Pathways that Harvest Chemical Energy

1. Pathways that Harvest Chemical Energy

2. Pathways that Harvest Chemical Energy 9.1 How Does Glucose Oxidation Release Chemical Energy? 9.2 What Are the Aerobic Pathways of Glucose Metabolism? 9.3 How Does Oxidative Phosphorylation Form ATP? 9.4 How Is Energy Harvested from Glucose in the Absence of Oxygen? 9.5 How Are Metabolic Pathways Interrelated and Regulated?

3. 9.1 How Does Glucose Oxidation Release Chemical Energy? The sugar glucose (C6H12O6) is the most common form of fuel or energy molecule Cells obtain energy from glucose in a series of metabolic pathways Other fuel molecules are first converted to glucose or other intermediate

4. Energy and Electrons from Glucose Principles governing metabolic pathways: complex chemical transformations which occur in a series of separate reactions each reaction is catalyzed by a specific enzyme metabolic pathways are similar in all organisms eukaryote pathways often compartmentalized in organelles each pathway is regulated by key enzymes

5. Metabolic Pathways

6. 9.1 How Does Glucose Oxidation Release Chemical Energy? When burned in a flame or when metabolised completely, glucose releases heat, carbon dioxide, and water C6H12O6 + 6 O2 ? 6 CO2 + 6 H2O + energy Electrons have relatively high potential energy in C-H bonds, compared to other bonds (C-O or H-O) due to low electronegativity of both C and H

7. Energy and Electrons from Glucose About half of the energy stored in glucose is collected in ATP (remainder is lost as heat) Breakdown of glucose is highly exergonic, and drives the endergonic formation of ATP ?G from complete combustion of glucose = �686 kcal/mol

8. 9.1 How Does Glucose Oxidation Release Chemical Energy? Three metabolic processes are used in the breakdown of glucose for energy: Glycolysis Cellular respiration (Citric Acid Cycle, Respiratory Chain) Fermentation

9. Figure 9.1 Energy for Life

10. Energy and Electrons from Glucose Glycolysis produces some usable energy + 2 molecules of pyruvate (a 3-carbon sugar) Glycolysis begins glucose metabolism in all cells Glycolysis does not require O2 ? so its an anaerobic metabolic process

11. 9.1 How Does Glucose Oxidation Release Chemical Energy? Cellular respiration uses O2 and occurs in aerobic (oxygen-containing) environments Pyruvate is converted to CO2 and H2O Energy stored in covalent bonds of pyruvate used to make ATP molecules

12. Energy and Electrons from Glucose Fermentation does not involve O2 ? anaerobic process Pyruvate is converted into lactic acid or ethanol Breakdown of glucose is incomplete Less energy is released than by aerobic cellular respiration

13. 9.1 How Does Glucose Oxidation Release Chemical Energy? Oxidation-Reduction (Redox) reactions transfer electrons from one molecule to another� OIL RIG Oxidation is loss � something loses electrons Reduction is gain � something gains electrons Also applies to loss/gain of hydrogen atoms (H = H++ e-)

15. 9.1 How Does Glucose Oxidation Release Chemical Energy? Oxidation and reduction occur together (coupled) � energy is transferred Reactant that loses electrons or H is a reducing agent Reactant that gains electrons or H is an oxidizing agent During glucose metabolism, glucose is the reducing agent (is oxidized, i.e. loses e-), while oxygen is the oxidizing agent (is reduced, i.e. gains e-) C6H12O6 + 6 O2 ? 6 CO2 + 6 H2O + energy

16. Figure 9.2 Oxidation, Reduction, and Energy

17. 9.1 How Does Glucose Oxidation Release Chemical Energy? The coenzyme NAD is an essential electron carrier oxidized form: NAD+ reduced form: NADH (+ H+) FAD+ also an electron carrier Reduced form is FADH2

18. Figure 9.3 NAD+/NADH is an Electron Carrier in Redox Reactions

20. 9.2 What Are the Aerobic Pathways of Glucose Metabolism? With O2 present, four pathways operate: Glycolysis Pyruvate oxidation Citric acid (Kreb�s) cycle Respiratory chain (electron transport chain, or ETC)

21. Figure 9.4 Energy-Producing Metabolic Pathways

23. Glycolysis: From Glucose to Pyruvate Glycolysis (in cytosol) has 10 steps in two stages: Energy-investing reactions that use 2 ATPs Energy-harvesting reactions that release 4 ATPs

24. Glycolysis: From Glucose to Pyruvate Energy-investing reactions of glycolysis: Two ATP molecules are used to modify glucose Phosphates from ATP are added to carbons of glucose to �energize� the molecule End product of �investment� phase is two glyceraldehyde-3-phosphate (G3P) molecules (link to photosynthesis � Ch 8)

25. Figure 9.5 Glycolysis Converts Glucose into Pyruvate (Part 1)

26. Glycolysis: From Glucose to Pyruvate Energy-harvesting reactions of glycolysis: First reaction oxidizes G3P to release free energy that is used to reduce NAD+ to make two molecules of NADH + H+ one for each of the two G3P molecules

27. Glycolysis: From Glucose to Pyruvate Two other reactions each yield one ATP per G3P molecule � called substrate-level phosphorylation Final products are two 3-carbon molecules of pyruvate + 4 ATP�s

28. Figure 9.5 Glycolysis Converts Glucose into Pyruvate (Part 4)

29. Figure 9.6 Changes in Free Energy During Glycolysis and the Citric Acid Cycle

30. Glycolysis: From Glucose to Pyruvate Summary: per one glucose (6-carbons) Net of 2 ATPs produced 2 ATPs used (investing phase) 4 ATPs produced (harvesting phase) � substrate- level phosphorylation 2 NADH produced (one for each two G3Ps) � must pass into mitochondria to ETC 2 pyruvate (3-carbons)

31. Pyruvate Oxidation Each pyruvate (3-C) is converted to one acetyl CoA (2-C) One NADH generated One CO2 generated x2 for both pyruvates Catalyzed by enzyme complex attached to inner mitochondrial membrane ? mitochondrial matrix Energy released is captured by electron carrier NAD+ ? NADH; rest stored in acetyl CoA

32. Figure 9.7 Pyruvate Oxidation and the Citric Acid Cycle (Part 1)

33. Pyruvate Oxidation Summary: per one glucose (6-carbons) 2 pyruvates (3-C) are converted to one acetyl CoA (2-C) 2 NADH generated 2 CO2 generated

34. The Citric Acid Cycle Citric acid cycle begins when Coenzyme A is removed from acetyl CoA (2-C) and the two carbons are added to oxaloacetate Cyclical series of reactions begins & ends with oxaloacetate, which can be used for next cycle Majority of energy stored in carbon molecules is transferred to electron carriers in CA cycle NADH and FADH2 Some energy in ATP

35. Figure 9.7 Pyruvate Oxidation and the Citric Acid Cycle (Part 2)

36. The Citric Acid Cycle Summary: per one glucose (2 turns of cycle) ? 2 acetyl CoAs go in� 6 NADHs, 2 ATPs, 2 FADH2, 4 CO2 are produced At this point, all carbons have been released as CO2 All remaining usable energy is in ATPs or in electron carriers (NADH and FADH2) from all steps

37. Figure 9.6 Changes in Free Energy During Glycolysis and the Citric Acid Cycle

38. 9.2 What Are the Aerobic Pathways of Glucose Metabolism? The electron carriers that are reduced during the citric acid cycle must be reoxidized to take part in the cycle again Oxidative phosphorylation � O2 is present Fermentation � if no O2 is present

39. 9.3 How Does Oxidative Phosphorylation Form ATP? Oxidative phosphorylation � ATP synthesized as electron carriers are reoxidized in presence of O2

40. 9.3 How Does Oxidative Phosphorylation Form ATP? Two stages of oxidative phosphorylation: Electron transport chain (aka ETC or the respiratory chain) Flow of electrons used to transport protons across inner mitochondrial membrane Stores energy in a proton concentration gradient across membrane Chemiosmosis Protons diffuse back into mitochondrial matrix (via facilitated transport) Energy release drives ATP synthesis from ADP + Pi

41. Oxidation of Glucose Produces ATP Why does the electron transport chain have so many steps? Why not just � in one step?... Too much free energy would be released all at once � majority of energy could not be harnessed by cell (escape as heat) In this series of reactions, each reaction releases a small amount of energy that can be more efficiently captured by an endergonic reaction

42. Electron Transport Chain: Electrons, Protons, and ATP Production Electron transport chain Consists of four large protein complexes (I, II, III & IV) bound to inner mitochondrial membrane plus cytochrome c and ubiquinone (Q) Energy is released as electrons pass from one carrier to the next

43. Figure 9.9 The Respiratory Chain and ATP Synthase Produce ATP by a Chemiosmotic Mechanism (Part 2)

44. 9.3 How Does Oxidative Phosphorylation Form ATP? NADH and FADH2 pass electrons (hydrogen atoms) to the complexes in chain These electrons are passed to other, more electronegative, complexes in chain Oxygen unites with two hydrogen ions to form water O is the final electron acceptor

45. Figure 9.8 The Oxidation of NADH and FADH2 in the Respiratory Chain

46. Figure 9.9 The Respiratory Chain and ATP Synthase Produce ATP by a Chemiosmotic Mechanism (Part 1)

47. 9.3 How Does Oxidative Phosphorylation Form ATP? As electrons pass through respiratory chain� Protons moved across membrane by three pumps (active transport) into intermembrane space � against their concentration gradient H+ gradient results in a difference in electric charge across inner membrane, storing energy Potential energy generated is proton-motive force

48. 9.3 How Does Oxidative Phosphorylation Form ATP? Chemiosmosis couples the proton-motive force and ATP synthesis NADH and FADH2 yield energy when oxidated Energy of transferred electrons is used to pump H+ into inter-membrane space NADH ? 10 protons pumped FADH2 ? 6 protons pumped

49. Figure 9.9 The Respiratory Chain and ATP Synthase Produce ATP by a Chemiosmotic Mechanism (Part 2)

50. H+ trapped in intermembrane space store energy in form of concentration gradient H+ flows down gradient (via facilitated diffusion) through ATP synthase (F0 unit) back into mitochondrial matrix Potential energy from proton-motive force is transformed into flow of H+ (kinetic energy of diffusion), harnessed by ATP synthase (F1 unit) to synthesize ATP from ADP + Pi

51. Requires 4 H+ to produce one ATP (3 H+ for ATP synthase +1 H+ for ADP+Pi to enter mitochondria via ATP-ADP translocase) NADH: 10 H+ pumped/4 H+ for ATP production = 2.5 ATP/NADH FADH2: 6 H+ pumped/4 H+ for ATP production = 1.5 ATP/ FADH2

52. Electron Transport Chain animation from Virtual Cell http://vcell.ndsu.edu/animations/etc/movie-flash.htm (In the animation, the number of protons pumped per NADH differ from text but the concept is well illustrated) ATP Synthase animation from Virtual Cell http://vcell.ndsu.edu/animations/atpgradient/movie.htm

53. 9.3 How Does Oxidative Phosphorylation Form ATP? Synthesized ATP is transported out of mitochondrial matrix as quickly as it is made ATP exits mitochondria via ATP-ADP Translocase, and ADP and Pi enters� 1 H+ is used to power pump (see above)

54. Mitochondrial membrane is impermeable to NADH in some animals, so electrons of NADH from glycolysis are transported to either NAD+ or FAD inside mitochondria (see above) If it is to FAD, then this discounts energy output in ATPs by one each

55. 9.3 How Does Oxidative Phosphorylation Form ATP? Summary: ATPs per glucose molecule for cellular respiration Glycolysis ATPs 2 ATPs (net) 2 2 NADHs ? x2.5 for ATP � 2 discounted to cross membr. 3 to 5 Pyruvate oxidation 2 NADHs ? x2.5 for ATP 5 Citric Acid Cycle 6 NADHs ? x2.5 for ATP 15 2 FADH2 ? x1.5 for ATP 3 2 ATPs 2 Total ATPs 30 to 32

56. 9.3 How Does Oxidative Phosphorylation Form ATP? ATP synthesis can be uncoupled If a different H+ channel is inserted into mitochondrial membrane, H+ gradient�s energy is lost as heat rather than used to make ATP Thermogenin or uncoupling protein (UCP) occurs in brown fat of human infants and adults, and in hibernating animals to generate heat

57. 9.4 How Is Energy Harvested from Glucose in the Absence of Oxygen? Electron carriers that are reduced must be reoxidized before they can accept electrons again Oxidative phosphorylation � O2 is present Fermentation � if no O2 is present

58. Fermentation: ATP from Glucose, without O2 Without O2, ATP can be produced only by glycolysis and fermentation Fermentation is used to regenerate NAD+, allowing glycolysis to continue Occurs in cytosol Pyruvate is reduced by NADH + H+, thus regenerating NAD+

59. 9.4 How Is Energy Harvested from Glucose in the Absence of Oxygen? Lactic acid fermentation Enzyme converts pyruvate ? lactate Pyruvate is the electron acceptor, allowing NAD+ to be replenished Lactic acid fermentation occurs in some microorganisms and in muscle cells when starved for oxygen

60. Figure 9.12 Alcoholic Fermentation

61. 9.4 How Is Energy Harvested from Glucose in the Absence of Oxygen? Alcoholic fermentation Pyruvate (3-C) ? acetylaldehyde (2-C) + CO2 Acetylaldehyde reduced by NADH + H+ ? ethanol + NAD+ in yeasts, some plants

62. 9.4 How Is Energy Harvested from Glucose in the Absence of Oxygen? Glycolysis and cellular respiration Total of 30-32 ATPs can be generated from each glucose Glycolysis and fermentation Total of only 2 ATP can be generated from each glucose Fermentation�s by-products still contain lots of energy

63. 9.5 How Are Metabolic Pathways Interrelated and Regulated? Related metabolic processes: Glucose utilization pathways can yield more than just energy Interchanges for diverse biochemical traffic Intermediate molecules are substrates for synthesis of lipids, amino acids, nucleic acids, and other molecules

64. 9.5 How Are Metabolic Pathways Interrelated and Regulated? Catabolic interconversions� What happens if inadequate food molecules are available? Glycogen (polysaccharides) stores in muscle/liver are used first Fats (triglycerides) are used next After fats are depleted, proteins alone provide energy

65. Figure 9.14 Relationships among the Major Metabolic Pathways of the Cell

66. 9.5 How Are Metabolic Pathways Interrelated and Regulated? Catabolic interconversions: Polysaccharides hydrolyzed into glucose ? passes on to glycolysis

67. 9.5 How Are Metabolic Pathways Interrelated and Regulated? Catabolic interconversions: Lipids (triglycerides) converted ? Glycerol ? enters glycolysis fatty acids ? acetyl CoA (beta oxidation)

68. 9.5 How Are Metabolic Pathways Interrelated and Regulated? Catabolic interconversions: Proteins hydrolyzed into amino acids ? feed into glycolysis or citric acid cycle (deamination)

69. 9.5 How Are Metabolic Pathways Interrelated and Regulated? Anabolic interconversions : Most catabolic reactions are reversible Gluconeogenesis uses intermediates of glycolysis and citric acid cycle ? glucose Acetyl CoA ? fatty acids Intermediates can form amino acids. Citric acid cycle intermediate a-ketoglutarate is starting point for synthesis of purines and Oxaloacetate is starting point for pyrimidines

70. 9.5 How Are Metabolic Pathways Interrelated and Regulated? Levels of products and substrates of energy metabolism are remarkably constant (e.g. blood glucose levels) Cells regulate enzymes of catabolism and anabolism to maintain balance, or metabolic homeostasis Negative and positive feedback inhibition Allosteric control of enzymes

71. Figure 9.15 Regulation by Negative and Positive Feedback

72. Regulating Energy Pathways Balance of products in cell regulated by allosteric control of enzymes Main control point in glycolysis is enzyme phosphofructokinase activated by ADP Inhibited by ATP and citrate

74. Question 1 Which one of the following is not a product of glycolysis? 1. ATP 2. CO2 3. NADH + H+ 4. Pyruvate Answer: 2Answer: 2

75. Question 2 When eukaryotes carry out aerobic respiration, where in the cell is the majority of the ATP produced? 1. Outside the mitochondrion 2. In the intermembrane space of the mitochondrion 3. In the inner mitochondrial matrix Answer: 3Answer: 3

76. Question 3 Per glucose, glycolysis yields 2 ATP, the citric acid cycle yields 2 ATP, and the respiratory chain yields 28 ATP. How many of these total 30 ATP are produced inside the mitochondrion? 1. 2 2. 4 3. 18 4. 28 5. 30 Answer: 4Answer: 4