ANSWERS TO PROBLEM SET 9. The Citric Acid Cycle


1.Acetyl-CoA is oxidized to carbon dioxide and water by the citric acid cycle.
2. Citric acid is 2-hydroxypropane-1,2,3-tricarboxylic acid.
Predict three citric acid cycles. Which agrees with the data given?


For a detailed discussion, see Chapter 9 of CBM.

You are going to grade your answers. There are 100 possible points. >90= A; 75-90 = B, 55-74 = C; 30-54 = D; <30 = F.  If you scored less than 55, you need to study Part 1 of CBM more carefully.

1. Did you write a balanced half reaction that shows 4 oxidation steps are required to oxidize acetyl-CoA to carbon dioxide? (5 points) (section 9.1, pp. 211-212 of CBM).
2. Did you write a net reaction that show the water formed from oxidation of acetyl-CoA comes from reduction of molecular oxygen. (5 points) (p. 212 of CBM).
3. Were you able to write the structure of citric acid from the name you were given? (5 points)(Fig  9.1, p. 211)
4.  Did you recognize that citric acid cannot be oxidized because it is a tertiary alcohol? (5 points).
5. Did you recognize that citric acid can undergo either elimination of water or a retrograde enolate condensation? (5 points).
6. Did you work backwards to succinic acid? (3 points) ( section 9.2, pp. 212-217)
7. Working backwards did you conclude that oxaloacetic acid, malic acid, fumaric acid (or maleic acid) and succinic acid are part of the citric acid cycle? (5 points) (Fig 9.11, p. 217)
8. Did you recognize that conversion of succinic acid to oxaloacetic acid accounts for two of the four oxidations required for oxidation of acetyl-CoA to carbon dioxide and water? (2 points)(Fig 9.11, p. 217).
9. Did you recognize that condensation of acetyl-CoA and oxaloacetic acid gives citroyl-CoA? (5 points)(Fig  9.3, p. 213).
10. Did you recognize that citroyl-CoA can be converted to citric acid by either hydrolysis or substrate level phosphorylation of ADP? (3 points)(Figs 9.4 and 9.5, p 214)
11. Did you conclude that elimination of water from citric acid produces aconitic acid? (5 points)( Fig 9.12, p. 217)
12. Did you conclude that hydration of aconitic acid could produce either citric acid or isocitric acid? (5 points) (p.218).
13. Did you recognize that isocitric acid can undergo either oxidation and decarboxylation to give α-ketoglutaric acid or a retrograde enolate condensation to give succinic acid and glyoxylic acid? (5 points) (p. 218).
14. Did you recognize that oxidation of isocitric acid adds a third oxidation step to the citric acid cycle? (3 points) (Fig 9.14, p 219).
15. Did you recognize that α-ketoglutaric acid  can undergo oxidative decarboxylation  to give succinyl-CoA and carbon dioxide? (5 points)(Fig  9.16,  p. 219)
16. Did you recognize that succinyl-CoA can be converted  to succinic acid by either hydrolysis or substrate phosphorylation? (2 points)(Figs 9.17 and 9.18, p. 220)
17. Did you write a possible citric acid cycle at this point? (10 points)(Fig 9.19, p. 221)
18. Does this cycle agree with the balanced reaction  you wrote at the start? (3 points)(Fig 9.21, p.221)
19. Did you recognize that a retrograde enolate condensation of isocitric acid produces an alternative   citric acid cycle. (5 points)(section 9.6,  p. 223)
20. Did you recognize that a second molecule of acetyl-CoA could react with glyoxylic acid to form malic acid? (10 points)(Fig  9.27, p. 225)
21. Did you recognize that condensation of acetyl-CoA with glyoxylic acid completes another alternative  citric acid cycle? (3 points) (Fig 9.28, p. 225)
22. Did you conclude that neither of the alternative  cycles agree with the information you were given or with the balanced reaction you wrote at the start?  (3 points)

We will explore these reactions in more detail in Problem Sets 10 and 11.

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