1. Prentice Hall c2002Chapter 121 Chapter 12 - The Citric Acid Cycle The citric acid cycle is involved in the aerobic catabolism of carbohydrates, lipids and amino acids Intermediates of the cycle are starting points for many biosynthetic reactions Enzymes of the cycle are in the mitochondria of eukaryotes Energy of the oxidation reactions is largely conserved as reducing power (stored electrons) Coenzymes reduced: NAD + NADH FAD FADH 2 Ubiquinone (Q)Reduced Ubiquinone (QH 2 )

2. Prentice Hall c2002Chapter 122 Transport of Pyruvate from the cytosol into the Mitochondria Fig 12.1 Pyruvate translocase transports pyruvate into the mitochondria in symport with H + Pyruvate dehydrogenase complex

3. Prentice Hall c2002Chapter 123 Conversion of Pyruvate to Acetyl CoA Pyruvate dehydrogenase complex is a multienzyme complex containing: 3 enzymes + 5 coenzymes + other proteins E 1 = pyruvate dehydrogenase E 2 = dihydrolipoamide acetyltransferase E 3 = dihydrolipoamide dehydrogenase

4. Prentice Hall c2002Chapter 124 Table 12.1 Components of the PDH Complex

5. Prentice Hall c2002Chapter 125 Fig 12.2 Reactions of the PDH complex

6. Prentice Hall c2002Chapter 126 Fig 12.2 Reactions of the PDH complex

7. Prentice Hall c2002Chapter 127 Fig 12.2 Reactions of the PDH complex Acetylated lipoamide

8. Prentice Hall c2002Chapter 128 Fig 12.2 Reactions of the PDH complex TCA cycle Reduced lipoamide

9. Prentice Hall c2002Chapter 129 Fig 12.2 Reactions of the PDH complex Oxidized lipoamide

10. Prentice Hall c2002Chapter 1210 Fig 12.2 Reactions of the PDH complex Oxidized lipoamide

11. Prentice Hall c2002Chapter 1211 Fig 12.2 Reactions of the PDH complex Acetylated lipoamide

12. Prentice Hall c2002Chapter 1212 Fig 12.2 Reactions of the PDH complex TCA cycle Reduced lipoamide

13. Prentice Hall c2002Chapter 1213 Fig 12.2 Reactions of the PDH complex Oxidized lipoamide

14. Prentice Hall c2002Chapter 1214 The Citric Acid Cycle Oxidizes AcetylCoA Table 12.2

15. Prentice Hall c2002Chapter 1215 Summary of the citric acid cycle For each acetyl CoA which enters the cycle: (1) Two molecules of CO 2 are released (2) Coenzymes NAD + and Q are reduced to NADH and QH 2 (3) One GDP (or ADP) is phosphorylated (4) The initial acceptor molecule (oxaloacetate) is reformed

16. Prentice Hall c2002Chapter 1216 Fig 12.4 Citric acid cycle

17. Prentice Hall c2002Chapter 1217 Fig 12.4

18. Prentice Hall c2002Chapter 1218 Fig 12.4

19. Prentice Hall c2002Chapter 1219 6. The Succinate Dehydrogenase (SDH) Complex Located on the inner mitochondrial membrane, in contrast to other enzymes of the TCA cycle which are dissolved in the mitochondrial matrix Complex of polypeptides, FAD and iron-sulfur clusters Electrons are transferred from succinate to FAD, forming FADH 2, then to ubiquinone (Q), a lipid-soluble mobile carrier of electrons Reduced ubiquinone (QH 2 ) is released as a mobile product

20. Prentice Hall c2002Chapter 1220 Fig 12.4

21. Prentice Hall c2002Chapter 1221 Fig 12.5 Fates of carbon atoms in the cycle 6C  5C  4C

22. Prentice Hall c2002Chapter 1222 Fig 12.6 Energy conservation by the cycle Energy is conserved in the reduced coenzymes NADH, QH 2 and one GTP NADH, QH 2 can be oxidized to produce ATP by oxidative phosphorylation

23. Prentice Hall c2002Chapter 1223 Reduced Coenzymes Fuel the Production of ATP Each acetyl CoA entering the cycle nets: (1) 3 NADH (2) 1 QH 2 (3) 1 GTP (or 1 ATP) Oxidation of each NADH yields 2.5 ATP Oxidation of each QH 2 yields 1.5 ATP Complete oxidation of 1 acetyl CoA = 10 ATP

24. Prentice Hall c2002Chapter 1224 Fig 12.11 Glucose degradation via glycolysis, citric acid cycle, and oxidative phosphorylation

25. Prentice Hall c2002Chapter 1225 Regulation of the Citric Acid Cycle The citric acid cycle is controlled by: (1) Allosteric modulators (2) Covalent modification of cycle enzymes (3) Supply of acetyl CoA (4) Regulation of pyruvate dehydrogenase complex controls acetyl CoA supply

26. Prentice Hall c2002Chapter 1226 Fig 12.12 Regulation of the pyruvate dehydrogenase complex Increased levels of acetyl CoA and NADH inhibit E 2, E 3 Increased levels of CoA and NAD + activate E 2, E 3

27. Prentice Hall c2002Chapter 1227 Fig 12.13 Regulation of mammalian PDH complex by covalent modification Phosphorylation/dephosphorylation of E 1

28. Prentice Hall c2002Chapter 1228 Regulation of isocitrate dehydrogenase Mammalian ICDH Activated by calcium (Ca 2+ ) and ADP Inhibited by NADH NAD + NADH ++ (-)

29. Prentice Hall c2002Chapter 1229 Fig 12.16 Regulation of the citric acid cycle

30. Prentice Hall c2002Chapter 1230 Entry and Exit of Metabolites Intermediates of the citric acid cycle are precursors for carbohydrates, lipids, amino acids, nucleotides and porphyrins Reactions feeding into the cycle replenish the pool of cycle intermediates

31. Prentice Hall c2002Chapter 1231 Fig 12.17

32. Prentice Hall c2002Chapter 1232 1. Citrate Synthase Citrate formed from acetyl CoA and oxaloacetate Only cycle reaction with C-C bond formation

33. Prentice Hall c2002Chapter 1233 2. Aconitase Elimination of H 2 O from citrate to form C=C bond of cis-aconitate Stereospecific addition of H 2 O to cis-aconitate to form 2R,3S-Isocitrate

34. Prentice Hall c2002Chapter 1234 3. Isocitrate Dehydrogenase Oxidative decarboxylation of isocitrate to  -ketoglutarate (a metabolically irreversible reaction) One of four oxidation-reduction reactions of the cycle Hydride ion from the C-2 of isocitrate is transferred to NAD + to form NADH

35. Prentice Hall c2002Chapter 1235 4. The  -Ketoglutarate Dehydrogenase Complex Similar to pyruvate dehydrogenase complex E 1 -  -ketoglutarate dehydrogenase (with TPP) E 2 - succinyltransferase (with flexible lipoamide prosthetic group) E 3 - dihydrolipoamide dehydrogenase (with FAD)

36. Prentice Hall c2002Chapter 1236 5. Succinyl-CoA Synthetase Free energy in thioester bond of succinyl CoA is conserved as GTP (or ATP in plants and some bacteria)

37. Prentice Hall c2002Chapter 1237 6. The Succinate Dehydrogenase (SDH) Complex Located on the inner mitochondrial membrane, in contrast to other enzymes of the TCA cycle which are dissolved in the mitochondrial matrix Complex of polypeptides, FAD and iron-sulfur clusters Electrons are transferred from succinate to FADH 2, then to ubiquinone (Q), a lipid-soluble mobile carrier of electrons Reduced ubiquinone (QH 2 ) is released as a mobile product

38. Prentice Hall c2002Chapter 1238 7. Fumarase Addition of water to the double bond of fumarate to form malate

39. Prentice Hall c2002Chapter 1239 8. Malate Dehydrogenase Oxidation of malate to oxaloacetate, with transfer of electrons to NAD + to form NADH

40. Prentice Hall c2002Chapter 1240 The Glyoxylate Cycle Pathway for the formation of glucose from noncarbohydrate precursors in plants, bacteria and yeast (not animals) Glyoxylate cycle leads from 2-carbon compounds to glucose In animals, acetyl CoA is not a carbon source for the net formation of glucose (2 carbons of acetyl CoA enter cycle, 2 are released as 2 CO 2 ) Allows for the formation of glucose from acetyl CoA Ethanol or acetate can be metabolized to acetyl CoA and then to glucose via the glyoxylate cycle Stored seed oils in plants are converted to carbohydrates during germination

41. Prentice Hall c2002Chapter 1241 Fig 12.18 The Glyoxylate Cycle bypasses the two decarboxylation steps of the citric acid cycle, conserving the carbon atoms as glyoxylate for synthesis of glucose. Germinating seeds use this pathway to synthesize sugar (glucose) from oil (triacylglycerols).

42. Prentice Hall c2002Chapter 1242 Glyoxylate cycle in germinating castor beans Conversion of acetyl CoA to glucose requires the transfer of metabolites among three metabolic compartments (1) The glyoxysome (2) The cytosol (3) The mitochondria Figure 12.21

43. Prentice Hall c2002Chapter 1243 Fig 12.19 Isocitrate lyase: first bypass enzyme of glyoxylate

44. Prentice Hall c2002Chapter 1244 Fig 12.20 Malate synthase: second bypass enzyme of glyoxylate

45. Prentice Hall c2002Chapter 1245 Bypass reactions of glyoxylate cycle Citric Acid Cycle