Complex III (cytochrome bc₁) accepts two electrons from ubiquinol and shuttles them to cytochrome c. Such a transfer is quite complex, since ubiquinol carries two electrons while cytochromec carries but one.

There are two electron transfer pathways in comlex III. Ubiquinol donates one electron to the , a special (His-ligated) Fe-S cluster. The protons released by ubiquinol oxidation are transferred to the intermembrane space. The electron then moves to a , heme , and thence to . cytochrome c.

The second electron is transferred to a heme and then (through another heme) to an ubiquinone molecule, reducing it to the semiquinone form.

Q + e^-+ H⁺ -> semiquinone

The proton needed for this half-reaction is supplied by the mytochondrial matrix. In the next cycle, the semiquinone accepts another electron (and another proton from the mytochondrial matrix), and is converted to ubiquinol:

semiquinone + e^- + H⁺ -> QH₂

The electron-transfer mechanism in compex III can therefore be represented in the following form:

2 QH₂ + Q + 2 cyt c_ox + 2 H ⁺_matrix -> Q + 2 cyt c_red+ QH₂ + 4 H ⁺_intermembr

The protons released by ubiquinol oxidation are transferred to the intermembrane space. On the other hand, the protons needed to reduce ubiquinone to semiquinone are supplied by the mytochondrial matrix. This process allows net proton transfer across the membrane even though no direct internal proton channel exists in complex III.

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