1. Citric Acid Cycle 2
C483 Spring 2013

2. 1. A metabolic pathway that is involved in both energy production and biosynthesis is
Anaplerotic
amphibolic
C) Duplibolic
Cataplerotic
2. Carbons from acetyl CoA are transferred to the citric acid cycle. Which is the first round of the citric acid cycle that could possibly release a carbon atom originating from this acetyl CoA?
A) First round.
B) Second round.
C) Third round.
D) Fourth round.
3. What type of enzyme is involved in all four redox reactions of the citric acid cycle?

3. What is the thermodynamic driving force for formation of citrate?
High levels of oxaloacetate
C-C bond formation through condensation
Loss of carbon dioxide
Hydrolysis of a high energy bond
5. True or false? Both glycolysis and the citric acid cycle produce a high energy bond starting from inorganic phosphate through a process called substrate level phosphorylation.

4. Overview

5. Carbon Flow Each cycle is net oxidation of acetyl CoA
C-13 incorporation experiments
4-carbon compounds act “catalytically” in oxygen consumption
Cyclic pathway!

6. Anaplerotic Reactions
“Filling up” reactions
Formation of oxaloacetate by pyruvate carboxylase
No net carbohydrates from Acetyl CoA in mammals

7. 1. Citrate Synthase Highly exothermic—lysis of high energy bond
Used to drive reaction in presence of small [oxaloacetate]

8. 2. Aconitase Citrate is achiral and prochiral
Green represents carbon from acetyl CoA
How can enzyme distinguish prochirality?

9. Prochirality
Only one compound produced X

10. 3. Isocitrate Dehydrogenase
Oxidative decarboxylation
Spontaneous in b-ketoacids
NADH

11. 4. a-Ketogluterate Dehydrogenase Complex
Analogous to pyruvate dehydrogenase complex
Second decarboxylation, but this is a-decarboxylation
High energy bond retained

12. 5. Succinyl CoA Synthetase
Synthetase means ATP (GTP) involved
High energy bond used to do substrate-level phosphorylation
Good leaving group to activate Pi
Covalent catalysis
GDP  GTP

13. Notice: symmetrical
Product! We lose track of which carbons are from acetyl CoA!

14. 6. Succinate Dehydrogenase
Oxidation to form C=C releases less energy
FAD is bound redox reagent
In turn, Q is reduced
Membrane enzyme—revisit in chapter 14!

15. 7. Fumarase Another prochiral molecule
Hydration reaction sets up another oxidation

16. 8. Malate Dehydrogenase Large standard free energy
Driven by low [oxaloacetate]

17. Carbon Flow
Practice C-14 labeling problems given basic chart

18. Answers B
Dehydrogenase D T