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Citric Acid Cycle 2
C483 Spring 2013
1. A metabolic pathway that is involved in both energy production and
biosynthesis is
A) Anaplerotic
B) amphibolic
C) Duplibolic
D) 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?
4.
A)
B)
C)
D)
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.
Overview
Carbon Flow
• Each cycle is net
oxidation of acetyl CoA
• C-13 incorporation
experiments
• 4-carbon compounds
act “catalytically” in
oxygen consumption
– Cyclic pathway!
Anaplerotic Reactions
• “Filling up”
reactions
• Formation of
oxaloacetate by
pyruvate
carboxylase
• No net
carbohydrates
from Acetyl CoA
in mammals
1. Citrate Synthase
• Highly exothermic—lysis of
high energy bond
• Used to drive reaction in
presence of small
[oxaloacetate]
2. Aconitase
• Citrate is achiral and prochiral
• Green represents carbon from acetyl CoA
– How can enzyme distinguish prochirality?
Prochirality
• Only one
compound
produced
X
3. Isocitrate
Dehydrogenase
• Oxidative
decarboxylation
• Spontaneous in
b-ketoacids
• NADH
4. a-Ketogluterate Dehydrogenase
Complex
• Analogous to
pyruvate
dehydrogenase
complex
• Second
decarboxylation, but
this is adecarboxylation
• High energy bond
retained
5. Succinyl CoA Synthetase
• Synthetase means ATP (GTP) involved
• High energy bond used to do substratelevel phosphorylation
– Good leaving group to activate Pi
– Covalent catalysis
– GDP  GTP
Notice: symmetrical
Product! We lose track of
which carbons are from
acetyl CoA!
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!
7. Fumarase
• Another prochiral molecule
• Hydration reaction sets up another oxidation
8. Malate Dehydrogenase
• Large standard free energy
• Driven by low [oxaloacetate]
Carbon Flow
• Practice C-14 labeling
problems given basic
chart
Answers
1.
2.
3.
4.
5.
B
B
Dehydrogenase
D
T