Citric Acid Cycle 2

C483 Spring 2013

1. A metabolic pathway that is involved in both energy production and biosynthesis isA) AnapleroticB) amphibolicC) DuplibolicD) 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. What is the thermodynamic driving force for formation of citrate?A) High levels of oxaloacetateB) C-C bond formation through condensationC) Loss of carbon dioxideD) 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 a-decarboxylation

• High energy bond retained

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

Notice: symmetricalProduct! 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!

CO2

O2C

CO2

O2C

FAD

FADH2

O

O

H3CO

H3CO

CH3

R

OH

OH

H3CO

H3CO

CH3

R

Q

QH2

succinate

fumarate

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. B2. B3. Dehydrogenase4. D5. T