In the electron transport chain, ubiquinol (CoQH2) donates electrons to Complex III (cytochrome bc1). The Q cycle mechanism results in 4 protons being translocated per 2 electrons passed through Complex III. If the P/O ratio for NADH oxidation is approximately 2.5 ATP per FADH2 pair, what is the correct sequence of electron carriers within Complex III from ubiquinol to cytochrome c?

• A CoQH2 → Rieske iron-sulfur protein → cytochrome b (bL/bH) → cytochrome c1 → cytochrome c
• B CoQH2 → cytochrome b (bL) → Rieske iron-sulfur protein → cytochrome c1 → cytochrome c
• C CoQH2 → Rieske iron-sulfur protein → cytochrome c1 → cytochrome c, with cytochrome b recycling the semiquinone ✓
• D CoQH2 → cytochrome c1 → Rieske iron-sulfur protein → cytochrome b → cytochrome c

Explanation

In the Q cycle mechanism of Complex III, ubiquinol (QH2) at the Qo (outer) site bifurcates its two electrons. One electron flows via the high-potential chain: Rieske 2Fe-2S protein → cytochrome c1 → cytochrome c (released to mobile pool). The second electron flows via the low-potential chain: cytochrome bL → cytochrome bH → semiquinone at the Qi (inner) site. After two turnovers of the cycle, one ubiquinone is fully reduced to ubiquinol at the Qi site, regenerating QH2. This cycling mechanism doubles the proton pumping efficiency of Complex III from 2H+/electron to 4H+/2 electrons across the inner mitochondrial membrane. Cytochrome b contains both bL and bH centers and functions in the recycling arm, not the main electron-to-cytochrome c path.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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