Question 1

Which enzyme catalyzes the first reaction of the citric acid cycle, and what are the reactants and products of this reaction? Write the overall reaction equation.

Accepted Answer

Citrate synthase; Acetyl-CoA, oxaloacetate, and H$$_2O $$are converted to citrate, CoA-SH, and H$$^+$$. $$\mathrm{Acetyl{-}CoA + Oxaloacetate + H_2O \rightarrow Citrate + CoA{-}SH + H^+}$$

Question 2

What is the significance of the hydrolysis of the thioester bond in the first step of the citric acid cycle?

Accepted Answer

It provides the energy needed to drive the condensation of acetyl-CoA and oxaloacetate, making the reaction highly exergonic. $$\Delta G'^{\circ} = -32.2\ \mathrm{kJ/mol}$$

Question 3

Which of the following best describes the fate of the two carbons introduced into the citric acid cycle by acetyl-CoA?

Accepted Answer

They are incorporated into citrate and remain in the cycle for at least one full turn before being released as CO$$_2.$$

Question 4

Why is succinate not highlighted after step 5 in the citric acid cycle diagram?

Accepted Answer

Because succinate is symmetrical, making it impossible to track the fate of individual carbons from acetyl-CoA.

Question 5

How many NADH, FADH$$_2$$, and GTP (or ATP) molecules are produced per turn of the citric acid cycle?

Accepted Answer

Three NADH, one FADH$$_2$$, and one GTP

Question 6

Which statement best explains why the citric acid cycle can proceed even at low concentrations of oxaloacetate?

Accepted Answer

The hydrolysis of the thioester bond in acetyl-CoA provides a large negative $$\Delta G'^{\circ}$$, driving the reaction forward.

Question 7

What is the role of citrate synthase in the citric acid cycle?

Accepted Answer

It catalyzes the condensation of acetyl-CoA and oxaloacetate to form citrate.

Question 8

Which of the following best describes the structural change in citrate synthase upon substrate binding?

Accepted Answer

Binding of oxaloacetate and acetyl-CoA induces a closed, catalytically competent conformation.

Question 9

What is meant by citrate being a 'prochiral' molecule in the context of the citric acid cycle?

Accepted Answer

It can become chiral if one of its carboxymethyl groups is changed, allowing for the formation of a chiral center.

Question 10

Which enzyme catalyzes the isomerization of citrate to D-isocitrate, and what is the significance of this reaction?

Accepted Answer

Aconitase; it converts a prochiral molecule to a chiral molecule.

Question 11

Which of the following is a correct description of the overall stoichiometry for one turn of the citric acid cycle starting from acetyl-CoA?

Accepted Answer

$$\mathrm{Acetyl{-}CoA + 3 NAD^+ + FAD + GDP + P_i + 2 H_2O \rightarrow 2 CO_2$ + 3 NADH + 3 H^+ + $$FADH_2$$ + GTP + CoA{-}SH}$$

Question 12

A mutation in the active site of citrate synthase prevents the hydrolysis of citroyl-CoA. What is the most likely consequence?

Accepted Answer

The condensation of acetyl-CoA and oxaloacetate will not proceed efficiently, halting the citric acid cycle.

Question 13

Which of the following statements about the energetics of the first reaction of the citric acid cycle is correct?

Accepted Answer

The reaction is highly exergonic, with $$\Delta G'^{\circ} = -32.2\ \mathrm{kJ/mol}$$, favoring citrate formation.

Question 14

Which of the following best explains why the citric acid cycle is considered a cycle?

Accepted Answer

Because the product of the eighth reaction (oxaloacetate) is also a reactant for the first reaction, allowing the process to repeat.

Citric Acid Cycle: Structure, Mechanism, and Fate of Carbon

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