Question

Nonenzyme-bound FAD is a stronger oxidizing agent than ${\mathbf{NAD}}^{\mathbf{+}}$. How, then, can ${\mathbf{NAD}}^{\mathbf{+}}$oxidize the reduced flavoenzyme in the pyruvate dehydrogenase

complex?

Short answer

Oxidation potential of FAD changes when the enzyme binds with FAD.

Step-by-step solution

Binding of enzyme with FAD

Moreover, FAD has the ability to oxidize NADH to ${\mathrm{NAD}}^{+}$. When the enzyme gets bound with FAD, oxidation potential of FAD will be changed and becomes less than that of ${\mathrm{NAD}}^{+}$. So${\mathrm{NAD}}^{+}$ can oxidize enzyme bound${\mathrm{FADH}}_2$ to FAD

Step 2: Oxidation of FADH2 by NAD+

More ${\mathrm{NAD}}^{+}$is present in mitochondria than the enzyme bound ${\mathrm{FADH}}_2$. The oxidation of ${\mathrm{FADH}}_2$by ${\mathrm{NAD}}^{+}$takes place in mitochondria.

Thus, using Le Chateliers principle, equilibrium is favoured to the oxidation of ${\mathrm{FADH}}_2$by ${\mathrm{NAD}}^{+}$.

$\mathrm{E}-{\mathrm{FADH}}_2+{\mathrm{NAD}}^{+}\mathrm{E}-\mathrm{FAD}+\mathrm{NADH}+{\mathrm{H}}^{+}$

excess