Q.37. The prosthetic groups/cofactors involved in both 𝟏𝐞−and 𝟐𝐞−transfer in the mitochondrial
electron transport chain are
(A) NAD and NADP
(B) NAD and FAD
(C) Heme and FMN
(D) Coenzyme Q and FMN

Answer: NAD and FAD

Explanation:

• In the mitochondrial electron transport chain (ETC), electrons are shuttled through a series of protein complexes and carriers. The two main carriers that participate in both one-electron and two-electron transfer processes are NAD (as NADH/NAD+) and FAD (as FADH2/FAD).

• NAD is a primary electron donor in Complex I (NADH dehydrogenase). It donates electrons as a two-electron carrier but transfers them one at a time within the complex, ultimately feeding the chain through ubiquinone (Coenzyme Q). Thus, NAD/NADH serves in both one-electron and two-electron steps depending on the transfer context.

• FAD is involved in Complex II (succinate dehydrogenase) and also as a prosthetic group in other flavoproteins. FADH2 donates electrons to the ETC via Coenzyme Q. Like NADH, FAD participates in transfers that can be considered single-electron steps within the protein’s flavin chemistry, while overall the process moves electrons in two-electron equivalents from substrates to the chain.

• The other options:

  • (A) NAD and NADP: NADP is primarily used in anabolic reactions (photosynthesis in plants, oxidative biosynthesis) and not a central electron donor in the mitochondrial ETC. It is not the typical prosthetic pair for mitochondrial electron transfer.

  • (C) Heme and FMN: Heme groups (in cytochromes) and FMN (a flavin) are key prosthetic groups in the ETC, but they do not together represent the principal pair responsible for both 1e− and 2e− transfer in the mitochondria as a primary, universal pair the way NAD and FAD do.

  • (D) Coenzyme Q and FMN: Coenzyme Q (ubiquinone) and FMN (flavin mononucleotide) participate in electron transport, but the question asks for the prosthetic groups involved in both 1e− and 2e− transfer in the mitochondria. The canonical pairing highlighting the main donors is NAD and FAD, with CoQ and FMN acting within specific complexes rather than as the two universal donors for both transfer modes.

Why NAD and FAD are correct:

• NADH and FADH2 are classic electron donors feeding electrons into the ETC at Complex I and Complex II, respectively. Their redox chemistry supports both one-electron and two-electron transfer steps as electrons pass through the chain and via intermediates such as ubiquinone and cytochromes, coordinating energy production through oxidative phosphorylation.

Tips for exam preparation:

• Remember: NAD+/NADH primarily enters the chain at Complex I, while FAD/FADH2 feeds into the chain at Complex II. Both participate in reactions where electrons are transferred in either one-electron or two-electron fashion depending on the redox chemistry of the specific carrier.

• Distinguish between prosthetic groups (non-covalently bound to proteins) like NAD/NADH and FAD/FADH2 versus mobile carriers like Coenzyme Q (ubiquinone) and cytochrome c.

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