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+ -> semiquinoneThe 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 cox + 2 H +matrix -> Q + 2 cyt cred+ 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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