Use of FAD Rather Than NAD^+ in Succinate Oxidation All the...

Use of FAD Rather Than $\mathrm{NAD}^{+}$ in Succinate Oxidation All the dehydrogenases of glycolysis and the citric acid cycle use $\mathrm{NAD}^{+}(E^{\prime \circ} \text { for } \mathrm{NAD}^{+} / \mathrm{NADH} \text { is }-0.32 \mathrm{V}$ as electron . acceptor except succinate dehydrogenase, which uses covalently bound FAD $(E^{\prime \circ}$ for . FAD/FADH $_{2}$ in this enzyme is 0.050 $\mathrm{V}$ ). Suggest why FAD is a more appropriate electron acceptor than $\mathrm{NAD}^{+}$ in the dehydrogenation of succinate, based on the $E^{\prime \circ}$ values of fumarate/succinate $\left(E^{\circ}=0.031 \mathrm{V}\right), \mathrm{NAD}^{+} / \mathrm{NADH},$ and the succinate dehydrogenase FAD/FADH $_{2}.$

Key Concepts

Standard Reduction Potential

This concept refers to the quantitative measure (E°') of a species’ tendency to acquire electrons and thus be reduced. It is a critical factor in determining whether a redox reaction is thermodynamically favorable. Comparing E°' values allows one to predict the direction of electron flow between different redox couples based on their relative potentials.

Redox Driving Force

The redox driving force is the energy difference between the electron donor and acceptor, indicated by their standard reduction potentials. A greater difference in potential makes electron transfer more thermodynamically favorable. In cases where the oxidation of a substrate occurs with only a small positive potential, the choice of an appropriate electron acceptor (such as FAD) becomes essential to effectively drive the reaction forward.

Cofactor Specificity in Enzymatic Reactions

Enzymes often select specific cofactors based on the thermodynamic and kinetic requirements of the reaction they catalyze. FAD is used in certain dehydrogenases, like succinate dehydrogenase, because its redox potential aligns well with that of the substrate’s redox couple. This specificity ensures efficient electron transfer where the standard reduction potential of the substrate/product pair offers only a small driving force for reduction by alternate acceptors like NAD+.

Flavoproteins and FAD

Flavoproteins are enzymes that contain a flavin moiety, such as FAD, as a cofactor. FAD's ability to participate in redox reactions over a range of potentials makes it particularly useful in reactions where the substrate’s redox couple has a higher potential. Its covalent binding within enzymes like succinate dehydrogenase stabilizes electron transfer intermediates, thereby facilitating the oxidation of substrates that might not effectively transfer electrons to other cofactors like NAD+.

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