Chapter 19 - Citric Acid Cycle Electron Harvest

Question 1

The complete oxidation of one molecule of acetyl CoA by the citric acid cycle results in:

a. production of one molecule of citrate

b. generation of two ATP molecules

c. consumption of one molecule of oxaloacetate

d. generation of two molecules of CO2

Answer 1

d. Generation of two molecules of CO2

Question 2

What is the net equation of the citric acid cycle?

Answer 2

Acetyl-CoA + 3 NAD⁺ + FAD + ADP + Pi —> 2 CO₂ + 3 NADH + 3 H⁺ + FADH₂ + ATP + CoA

Question 3

The citric acid cycle is part of aerobic respiration, but no O2 is required for the cycle. Explain this paradox.

Answer 3

The citric acid cycle depends on a steady supply of NAD+ and FAD, which are typically generated from NADH and FADH2 by reaction of these electron carriers with oxygen. If there is no oxygen to accept the electrons, the citric acid cycle will cease to operate.

Question 4

The citric acid cycle can be though of as taking place in two stages. What are the two stages?

Answer 4

First Stage - 2 carbon atoms are introduced into the cycle by reacting with oxaloacetate to form citrate. Two carbon atoms are then oxidized to CO₂

Second Stage - the resulting four-carbon molecule is metabolized to regenerate oxaloacetate.

High-energy electrons are generated in both stages.

Question 5

The synthesis of citrate from acetyl CoA and oxaloacetate is a biosynthetic reaction. What is the energy source that drives the formation of citrate?

Answer 5

Powered by the hydrolysis of a thioester.

Acetyl CoA provides the thioester that is converted into citryl CoA. Then this thioester is hydrolyzed, citrate is formed in an irreversible reaction.

Question 6

How is the wasteful hydrolysis of acetyl CoA prevented by citrate synthase?

Answer 6

Acetyl CoA does not bind citrate synthase until oxaloacetate is bound and ready for the synthesis of citrate. In addition, the catalytic groups required fro the hydrolysis of the thioester are not positioned for catalysis until citryl coA has formed.

Question 7

How is succinate dehydrogenase unique compared with the other enzymes in the citric acid cycle?

Answer 7

Succinate Dehydrogenase is the only enzyme in the citric acid cycle that is embedded in the mitochondrial membrane in association with the electron transport chain.

Question 8

What step in the citric acid cycle requires a molecule of inorganic phosphate?

Answer 8

The reaction Catalyzed by succinyl CoA Synthetase

Succinyl CoA + Pi + ADP —> Succinate + CoA + ATP

Question 9

What reaction in the citric acid cycle results in the direct formation of a molecule of ATP?

Answer 9

The reaction Catalyzed by succinyl CoA Synthetase

Succinyl CoA + Pi + ADP —> Succinate + CoA + ATP

Question 10

Which enzymes are the key regulatory enzymes of the citric acid cycle itself?

Answer 10

Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase