Case study- Mutation in complex II of ETC

An -8 year-old boy is seen by an ophthalmologist for difficulties in seeing in all visual fields as well as slow eye movements. The ophthalmologist finds pigmentary retinopathy and ophthalmoplegia. The child is suspected to have Kearns- Sayre syndrome, a disorder due to a mutation in complex II of ETC. The electron transport from which substance would be impaired?

A. Malate

B. Isocitrate

C. Succinate

D. Pyruvate

E. Alpha Keto glutarate

Answer- The correct answer is C- Succinate.

Electrons flow through the respiratory chain through a redox span of 1.1 V from NAD+/NADH to O2/2H2O passing through three large protein complexes;

1) NADH-Q oxidoreductase (Complex I), where electrons are transferred from NADH to coenzyme Q (Q) (also called ubiquinone)

2) Q-cytochrome c oxidoreductase (Complex III), which passes the electrons on to cytochrome c; and

3) Cytochrome c oxidase (Complex IV), which completes the chain, passing the electrons to O2 and causing it to be reduced to H2O .

Some substrates with more positive redox potentials than NAD+/NADH (e.g., succinate) pass electrons to Q via, succinate Q reductase (Complex II), rather than Complex I.

The four complexes are embedded in the inner mitochondrial membrane, but Q and cytochrome c are mobile. Q diffuses rapidly within the membrane, while Cytochrome c is a soluble protein.


Figure- Flow of electrons through the respiratory chain complexes.

Malate, Isocitrate and alpha ketoglutarate are intermediates of TCA cycle, the electrons flow from them to NAD+ forming NADH that is regenerated back to its oxidized form by passing electrons to complex I of ETC. Similarly electron flow from Pyruvate is also through complex I.

Electron from Succinate pass to FAD forming FADH2 that is regenerated back to its oxidized form after passing its electrons to complex II of ETC.

Thus the correct answer for the given problem is Succinate.





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