Compare the ΔG^∘' values for the oxidation of succinate by...

Compare the $\Delta G^{\circ \prime}$ values for the oxidation of succinate by $\mathrm{NAD}^{+}$ and by FAD. Use the data given in Table 18.1 to find the $E_{0}^{\prime}$ of the $\mathrm{NAD}^{+}=$ NADH and fumarate-succinate couples, and assume that $E_{0}^{\prime}$ for the $\mathrm{FAD}-\mathrm{FADH}_{2}$ redox couple is nearly $0.05 \mathrm{V}$ Why is FAD rather than $\mathrm{NAD}^{+}$ the electron acceptor in the reaction catalyzed by succinate dehydrogenase?

Key Concepts

Role of FAD in Succinate Dehydrogenase

Succinate dehydrogenase specifically uses FAD as the electron acceptor because its redox potential is appropriately matched to the oxidation of succinate. The relatively low E0' value of FAD (around 0.05 V) makes the reaction's ?G°' less unfavorable compared to using NAD+, whose higher reduction potential would not align as well with the available energy from succinate oxidation. This specificity ensures that electron transfer proceeds efficiently within the mitochondrial electron transport chain.

Choice of Electron Acceptors in Metabolic Pathways

Metabolic reactions are finely tuned based on the energy changes associated with electron transfers. The selection of an electron acceptor, such as FAD versus NAD+, is based on the redox potential that makes the reaction energetically favorable. In instances where the available energy difference is small, an electron carrier with a lower standard reduction potential (like FAD) is used to match the energetic output, ensuring efficient catalysis.

Standard Reduction Potential (E0')

This concept refers to the inherent tendency of a redox couple to gain or lose electrons and is measured under standard conditions. It quantitatively expresses how strongly a compound will accept electrons. In the context of bioenergetics, different redox couples have unique E0' values, which dictate the direction and feasibility of electron transfers in metabolic reactions.

Gibbs Free Energy Change (?G°') in Redox Reactions

?G°' is a thermodynamic parameter that indicates whether a reaction is spontaneous. In redox reactions, it is determined by the difference in the standard reduction potentials of the electron donor and acceptor, with the relationship ?G°' = -nF?E°'. This relationship provides insight into how much energy is available or required for a particular electron transfer, influencing which electron carrier is favored in a given reaction.

Transcript

00:01 This question wants us to compare the oxidation of succinate by nad plus and fad in order to determine why fad is the electron acceptor in this reaction catalyzed by succinate dehydrogenase.

00:13 So the relevant half reactions for the oxidation of succinate by nad is the half reaction of sexinate, which forms fumerate.

00:24 It is the opposite of the equation given in table 18 .1.

00:31 So in this reaction, succinate is being oxidized to form fumarate, two protons, and two electrons, with two electrons being passed, and the standard redox potential for this reaction is negative 0 .03 volts.

01:00 So we can use this half reaction to catalyze the free energy.

01:05 For this half reaction, using this equation, which relates the number of oxygens times faraday's constant times the redox potential to the free energy change.

01:17 So with this equation, and knowing our known variables here, and knowing that faraday's constant is equal to 96 .48 kilojoules per mole, we'll find that the delta g, the change in free energy for this half reaction, is equal to 5 .788 kilojoules per mole.

01:45 The relevant half reaction with nad is the reduction of nad to form nad.

01:59 And in this equation, and this reaction two electrons are carried, and it has a standard redux potential of negative 0 .32 volts.

02:17 So similarly, we can calculate the free energy change for this reaction, for this half reaction of nedh, and we will find it to be equal to 61 .7472 kilojoules per mole.

02:36 So the net reaction for the oxidation of succinate by nad will be given by succinate reacting with nad plus in order to form fumerate a proton and nadh.

03:05 And we can calculate the change in free energy for this overall reaction by summing the change in free energies for for the half reactions, and we will find that the change in free energy for the oxidation of succinate by nad is equal to positive 67 .536 kilojoules per mole, which is a positive delta g, which is an endergonic process, which is not that favorable to occur...

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