The complex II of mitochondrial electron transport chain is also known as

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Question

The complex II of mitochondrial electron transport chain is also known as

Accepted Answer

Correct Answer: C — Succinate dehydrogenaseStep-by-step Solution:The mitochondrial Electron Transport Chain (ETC), located in the inner mitochondrial membrane, consists of four protein complexes and two mobile electron carriers. Each complex is a large multi-protein assembly with specific functions:Complex I — NADH Dehydrogenase (NADH:ubiquinone oxidoreductase): The largest complex; accepts electrons from NADH, transfers them to ubiquinone (Coenzyme Q/CoQ); pumps 4H⁺ across the inner membrane per NADH; contains FMN (flavin mononucleotide) and multiple Fe-S clusters; inhibited by rotenone, amytal.Complex II — Succinate Dehydrogenase (Succinate:ubiquinone oxidoreductase) — CORRECT: Catalyzes the oxidation of succinate to fumarate (a TCA cycle reaction) and transfers the electrons from FADH₂ to ubiquinone (CoQ). Note: Complex II does NOT pump protons across the membrane (no energy is conserved as a proton gradient at this step). Contains FAD (covalently bound as prosthetic group), iron-sulfur clusters. The electrons from FADH₂ enter the ETC at Complex II (bypassing Complex I), which is why FADH₂ generates fewer ATP than NADH. Inhibited by malonate (competitive inhibitor of succinate dehydrogenase — succinate vs. malonate competition — a classic competitive inhibition example).Complex III — Cytochrome bc1 (Ubiquinol:cytochrome c oxidoreductase): Accepts electrons from reduced CoQ (CoQH₂/ubiquinol); transfers to cytochrome c (mobile carrier); pumps 4H⁺ per 2e⁻ (Q cycle mechanism); contains cytochrome b, cytochrome c1, and Rieske Fe-S protein; inhibited by antimycin A.Complex IV — Cytochrome c oxidase (Cytochrome c:O₂ oxidoreductase): Terminal oxidase; accepts electrons from cytochrome c; reduces O₂ to H₂O (the final electron acceptor); pumps 2H⁺ per 2e⁻; contains copper centers (CuA, CuB) and cytochrome a/a3; inhibited by cyanide (CN⁻), carbon monoxide (CO), azide (N₃⁻).Extended Explanation:ATP yield from ETC: NADH → enters at Complex I → 10H⁺ pumped → ~2.5 ATP via ATP synthase; FADH₂ → enters at Complex II → 6H⁺ pumped → ~1.5 ATP. Total ATP from complete oxidation of glucose: ~30–32 ATP (modern estimates, replacing older values of 36–38). ATP synthase (Complex V / F₁F₀-ATPase): not part of the ETC per se but uses the proton gradient (proton motive force) created by Complexes I, III, IV to drive ATP synthesis from ADP + Pi. Mobile carriers: Ubiquinone (CoQ) — lipid-soluble, moves in the membrane; carries electrons from I and II to III; Cytochrome c — water-soluble, peripheral membrane protein; carries electrons from III to IV. The ETC is the site of action of many toxins and drugs — understanding inhibition sites is important for pharmacology and toxicology.Key Concepts: Electron transport chain, Complex I (NADH dehydrogenase), Complex II (succinate dehydrogenase — FAD, succinate to fumarate, no proton pumping), Complex III (cytochrome bc1), Complex IV (cytochrome c oxidase), ubiquinone, cytochrome c, proton gradient, ATP synthesis.

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