Chapter 20, Problem 13RE

MATHEMATICAL Which is more favorable energetically, the oxidation of succinate to fumarate by ${\text{NAD}}^{+}$ or by FAD? Give the reason for your answer.

Interpretation Introduction

Interpretation:

The reason for the oxidation of succinate to fumarate using NAD+and FADis to be explained.

Concept introduction:

Oxidation is the addition of an electronegative element or the removal of an electropositive element in a chemical reaction.

Reduction is the addition of an electropositive element or the removal of an electronegative element in a chemical reaction

The chemical reaction in which oxidation and reduction take place simultaneously is called a redox reaction.

Cytochromes are the types of proteins that consists of iron as the heme group.

The coenzyme Q is the redox coenzyme in the mitochondrial electron transport.

NADH−CoQ oxidoreductase is the first complex in the electron transport chain and it catalyzes the transfer of electrons from NADHtocoenzyme Q.

The succinate−CoQ oxidoreductase is the second complex of the electron transport chain, which catalyzes the transfer of electrons from the succinates to coenzyme Q. The succinates are the substrates from the citric acid cycle.

Answer to Problem 13RE

Solution:

The oxidation of succinate to fumarate using NAD+and FAD is energetically unfavorable due to the negative values of standard reduction potential.

Explanation of Solution

The half reactions that are paired during the redox reaction are shown below, along with their standard reduction potentials:

Reaction−1: Fumarate+  2 H+  +2 e−→Succinate Eo' = 0.031 V

Reaction−2: FAD + 2 H++2 e−→FADH2 Eo' = −0.219 V

Reaction−3: NAD+ + 2 H++2 e−→NADH + H+ Eo' = −0.320V

Case−I: The aim reaction for the oxidation of succinate to fumarate using FAD is as follows:

Succinate+  FAD  →Fumarate + FADH2

As the above aim reaction will get by subtracting reaction−1from reaction−2 that are mentioned above, the overall voltage for the aim reaction will get by subtracting the standard reduction potential values of the two half-reactions mentioned above.

E∘' = −0.219V−(0.031V)E∘' = −0.219V−0.031VE∘' = −0.250V.

Hence, the overall voltage of the given biological reaction will be calculated.

Case−II: The aim reaction for the oxidation of succinate to fumarate usingNAD+ is as follows:

Succinate+  NAD+  →Fumerate + NADH

As the above aim reaction will get by subtracting reaction−1from reaction−3 that are mentioned above, the overall voltage for the aim reaction will get by subtracting the standard reduction potential values of the two half-reactions above.

E∘' = −0.320V−(0.031V)E∘' = −0.320V−0.031VE∘' = −0.351 V.

Hence, the overall voltage of the given biological reaction will be calculated.

The value of the standard reduction potential in each case is negative. Thus, the oxidation of succinate to fumarate using NAD+and FADis energetically unfavorable in both the cases. There are two main reasons behind this. The first is that these two reactions are non-spontaneous under standard conditions. The second reason is that the reducing agents in the two reactions act as electron carriers and get reoxidized.

Conclusion

The oxidation of succinate to fumarate using NAD+and FADis energetically unfavorable due to the negative values of standard reduction potential.