Question

In the electron-transport chain, electrons are handed off from low reduction potential carriers to increasingly higher reduction potential carriers. At some steps this exchange of electrons is sufficiently exergonic to drive proton pumping. Based on this, explain why FADH2 results in less ATP generation per mole compared with NADH.

Answer 1

In electron-transport chain, there are four complexes in the following order: Complex I, complex II, complex III and complex IV

NADH is an electron donor that releases high energy electrons at Complex I. Complex I pumps four protons into the intermembrane space of mitochondria for each NADH oxidized and then passes the electrons to Complex III. The complex III in turn pumps four protons to the intermembrane space. Again, the complex III passes these electrons to Complex IV and pumps four electrons to the intermembrance space. The complex IV then pumps 2 protons into the intermembrane space. Hence, a total of 10 protons are pumped for each NADH oxidized. The flow of these protons down their concentration gradient is coupled with ATP synthesis through the ATP synthase.

FADH2 is an electron donor that donates its high energy electrons to Complex II. Complex II does not pump any proton. It passes on the electrons to Complex III. The Complex III pumps 4 protons into the intermembrane space and then passes on the electrons to complex IV which in turn pumps 2 protons into the intermembrane space. Hence, a total of 6 protons are pumped for each FADH2 oxidized.

As we can see, the total number of protons pumped in case of NADH and FADH2 are different. In case of FADH2, number of protons pumped is lower and hence the amount of ATP generated is also lower.

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