Question

How many ATP are produced when one molecule of $FAD{H_2}$ is oxidized to FAD through an electron transport system?
A. $2$
B. $3$
C. $1$
D. $4$

Answer 1

$FAD{H_2}$ is involved in the electron transport chain. Electron transport chain-ETC is located in the inner mitochondrial membrane. The inner mitochondrial membrane has groups of several proton (${H^ + }$) and electron (${e^ - }$) acceptors, which are arranged in series called Electron Transport chain (ETC).

Complete Answer:
The various components of electron transport system includes:
Cytochrome b, 2 types of cytochrome C, Ubiquinone, flavoprotein (FMN or FAD), $Fe - S$protein ($Fe - S$) and enzyme cytochrome oxidase. These components are arranged in four kinds of complexes:
A) Complex I (NADH dehydrogenase complex).
B) Complex II (Succinate dehydrogenase complex).
C) Complex III (Cytochrome$b{c_1}$, complex).
D) Complex IV (Cytochrome oxidase complex).
E) A fifth complex, ATP synthase complex is also there (oxidative phosphorylation).

During Krebs cycle two hydrogens removed from succinate reduce FAD to $FAD{H_2}$. This $FAD{H_2}$ enters through Complex II into the electron transport system (ETS), at this stage by transferring its 2H to Ubiquinone (UQ) the UQ is reduced to $UQ{H_2}$. They are oxidised during ETC and used for ATP synthesis.

The correct option is (A) $2$.

Note: The enzyme involved in ETC oxidase is tightly bound to the inner mitochondrial membrane. This contains inseparable $Cyt{\text{ }}a$ and $Cyt{\text{ }}{a_3}$ components and also some polypeptide that contain two $Cu$ ion.
3 ATP are formed for every pair of electrons that enter by way of NADH and 2 ATP are formed that enter by way of $FAD{H_2}$.
Complex V i.e. ATP synthase complex is involved in ATP synthesis.