Q cycle

Meaning: Complex III (cytochrome bc₁) has to hand electrons from QH₂, which carries two, to cytochrome c, which carries only one. QH₂ is oxidized at a site facing the intermembrane space. One electron goes through the Fe-S protein and cytochrome c₁ to cytochrome c. The other goes through the b hemes to a second site near the matrix, where it reduces Q to a semiquinone. A second QH₂ repeats this, and its recycled electron finishes reducing the semiquinone to QH₂, which takes up 2 H⁺ from the matrix. The protons from the oxidized QH₂ are released into the intermembrane space.

Equation: QH₂ + 2 cyt c (Fe³⁺) + 2 H⁺(matrix) → Q + 2 cyt c (Fe²⁺) + 4 H⁺(intermembrane space). QH₂ is ubiquinol, Q is ubiquinone, and cyt c is cytochrome c in its oxidized (Fe³⁺) or reduced (Fe²⁺) form. The coefficients are amounts in mol.

On the MCAT: Passages describe an inhibitor or mutation at Complex III. Predict that carriers upstream (QH₂) stay reduced, cytochrome c downstream stays oxidized, and the proton gradient, O₂ consumption and ATP synthesis fall. Questions may also ask which side of the inner membrane gains H⁺ (the intermembrane space, so it becomes more acidic) or why a one-electron carrier follows a two-electron carrier.

Common confusion: The Q cycle is not a cycle of metabolites like the citric acid cycle. Q is reused within one complex. Protons are not moved through a channel. The gradient comes from QH₂ releasing H⁺ on one side and Q picking up H⁺ on the other. Textbooks count either 4 H⁺ released or 2 net per 2 electrons, so check the passage.