FAD is a stronger oxidant than NAD+; FAD has a higher standard reduction potential than NAD+. Yet in the last reaction of the pyruvate dehydrogenase complex, FADH2 bound to the E3 subunit is oxidized by NAD+.
Added by Michael B.
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FAD and NAD+ are both electron carriers that play crucial roles in redox reactions, with FAD typically involved in reactions that require a stronger oxidant. Show more…
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Nonenzyme-bound FAD is a stronger oxidizing agent than $\mathrm{NAD}^{+} .$ How, then, can $\mathrm{NAD}^{+}$ oxidize the reduced flavoenzyme in the pyruvate dehydrogenase complex?
Role of FAD as Electron Acceptor Acyl-CoA dehydrogenase uses enzyme-bound FAD as a prosthetic group to dehydrogenate the $\alpha$ and $\beta$ carbons of fatty acyl-CoA. What is the advantage of using FAD as an electron acceptor rather than $\mathrm{NAD}^{+}$ ? Explain in terms of the standard reduction potentials for the Enz-FAD/FADH $_{2}\left(E^{\prime \circ}=-0.219 \mathrm{V}\right)$ and NAD' $^{+}$ NADH $\left(E^{\circ}=-0.320 \mathrm{V}\right)$ half-reactions.
The dehydrogenases used in glycolysis as well as most of the dehydrogenases of the citric acid cycle utilize NAD+ as the electron acceptor. The one exception is succinate dehydrogenase in the citric acid cycle that transfers electrons onto FAD to make FADH2. Looking at the standard reduction potential for the redox pair NAD+/NADH the E°' = -0.32V, for FAD/FADH2 the E°' = 0.050V and for the redox pair fumarate/succinate the E°' = 0.031. Given this information, explain why succinate dehydrogenase preferably places electrons on FAD instead of NAD+ as the electron acceptor. Support your explanation with reference to E°' and ΔG.
Sri K.
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