Chapter 19. Problem 19.25

Problem-Solving Video Assessment

true

false

Problem 19.25: Coupling Reactions

1.

The second step of the citric acid cycle involves a two-part isomerization of citrate to form isocitrate via the enzyme aconitase. The ΔG°^' = +13.3 kJ/mol for this reaction. Which of the following statements best explains why this reaction proceeds in the forward direction?

This reaction is highly endergonic.

This reaction is highly exergonic.

Isocitrate is rapidly used in the downstream formation of α-ketoglutarate.

The concentration of citrate in the cell is very low.

The next reaction in the citric acid cycle is highly endergonic.

_feedback_correct: Correct. _feedback_incorrect: Incorrect.

The last step of the citric acid cycle, oxidation of malate to form oxaloacetate, is endergonic under standard conditions, with a ΔG°^' = +29 kJ/mol:

malate + NAD⁺ ⇄ oxaloacetate + NADH + H⁺

2.

Assume that inside of a cell at pH 7, the [NADH]/[NAD⁺] ratio is 0.07 and the [malate]/[oxaloacetate] ratio is 1.0 x 10⁴. What is the actual free-energy change (ΔG) for this reaction?

-40.2 kJ/mol

-0.4 kJ/mol

+12.8 kJ/mol

+45.2 kJ/mol

+58.4 kJ/mol

_feedback_correct: Correct. _feedback_incorrect: Incorrect.

3.

Which of the following statements is correct concerning the fate of this reaction under the physiological conditions outlined in question 2?

The reaction will proceed toward malate formation due to an increase in free energy.

The reaction will proceed toward malate formation due to a loss of free energy.

The reaction will proceed toward oxaloacetate formation due to an increase in free energy.

The reaction will proceed toward oxaloacetate formation due to a loss of free energy.

The reaction has reached equilibrium.

_feedback_correct: Correct. _feedback_incorrect: Incorrect.

Activity results are being submitted...

Chapter 19. Problem 19.25

Introduction

1.

2.

3.