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Question: Oxidative phosphorylation is carried out by electron transport chain (ETC) present in the mitochondr...

Oxidative phosphorylation is carried out by electron transport chain (ETC) present in the mitochondria of aerobes. The NADH or FADH2_2 produced in TCA (Kreb's) cycle transfers their electron to the ETC to finally reduce O2_2. Based on this information, the correct option is:

A

Complex I of ETC is also an enzyme of the TCA cycle.

B

Complex I transfers electrons to complex II in the ETC.

C

Complex IV is an ATP synthase.

D

Complex II does not transfer protons into the inter-membrane space.

Answer

Complex II does not transfer protons into the inter-membrane space.

Explanation

Solution

The electron transport chain (ETC) in mitochondria consists of four major protein complexes (Complex I, II, III, and IV) and ATP synthase (Complex V).

  • Complex I is NADH dehydrogenase. It oxidizes NADH and transfers electrons to ubiquinone. It is not an enzyme of the TCA cycle. Complex II (succinate dehydrogenase) is the only complex that is part of both the TCA cycle and the ETC. It catalyzes the oxidation of succinate to fumarate in the TCA cycle and transfers electrons from FADH2_2 to ubiquinone in the ETC.

  • Complex I (NADH dehydrogenase) transfers electrons to ubiquinone (Q). Complex II (succinate dehydrogenase) also transfers electrons to ubiquinone (Q). Electrons from Complex I do not directly flow to Complex II. Both complexes feed electrons into the ubiquinone pool.

  • Complex IV is cytochrome c oxidase, which catalyzes the final transfer of electrons to molecular oxygen, reducing it to water. ATP synthase (also known as Complex V or F0_0F1_1-ATPase) is the enzyme responsible for synthesizing ATP using the proton gradient established by Complexes I, III, and IV.

  • Complexes I, III, and IV are proton pumps that actively transport protons from the mitochondrial matrix to the inter-membrane space, generating the proton motive force. Complex II (succinate dehydrogenase) accepts electrons from FADH2_2 and transfers them to ubiquinone, but it does not pump protons across the inner mitochondrial membrane. It is unique in this aspect among the first four complexes.

Therefore, Complex II is the only one among the first four complexes that does not pump protons.