to get the pathway started • Not reversibleReaction 2• Product of reaction 1 is rearranged to the structural isomer fructose-6-phosphate by enzyme phosphoglucose isomerase• Product has an “exposed” C-1, no longer part of the ring structure • Converts and aldose to a ketose
  10. 10. Glycolysis: Step 1, 2Glucose Glucose-6-phosphate CH2 OH 2- H O H CH2 OPO3 H Hexokinase H O H OH H H HO OH OH H H OH HO OH H OH + ATP 2- + CH2OPO3 ADP O CH2OH H HO Fructose-6- H OH Phosphoglucose phosphate OH H isomerase
  11. 11. Glycolysis: Step 2
  12. 12. Glycolysis Reaction 3Reaction 3 (INVESTMENT 2)• Substrate fructose-6-phosphate is phosphorylated by phosphofructokinase• Product is fructose-1,6-bisphosphate • Source of the phosphoryl group is ATP • Again the expenditure of ATP early in the pathway works as energy “debt” necessary to get the pathway started • Step 1 and 3 considered the first committed steps of glycolysis, not reversible
  13. 13. Glycolysis: Step 3Fructose-6-phosphate 2- CH2OPO3 O CH2OH Phosphofructokinase H HO H CH2O PO3 2- OH 2- OH H O CH2OPO3 H HO + ATP H OH + ADP OH H Fructose-1,6-bisphosphate
  14. 14. Phosphofructokinase is a Key RegulatoryEnzyme in glycolysis• Phosphofructokinase (PFK): • Exists as a tetramer and subject to allosteric feedback • The tetramer is composed of L and M subunits • M4, M3L, M2L2, ML3, and L4 all exist. Combinations of these subunits are called isozymes • Muscles are rich in M4; the liver is rich in L4 • ATP is an allosteric effector; high levels inhibit the enzyme, low levels activate it • Fructose-1,6-bisphosphate is also an allosteric effector
  15. 15. PFK can exist in Many Isozyme Forms
  16. 16. Glycolysis Reactions 4 and 5Reaction 4• Product of reaction 3 is split into two 3-carbon intermediates by the enzyme aldolase forming: • Glyceraldehyde-3-phosphate (substrate of next reaction) • Dihydroxyacetone phosphateReaction 5• Dihydroxyacetone phosphate is rearranged into a second glyceraldehyde-3-phosphate by the enzyme triose phosphate isomerase • Glyceraldehyde-3-phosphate is the only substrate for the next reaction
  17. 17. Glycolysis: Steps 4 and 5 D-glyceraldehyde- Fructose-1,6-bisphosphate 3-phosphate 2- CH2OPO3 HC O 2- O CH2OPO3 H C OH 2- H HO Aldolase CH2OPO3 H OH + 2- OH H CH2O PO3 Triosephosphate C O isomerase CH2OH HC O Dihydroxyaceton H C OH e 2-D-glyceraldehyde- CH2OPO3 phosphate 3-phosphate
  18. 18. Glycolysis: Steps 4 and 5
  19. 19. Glycolysis Reaction 6Reaction 6 (remember this reaction and 7,8,9,10 all happen twice per glucose)• Substrate glyceraldehyde-3-phosphate is oxidized to a carboxylic acid by glyceraldehyde- 3-phosphate dehydrogenase • Reduces NAD+ to NADH (substrate is oxidized, but not oxidative phosphorylation|) • Transfers an inorganic phosphate group to the carboxyl group• First step in glycolysis to “harvest” energy (x2)• Product is 1,3-Bisphosphoglycerate • New phosphate group attached with a “high-energy” bond • This and all subsequent steps occur twice for each G-3-P
  20. 20. Glycolysis: Step 6 Glyceraldehyde HC O 3-phosphate dehydrogenase OH C OH 2- CH2OPO3 2- C OPO3 + NAD+ H C OH 2- + HPO42- CH2OPO3 Glyceraaldehyde- + NADH + H+ 3-phosphate Glycerate- 1,3-bisphosphate
  21. 21. Glycolysis: Step 6
  22. 22. Glycolysis Reactions 7 and 8Reaction 7• Harvest energy in the form of ATP• 1,3-Bisphosphoglycerate high energy phosphate group is transferred to ADP by phosphoglycerate kinase: • 3-Phosphoglycerate • ATP• This is the first substrate level phosphorylation of glycolysis (we are now out of debt!)Reaction 8• 3-Phosphoglycerate is isomerized into 2- phosphoglycerate by the enzyme phosphoglycerate mutase • Moves the phosphate group from carbon-3 to carbon-2
  23. 23. Glycolysis: Steps 7 and 81,3-Bisphosphoglycerate O 3-Phosphoglycerate 2- O C OPO3 Phosphoglycerate C O H C OH kinase + ATP 2- H C OH CH2OPO3 CH2OPO3 2- + ADP O C O Phosphoglycerate 2- mutase H C OPO3 CH2OH 2-Phosphoglycerate
  24. 24. Glycolysis Reactions 9 and 10Reaction 9• The enzyme enolase catalyzes dehydration of 2- phospholgycerate • Phosphoenolpyruvate • Energy rich – highest energy phosphorylated compound in metabolismReaction 10• Final substrate-level dehydration in the pathway• Phosphoenolpyruvate serves as donor of the phosphoryl group transferred to ADP by pyruvate kinase making ATP and releasing water • Pyruvate is the final product of glycolysis • A coupled reaction in which hydrolysis of the phosphoester bond provides energy for the formation of the phosphoanhydride bond of ATP
  25. 25. Glycolysis: Steps 9 and 10 “High 2-Phosphoglycerate O energy O C O bond” C O 2- Enolase 2-H C OPO3 C O PO3 CH2 OH CH2 + H2O Phosphoenolpyruvate O C O Pyruvate C O + ATP kinase CH3 Pyruvate
  26. 26. Summary• In the first stages of glycolysis, glucose is converted to two molecules of glyceraldehyde-3-phosphate• The key intermediate in this series of reactions is fructose-1,6-bisphosphate. The enzyme that catalyzes this reaction, phosphofructokinase, is subject to allosteric control
  27. 27. Anaerobic Metabolism of Pyruvate• 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
  28. 28. NAD+ Needs to be Recycled to PreventDecrease in Oxidation Reactions
  29. 29. Fermentation• From glycolysis pyruvate remains for further degradation and NADH must be reoxidized• In aerobic conditions, both will occur in cellular respiration (mitochondria, TCA, Ox-phos)• Under anaerobic conditions, fermentation reactions accomplish this • Fermentation reactions are catabolic reactions occurring with no net oxidation • Major fermentation pathways: • Lactate fermentation • Alcohol fermentation
  30. 30. Lactate Fermentation• Lactate fermentation is the anaerobic metabolism that occurs in exercising muscle • Bacteria also use lactate fermentation • Production of yogurt and cheese• This reaction produces NAD+ and degrades pyruvate (allowing glycolysis to continue)
  31. 31. Alcohol Fermentation• Yeast ferment sugars of fruit and grains anaerobically, using pyruvate from glycolysis • Pyruvate decarboxylase removes CO2 from the pyruvate producing acetaldehyde • Alcohol dehydrogenase catalyzes reduction of acetaldehyde to ethanol, releasing NADH in the process